1
LTC1286/LTC1298
Micropower Sampling
12-Bit A/D Converters In
S0-8 Packages
■
12-Bit Resolution
■
8-Pin SOIC Plastic Package
■
Low Cost
■
Low Supply Current: 250µA Typ.
■
Auto Shutdown to 1nA Typ.
■
Guaranteed ±3/4LSB Max DNL
■
Single Supply 5V to 9V Operation
■
On-Chip Sample-and-Hold
■
60µs Conversion Time
■
Sampling Rates:
12.5 ksps (LTC1286)
11.1 ksps (LTC1298)
■
I/O Compatible with SPI, Microwire, etc.
■
Differential Inputs (LTC1286)
■
2-Channel MUX (LTC1298)
■
3V Versions Available: LTC1285/LTC1288
The LTC1286/LTC1298 are micropower, 12-bit, succes-
sive approximation sampling A/D converters. They typi-
cally draw only 250µA of supply current when converting
and automatically power down to a typical supply current
of 1nA whenever they are not performing conversions.
They are packaged in 8-pin SO packages and operate on
5V to 9V supplies. These 12-bit, switched-capacitor, suc-
cessive approximation ADCs include sample-and-holds.
The LTC1286 has a single differential analog input. The
LTC1298 offers a software selectable 2-channel MUX.
On-chip serial ports allow efficient data transfer to a wide
range of microprocessors and microcontrollers over three
wires. This, coupled with micropower consumption, makes
remote location possible and facilitates transmitting data
through isolation barriers.
These circuits can be used in ratiometric applications or
with an external reference. The high impedance analog
inputs and the ability to operate with reduced spans (to
1.5V full scale) allow direct connection to sensors and
transducers in many applications, eliminating the need for
gain stages.
5V4.7µF
ANALOG INPUT
–IN
GND
V
CC
CLK
D
OUT
V
REF
LTC1286
MPU
(e.g., 8051)
P1.4
P1.3
P1.2
+IN
0V TO 5V RANGE
LTC1286/98 • TA01
CS/SHDN
6
5
8
7
3
4
1
2
SERIAL DATA LINK
■
Battery-Operated Systems
■
Remote Data Acquisition
■
Battery Monitoring
■
Handheld Terminal Interface
■
Temperature Measurement
■
Isolated Data Acquisition
SAMPLE FREQUENCY (Hz)
0.1k
1
SUPPLY CURRENT (µA)
10
100
1000
1k 10k 100k
LTC1286/98 • TA02
T
A
= 25°C
V
CC
= V
REF
= 5V
f
CLK
= 200kHz
DESCRIPTION
U
25µW, S0-8 Package, 12-Bit ADC
Samples at 200Hz and Runs Off a 5V Supply Supply Current vs Sample Rate
FEATURES
APPLICATIONS
U
TYPICAL APPLICATIONS N
U
2
LTC1286/LTC1298
1298
1298I
ORDER PART
NUMBER
LTC1286CN8
LTC1286IN8
T
JMAX
= 150°C, θ
JA
= 130°C/W T
JMAX
= 150°C, θ
JA
= 175°C/W
ORDER PART
NUMBER
PACKAGE/ORDER INFORMATION
W
UU
ABSOLUTE MAXIMUM RATINGS
W
WW
(Notes 1 and 2)
Power Dissipation..............................................500mW
Operating Temperature Range
LTC1286C/LTC1298C............................. 0°C to 70°C
LTC1286I/LTC1298I........................... –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................ 300°C
Supply Voltage (V
CC
) to GND................................... 12V
Voltage
Analog and Reference ................ –0.3V to V
CC
+ 0.3V
Digital Inputs.........................................–0.3V to 12V
Digital Output ............................. –0.3V to V
CC
+ 0.3V
PART MARKING
1286
1286I
ORDER PART
NUMBER
LTC1298CN8
LTC1298IN8
ORDER PART
NUMBER
PART MARKING
LTC1298CS8
LTC1298IS8
LTC1286CS8
LTC1286IS8
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
CC
Supply Voltage (Note 3) LTC1286 4.5 9.0 V
LTC1298 4.5 5.5 V
f
CLK
Clock Frequency V
CC
= 5V (Note 4) 200 kHz
t
CYC
Total Cycle Time LTC1286, f
CLK
= 200kHz 80 µs
LTC1298, f
CLK
= 200kHz 90 µs
t
hDI
Hold Time, D
IN
After CLK↑V
CC
= 5V 150 ns
t
suCS
Setup Time CS↓ Before First CLK↑ (See Operating Sequence) LTC1286, V
CC
= 5V 2 µs
LTC1298, V
CC
= 5V 2 µs
t
suDI
Setup Time, D
IN
Stable Before CLK↑V
CC
= 5V 400 ns
t
WHCLK
CLK High Time V
CC
= 5V 2 µs
t
WLCLK
CLK Low Time V
CC
= 5V 2 µs
t
WHCS
CS High Time Between Data Transfer Cycles V
CC
= 5V 2 µs
t
WLCS
CS Low Time During Data Transfer LTC1286, f
CLK
= 200kHz 75 µs
LTC1298, f
CLK
= 200kHz 85 µs
RECOM ENDED OPERATING CONDITIONS
UUUU W
W
Consult factory for military grade parts.
1
2
3
4
8
7
6
5
TOP VIEW
V
REF
+IN
–IN
GND
V
CC
CLK
D
OUT
N8 PACKAGE
8-LEAD PLASTIC DIP
CS/SHDN
1
2
3
4
8
7
6
5
TOP VIEW
CH0
CH1
GND
V
CC
(V
REF
)
CLK
D
OUT
D
IN
N8 PACKAGE
8-LEAD PLASTIC DIP
CS/SHDN 1
2
3
4
8
7
6
5
TOP VIEW
V
CC
(V
REF
)
CLK
D
OUT
D
IN
CH0
CH1
GND
S8 PACKAGE
8-LEAD PLASTIC SOIC
CS/SHDN
1
2
3
4
8
7
6
5
TOP VIEW
V
CC
CLK
D
OUT
V
REF
+IN
–IN
GND
S8 PACKAGE
8-LEAD PLASTIC SOIC
CS/SHDN
T
JMAX
= 150°C, θ
JA
= 130°C/W T
JMAX
= 150°C, θ
JA
= 175°C/W
3
LTC1286/LTC1298
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
S/(N +D) Signal-to-Noise Plus Distortion Ratio 1kHz/7kHz Input Signal 71/68 dB
THD Total Harmonic Distortion (Up to 5th Harmonic) 1kHz/7kHz Input Signal –84/–80 dB
SFDR Spurious-Free Dynamic Range 1kHz/7kHz Input Signal 90/86 dB
Peak Harmonic or Spurious Noise 1kHz/7kHz Input Signal –90/–86 dB
fSMPL = 12.5kHz (LTC1286), fSMPL = 11.1kHz (LTC1298) (Note 5)
CONVERTER AND MULTIPLEXER CHARACTERISTICS
UW U
(Note 5)
DYNAMIC ACCURACY
UW
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
IH
High Level Input Voltage V
CC
= 5.25V ●2V
V
IL
Low Level Input Voltage V
CC
= 4.75V ●0.8 V
I
IH
High Level Input Current V
IN
= V
CC
●2.5 µA
I
IL
Low Level Input Current V
IN
= 0V ●–2.5 µA
V
OH
High Level Output Voltage V
CC
= 4.75V, I
O
= 10µA●4.0 4.64 V
V
CC
= 4.75V, I
O
= 360µA●2.4 4.62 V
V
OL
Low Level Output Voltage V
CC
= 4.75V, I
O
= 1.6mA ●0.4 V
I
OZ
Hi-Z Output Leakage CS = High ●±3µA
I
SOURCE
Output Source Current V
OUT
= 0V –25 mA
I
SINK
Output Sink Current V
OUT
= V
CC
45 mA
R
REF
Reference Input Resistance CS = V
CC
5000 MΩ
(LTC1286) CS = GND 55 kΩ
I
REF
Reference Current (LTC1286) CS = V
CC
●0.001 2.5 µA
t
CYC
≥ 640µs, f
CLK
≤ 25kHz ●90 140 µA
t
CYC
= 80µs, f
CLK
= 200kHz ●90 140 µA
I
CC
Supply Current CS = V
CC
●0.001 ±3.0 µA
LTC1286, t
CYC
≥ 640µs, f
CLK
≤ 25kHz ●220 460 µA
LTC1286, t
CYC
= 80µs, f
CLK
= 200kHz ●260 500 µA
LTC1298, t
CYC
≥ 720µs, f
CLK
≤ 25kHz ●320 600 µA
LTC1298, t
CYC
= 90µs, f
CLK
= 200kHz ●360 640 µA
(Note 5)
1.5V to V
CC
+ 0.05V
1.5V to 5.55V
–0.05V to V
CC
+ 0.05V
LTC1286 LTC1298
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
Resolution (No Missing Codes) ●12 12 Bits
Integral Linearity Error (Note 6) ●±3/4 ±2±3/4 ±2 LSB
Differential Linearity Error ●±1/4 ±3/4 ±1/4 ±3/4 LSB
Offset Error ●3/4 ±33/4±3 LSB
Gain Error ●±2±8±2±8 LSB
Analog Input Range (Note 7 and 8) ●V
REF Input Range (LTC1286) 4.5 ≤ V
CC
≤ 5.5V V
(Notes 7, 8, and 9) 5.5V < V
CC
≤ 9V V
Analog Input Leakage Current (Note 10) ●±1±1µA
DIGITAL AND DC ELECTRICAL CHARACTERISTICS
U
4
LTC1286/LTC1298
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
t
SMPL
Analog Input Sample Time See Operating Sequence 1.5 CLK Cycles
f
SMPL(MAX)
Maximum Sampling Frequency LTC1286 ●12.5 kHz
LTC1298 ●11.1 kHz
t
CONV
Conversion Time See Operating Sequence 12 CLK Cycles
t
dDO
Delay Time, CLK↓ to D
OUT
Data Valid See Test Circuits ●250 600 ns
t
dis
Delay Time, CS↑ to D
OUT
Hi-Z See Test Circuits ●135 300 ns
t
en
Delay Time, CLK↓ to D
OUT
Enable See Test Circuits ●75 200 ns
t
hDO
Time Output Data Remains Valid After CLK↓C
LOAD
= 100pF 230 ns
t
f
D
OUT
Fall Time See Test Circuits ●20 75 ns
t
r
D
OUT
Rise Time See Test Circuits ●20 75 ns
C
IN
Input Capacitance Analog Inputs, On Channel 20 pF
Analog Inputs, Off Channel 5 pF
Digital Input 5 pF
AC CHARACTERISTICS
(Note 5)
The ● denotes specifications which apply over the full operating
temperature range.
Note 1: Absolute maximum ratings are those values beyond which the life
of a device may be impaired.
Note 2: All voltage values are with respect to GND.
Note 3: These devices are specified at 5V. For 3V specified devices, see
LTC1285 and LTC1288.
Note 4: Increased leakage currents at elevated temperatures cause the S/H
to droop, therefore it is recommended that f
CLK
≥ 120kHz at 85°C, f
CLK
≥
75kHz at 70° and f
CLK
≥ 1kHz at 25°C.
Note 5: V
CC
= 5V, V
REF
= 5V and CLK = 200kHz unless otherwise specified.
Note 6: Linearity error is specified between the actual end points of the
A/D transfer curve.
Note 7: Two on-chip diodes are tied to each reference and analog input
which will conduct for reference or analog input voltages one diode drop
below GND or one diode drop above V
CC
. This spec allows 50mV forward
bias of either diode for 4.5V ≤ V
CC
≤ 5.5V. This means that as long as the
reference or analog input does not exceed the supply voltage by more than
50mV the output code will be correct. To achieve an absolute 0V to 5V
input voltage range will therefore require a minimum supply voltage of
4.950V over initial tolerance, temperature variations and loading. For 5.5V
< V
CC
≤ 9V, reference and analog input range cannot exceed 5.55V. If
reference and analog input range are greater than 5.55V, the output code
will not be guaranteed to be correct.
Note 8: The supply voltage range for the LTC1286 is from 4.5V to 9V, but
the supply voltage range for the LTC1298 is only from 4.5V to 5.5V.
Note 9: Recommended operating conditions
Note 10: Channel leakage current is measured after the channel selection.
TYPICAL PERFORMANCE CHARACTERISTICS
UW
Shutdown Supply Current vs Clock
Rate with CS High and CS LowSupply Current vs Sample Rate
SAMPLE RATE (kHz)
0.1k
1
10
100
1000
1k 10k 100k
LT1286/98 G03
SUPPLY CURRENT (µA)
T
A
= 25°C
V
CC
= V
REF
= 5V
f
CLK
= 200kHz
LTC1286
LTC1298
Supply Current vs Temperature
TEMPERATURE (°C)
–55
200
SUPPLY CURRENT (µA)
250
350
400
450
–15 25 45 125
LT1286/98 G04
300
–35 5 65 85 105
TA = 25°C
VCC = VREF = 5V
fCLK = 200kHz
LTC1298 fSMPL =11.1kHz
LTC1286 fSMPL =12.5kHz
FREQUENCY (kHz)
1
0.002
SUPPLY CURRENT (µA)
5
1
0
15
20
25
35
20 100 140
LT1286/98 G01
10
30
80 180 200
40 60 120 160
CS = 0
(AFTER CONVERSION)
T
A
= 25°C
V
CC
= V
REF
= 5V
CS = V
CC
5
LTC1286/LTC1298
TYPICAL PERFORMANCE CHARACTERISTICS
UW
0
–0.05
–0.15
–0.2
–.25
–0.3
–0.5
–0.35
–0.1
–0.4
–0.45
REFERENCE VOLTAGE (V)
1
CHANGE IN LINEARITY (LSB)
2345
LT1286/98 G10
1.5 2.5 3.5 4.5
T
A
= 25°C
V
CC
= 5V
f
CLK
= 200kHz
f
SMPL
= 12.5kHz
FREQUENCY (kHz)
0
0
REFERENCE CURRENT (µA)
10
30
40
50
100
70
4810
LT1286/98 G06
20
80
90
60
2612 14
T
A
= 25°C
V
CC
= 5V
V
REF
= 5V
f
CLK
= 200kHz
TEMPERATURE (°C)
–55
92
REFERENCE CURRENT (µA)
92.5
93.5
94
94.5
–15 25 45 125
LT1286/98 G07
93
–35 5 65 85 105
95 VCC = VREF = 5V
fSMPL = 12.5kHz
fCLK = 200kHz
TA = 25°C
Reference Current vs Temperature
REFERENCE VOLTAGE (V)
1
0
CHANGE IN OFFSET (LSB = 1/4096 V
REF
)
0.5
1
1.5
2
2345
LT1286/98 G08
2.5
3
1.5 2.5 3.5 4.5
T
A
= 25°C
V
CC
= 5V
f
CLK
= 200kHz
f
SMPL
= 12.5kHz
Change in Offset vs
Reference Voltage
Change in Offset vs Temperature
TEMPERATURE (°C)
-55
-3
CHANGE IN OFFSET (LSB)
-2.5
-2
1.5
-1
-15 25 65
LT1286/98 G09
-0.5
0
-35 5 45 85
V
CC
= V
REF
= 5V
f
CLK
= 200kHz
f
SMPL
= f
SMPL
(MAX)
Change In Linearity vs
Reference Voltage Change In Gain vs
Reference Voltage
Reference Current vs
Sample Rate (LTC1286)
0
–1
–3
–4
–5
–6
–10
–7
–2
–8
–9
REFERENCE VOLTAGE (V)
1
CHANGE IN GAIN (LSB)
2345
LT1286/98 G11
1.5 2.5 3.5 4.5
TA = 25°C
VCC = 5V
fCLK = 200kHz
fSMPL = 12.5kHz
INPUT FREQUENCY (kHz)
1
0
EFFECTIVE NUMBER OF BITS (ENOBs)
8
7
10
9
12
11
10 100 1000
LTC 1286/98 G20
6
50
44
62
56
74
68
38
5
4
3
2
1
T
A
= 25°C
V
CC
= 5V
f
CLK
= 200kHz
f
SMPL
= 12.5kHz
Effective Bits and S/(N + D)
vs Input Frequency
Differential Nonlinearity vs Code
Peak-to-Peak ADC Noise vs
Reference Voltage
REFERENCE VOLTAGE (V)
1
ADC NOISE IN LBSs
1
1.5
5
LT1286/98 G15
0.5
0234
2T
A
= 25°C
V
CC
= 5V
f
CLK
= 200kHz
CODE
0
DIFFERENTIAL NONLINEARITY ERROR (LBS)
–1.0
–0.80
–0.60
–0.40
–0.20
0.40
0.60
0.80
1.0
0.20
0.00
2048 4096
6
LTC1286/LTC1298
TYPICAL PERFORMANCE CHARACTERISTICS
UW
INPUT FREQUENCY (Hz)
1 10k
100
ATTENUATION (%)
80
90
60
70
40
50
20
30
100k 1M 10M
LTC 1286/98 G26
0
10
T
A
= 25°C
V
CC
= V
REF
=
5V
f
SMPL
= 12.5kHz
Attenuation vs
Input Frequency
Spurious Free Dynamic Range
vs Frequency S/(N+D) vs Input Level
INPUT FREQUENCY (Hz)
1k
40
SPURIOUS FREE DYNAMIC RANGE (dB)
50
60
70
80
10k 100k 1M
LTC 1286/98 G27
30
20
10
0
90
100
T
A
= 25°C
V
CC
= V
REF
=
5V
f
SMPL
= 12.5kHz
INPUT LEVEL (dB)
–40
0
SIGNAL-TO-NOISE PLUS DISTORTION (dB)
20
10
40
30
60
50
80
70
–30 –20
LT1286/98 G25
–10 0
TA = 25°C
VCC = VREF = 5V
fIN = 1kHz
fSMPL = 12.5kHz
4096 Point FFT Plot Intermodulation Distortion Power Supply Feedthrough
vs Ripple Frequency
FREQUENCY (kHz)
0
–60
–40
0
35
LTC 1286/98 G21
–80
–100
12 467
–120
–140
–20
MAGNITUDE (dB)
T
A
= 25°C
V
CC
= V
REF
= 5V
f
IN
= 5kHz
f
CLK
= 200kHz
f
SMPL
= 12.5kHz
FREQUENCY (kHz)
0
–60
–40
0
35
LTC 1286/98 G24
–80
–100
12 467
–120
–140
–20
MAGNITUDE (dB)
T
A
= 25°C
V
CC
= V
REF
=
5V
f
1
= 5kHz
f
2
= 6kHz
f
SMPL
= 12.5kHz
RIPPLE FREQUENCY (kHz)
FEEDTHROUGH (dB)
–50
0
1 100 1000 10000
LTC 1286/98 G22
–100 10
T
A
= 25°C
V
CC
= 5V (V
RIPPLE
= 20mV)
V
REF
= 5V
f
CLK
= 200kHz
Maximum Clock Frequency vs
Source Resistance
SOURCE RESISTANCE (kΩ)
0.1
0
CLOCK FREQUENCY (kHz)
50
100
150
200
300
110
LT1286/98 G12
250
+INPUT
–INPUT
R
SOURCE–
V
IN
T
A
= 25°C
V
CC
= V
REF
= 5V
Sample and Hold Aquisition
Time vs Source Resistance
SOURCE RESISTANCE (Ω)
10 100 1000
LT1286/98 G16
10.1 10000
100
S&H ACQUISITION TIME (ns)
1000
10000
T
A
= 25°C
V
CC
= V
REF
= 5V
+INPUT
–INPUT
R
SOURCE+
V
IN
SUPPLY VOLTAGE (V)
59
LT1286/98 G13
678
250
CLOCK FREQUENCY (kHz)
260
270
280
300
290
TA = 25°C
VCC = VREF = 5V
Maximum Clock Frequency vs
Supply Voltage
7
LTC1286/LTC1298
TYPICAL PERFORMANCE CHARACTERISTICS
UW
TEMPERATURE (°C)
–55
CLOCK FREQUENCY (kHz)
100
150
25 45 65 85
LT1286/98 • G14
50
0–35 –15 5
200
VCC = VREF = 5V
SUPPLY VOLTAGE (V)
3
1
DIGITAL LOGIC THRESHOLD VOLTAGE (V)
2
3
4567
LTC 1286/98 G17
89
T
A
= 25°C
TEMPERATURE (°C)
–60
LEAKAGE CURRENT (nA)
1000
100
10
1
0.1
0.01 100
1196/98 G19
–20 20 60 140
–40 040 80 120
V
CC
= 5V
V
REF
= 5V
ON CHANNEL
OFF CHANNEL
Input Channel Leakage Current
vs Temperature
Digital Input Logic Threshold
vs Supply Voltage
Minimum Clock Frequency
for 0.1 LSB Error vs Temperature
PIN FUNCTIONS
UUU
LTC1286
V
REF
(Pin 1): Reference Input. The reference input defines
the span of the A/D converter.
IN
+
(Pin 2): Positive Analog Input.
IN
–
(Pin 3): Negative Analog Input.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
CS/SHDN (Pin 5): Chip Select Input. A logic low on this
input enables the LTC1286. A logic high on this input
disables and powers down the LTC1286.
D
OUT
(Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
CLK (Pin 7): Shift Clock. This clock synchronizes the serial
data transfer and determines conversion speed.
V
CC
(Pin 8): Power Supply Voltage. This pin provides
power to the A/D converter. It must be kept free of noise
and ripple by bypassing directly to the analog ground
plane.
LTC1298
CS/SHDN (Pin 1): Chip Select Input. A logic low on this
input enables the LTC1298. A logic high on this input
disables and powers down the LTC1298.
CH0 (Pin 2): Analog Input.
CH1 (Pin 3): Analog Input.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
D
IN
(Pin 5): Digital Data Input. The multiplexer address is
shifted into this input.
D
OUT
(Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
CLK (Pin 7): Shift Clock. This clock synchronizes the
serial data transfer and determines conversion speed.
V
CC
/V
REF
(Pin 8): Power Supply and Reference Voltage.
This pin provides power and defines the span of the A/D
converter. It must be kept free of noise and ripple by
bypassing directly to the analog ground plane.
8
LTC1286/LTC1298
BLOCK DIAGRAM
W
–
+
C
SAMPLE
V
CC
(V
CC
/V
REF
)
CS/SHDN
CLK
D
OUT
IN
+
(CH0)
IN
–
(CH1)
MICROPOWER
COMPARATOR
CAPACITIVE DAC
V
REF
GND PIN NAMES IN PARENTHESES
REFER TO THE LTC1298
(D
IN
)
BIAS AND
SHUTDOWN CIRCUIT
SAR
SERIAL PORT
TEST CIRCUITS
D
OUT
1.4V
3k
100pF
TEST POINT
LTC1286/98 • TC01
D
OUT
V
OL
V
OH
t
r
t
fLTC1286/98 • TC02
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
Load Circuit for tdDO, tr and tf
Load Circuit for tdis and ten
Voltage Waveforms for DOUT Delay Times, tdDO
CLK
D
OUT
V
IL
t
dDO
V
OL
V
OH
LTC1286/98 • TC03
D
OUT
3k
100pF
TEST POINT
V
CC
t
dis
WAVEFORM 2, t
en
t
dis
WAVEFORM 1
LTC1286/98 • TC04
9
LTC1286/LTC1298
1234
LTC1298
D
IN
CLK
D
OUT
START
t
en
B11
V
OL
LTC1286/98 • TC07
CS
TEST CIRCUITS
Voltage Waveforms for tdis Voltage Waveforms for ten
Voltage Waveforms for ten
D
OUT
WAVEFORM 1
(SEE NOTE 1)
V
IH
t
dis
90%
10%
D
OUT
WAVEFORM 2
(SEE NOTE 2)
CS
NOTE 1: WAVEFORM 1 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS HIGH UNLESS DISABLED BY THE OUTPUT CONTROL.
NOTE 2: WAVEFORM 2 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS LOW UNLESS DISABLED BY THE OUTPUT CONTROL.
LTC1286/98 • TC05
LTC1286/98 • TC06
CS
LTC1286
1
CLK
D
OUT
t
en
B11
V
OL
2
APPLICATION INFORMATION
WUUU
while the LTC1298 operates from a 4.5V to 5.5V supply.
Both the LTC1286 and the LTC1298 contain a 12-bit,
switched-capacitor ADC, a sample-and-hold, and a
serial port (see Block Diagram). Although they share
the same basic design, the LTC1286 and LTC1298
differ in some respects. The LTC1286 has a differential
input and has an external reference input pin. It can
measure signals floating on a DC common-mode volt-
age and can operate with reduced spans to 1V. Reduc-
ing the spans allows it to achieve 244µV resolution. The
LTC1298 has a two-channel input multiplexer and can
convert either channel with respect to ground or the
difference between the two. The reference input is tied
to the supply pin.
OVERVIEW
The LTC1286 and LTC1298 are micropower, 12-bit, suc-
cessive approximation sampling A/D converters. The
LTC1286 typically draws 250µA of supply current when
sampling at 12.5kHz while the LTC1298 nominally con-
sumes 350µA of supply current when sampling at
11.1 kHz. The extra 100µA of supply current on the
LTC1298 comes from the reference input which is inten-
tionally tied to the supply. Supply current drops linearly as
the sample rate is reduced (see Supply Current vs Sample
Rate). The ADCs automatically power down when not
performing conversions, drawing only leakage current.
They are packaged in 8-pin SO and DIP packages. The
LTC1286 operates on a single supply from 4.5V to 9V,
10
LTC1286/LTC1298
APPLICATION INFORMATION
WUUU
SERIAL INTERFACE
The 2-channel LTC1298 communicates with micropro-
cessors and other external circuitry via a synchronous,
half duplex, 4-wire serial interface. The single channel
LTC1286 uses a 3-wire interface (see Operating Sequence
in Figures 1 and 2).
Data Transfer
The CLK synchronizes the data transfer with each bit being
transmitted on the falling CLK edge and captured on the
rising CLK edge in both transmitting and receiving systems.
The LTC1286 does not require a configuration input word
and has no D
IN
pin. A falling CS initiates data transfer as
shown in the LTC1286 operating sequence. After CS falls
the second CLK pulse enables D
OUT
. After one null bit the
A/D conversion result is output on the D
OUT
line. Bringing
CS high resets the LTC1286 for the next data exchange.
The LTC1298 first receives input data and then transmits
back the A/D conversion result (half duplex). Because of
the half duplex operation, D
IN
and D
OUT
may be tied
together allowing transmission over just 3 wires: CS, CLK
and DATA (D
IN
/D
OUT
).
Data transfer is initiated by a falling chip select (CS) signal.
After CS falls the LTC1298 looks for a start bit. After the
start bit is received, the 3-bit input word is shifted into the
D
IN
input which configures the LTC1298 and starts the
conversion. After one null bit, the result of the conversion
is output on the D
OUT
line. At the end of the data exchange
CS should be brought high. This resets the LTC1298 in
preparation for the next data exchange.
CLK
CS
t
CYC
B11
B5
B6
B7
B8B9
B10B11 HI-Z
D
OUT
t
CONV
t
DATA
HI-Z
t
suCS
NULL
BIT B4 B3 B2 B1
POWER
DOWN
POWER DOWN
B0* NULL
BIT B10 B9 B8
t
SMPL
(MSB)
(MSB)
CLK
CS
t
CYC
B11*
B5
B6
B7
B8B9
B10B11 HI-Z
D
OUT
t
CONV
t
DATA
HI-Z
t
suCS
NULL
BIT
LTC1286/98 • F01
B4 B3 B3 B4 B5 B6 B7
B2 B2B1 B0 B1 B10
B9B8
t
SMPL
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT ZEROS INDEFINITELY.
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT LSB-FIRST DATA THEN FOLLOWED WITH ZEROS INDEFINITELY.
t
DATA
: DURING THIS TIME, THE BIAS CIRCUIT AND THE COMPARATOR POWER DOWN AND THE REFERENCE INPUT
BECOMES A HIGH IMPEDANCE NODE, LEAVING THE CLK RUNNING TO CLOCK OUT LSB-FIRST DATA OR ZEROES.
Figure 1. LTC1286 Operating Sequence
11
LTC1286/LTC1298
APPLICATION INFORMATION
WUUU
Figure 2. LTC1298 Operating Sequence Example: Differential Inputs (CH+, CH–)
CLK
CS
t
CYC
B5
B6
B7
B8B9
B10B11 HI-Z
D
OUT
t
CONV
t
DATA
HI-Z
t
suCS
NULL
BIT B4 B3 B2 B1
POWER
DOWN
B0*
t
SMPL
(MSB)
(MSB)
CLK
START ODD/
SIGN
SGL/
DIFF
CS
t
CYC
B11
B5
B6
B7
B8B9
B10B11 HI-Z
D
OUT
D
IN
t
CONV
t
DATA
HI-Z
t
suCS
NULL
BIT
MSBF
LTC1286/98 • F02
B4 B3 B3 B4 B5 B6 B7
B2 B2
B1 B0 B1 B10
B9B8
t
SMPL
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT ZEROS INDEFINITELY.
DON'T CARE
START ODD/
SIGN
D
IN
DON'T CARE
t
DATA
: DURING THIS TIME, THE BIAS CIRCUIT AND THE COMPARATOR POWER DOWN AND THE REFERENCE INPUT
BECOMES A HIGH IMPEDANCE NODE, LEAVING THE CLK RUNNING TO CLOCK OUT LSB-FIRST DATA OR ZEROES.
SGL/
DIFF MSBF
*
POWER DOWN
MSB-First Data (MSBF = 0)
MSB-First Data (MSBF = 1)
D
IN
1 D
IN
2
D
OUT
1 D
OUT
2
CS
SHIFT MUX
ADDRESS IN
1 NULL BIT SHIFT A/D CONVERSION
RESULT OUT
LTC1096/98 • AI01
12
LTC1286/LTC1298
Start Bit
The first “logical one” clocked into the D
IN
input after CS
goes low is the start bit. The start bit initiates the data
transfer. The LTC1298 will ignore all leading zeros which
precede this logical one. After the start bit is received, the
remaining bits of the input word will be clocked in. Further
inputs on the D
IN
pin are then ignored until the next CS
cycle.
Multiplexer (MUX) Address
The bits of the input word following the START bit assign
the MUX configuration for the requested conversion. For
a given channel selection, the converter will measure the
voltage between the two channels indicated by the + and
– signs in the selected row of the following tables. In
single-ended mode, all input channels are measured with
respect to GND.
APPLICATION INFORMATION
WUUU
Input Data Word
The LTC1286 requires no D
IN
word. It is permanently
configured to have a single differential input. The conver-
sion result appears on the D
OUT
line. The data format is
MSB first followed by the LSB sequence. This provides
easy interface to MSB or LSB first serial ports. For MSB
first data the CS signal can be taken high after B0 (see
Figure 1). The LTC1298 clocks data into the D
IN
input on
the rising edge of the clock. The input data words are
defined as follows:
MSBF bit is a logical zero, LSB first data will follow the
normal MSB first data on the DOUT line. (see Operating
Sequence)
Transfer Curve
The LTC1286/LTC1298 are permanently configured for
unipolar only. The input span and code assignment for
this conversion type are shown in the following figures.
MSB First/LSB First (MSBF)
The output data of the LTC1298 is programmed for
MSB first or LSB first sequence using the MSBF bit.
When the MSBF bit is a logical one, data will appear on
the DOUT line in MSB first format. Logical zeros will be
filled in indefinitely following the last data bit. When the
Operation with D
IN
and D
OUT
Tied Together
The LTC1298 can be operated with D
IN
and D
OUT
tied
together. This eliminates one of the lines required to
communicate to the microprocessor (MPU). Data is trans-
mitted in both directions on a single wire. The processor
pin connected to this data line should be configurable as
either an input or an output. The LTC1298 will take control
of the data line and drive it low on the 4th falling CLK edge
after the start bit is received (see Figure 3). Therefore the
processor port line must be switched to an input before
this happens to avoid a conflict.
In the Typical Applications section, there is an example of
interfacing the LTC1298 with D
IN
and D
OUT
tied together to
the Intel 8051 MPU.
OUTPUT CODE
•
•
•
1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0 0 0 0 0
INPUT VOLTAGE
V
REF
– 1LSB
V
REF
– 2LSB
•
•
•
1LSB
0V
INPUT VOLTAGE
(V
REF
= 5.000V)
4.99878V
4.99756V
•
•
•
0.00122V
0V
LTC1286/98 • AI05
Transfer Curve
0V
1LSB
V
REF
–2LSB
V
REF
4096
V
REF
–1LSB
V
REF
V
IN
0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 0
•
•
•
LTC1286/98 • AI04
1LSB =
MUX ADDRESS
SGL/DIFF
1
1
0
0
ODD/SIGN
0
1
0
1
CHANNEL #
0
+
+
–
1
+
–
+
GND
–
–
SINGLE-ENDED
MUX MODE
DIFFERENTIAL
MUX MODE
LTC1096/8 • AI03
SGL/
DIFF
ODD/
SIGN MSBFSTART
MUX
ADDRESS MSB FIRST/
LSB FIRST
LTC1096/9 • AI02
LTC1298 Channel Selection
Output Code
13
LTC1286/LTC1298
APPLICATION INFORMATION
WUUU
SAMPLE RATE (kHz)
0.1k
1
10
100
1000
1k 10k 100k
LT1286/98 G03
SUPPLY CURRENT (µA)
T
A
= 25°C
V
CC
= V
REF
= 5V
f
CLK
= 200kHz
LTC1286
LTC1298
input becomes high impedance at the end of each conver-
sion leaving the CLK running to clock out the LSB first data
or zeroes (see Figures 1 and 2). If the CS is not running rail-
to-rail, the input logic buffer will draw current. This current
may be large compared to the typical supply current. To
obtain the lowest supply current, bring the CS pin to
ground when it is low and to supply voltage when it is high.
When the CS pin is high (= supply voltage), the converter
is in shutdown mode and draws only leakage current. The
status of the D
IN
and CLK input have no effect on supply
current during this time. There is no need to stop D
IN
and
CLK with CS = high; they can continue to run without
drawing current.
Minimize CS Low Time
In systems that have significant time between conver-
sions, lowest power drain will occur with the minimum CS
low time. Bringing CS low, transferring data as quickly as
possible, and then bringing it back high will result in the
lowest current drain. This minimizes the amount of time
the device draws power. After a conversion the ADC
automatically shuts down even if CS is held low (see
Figures 1 and 2). If the clock is left running to clock out
LSB-data or zero, the logic will draw a small current.
Figure 5 shows that the typical supply current with CS =
ground varies from 1µA at 1kHz to 35µA at 200kHz. When
CS = V
CC
, the logic is gated off and no supply current is
drawn regardless of the clock frequency.
Shutdown
The LTC1286/LTC1298 are equipped with automatic shut-
down features. They draw power when the CS pin is low
and shut down completely when that pin is high. The bias
circuit and comparator powers down and the reference
ACHIEVING MICROPOWER PERFORMANCE
With typical operating currents of 250µA and automatic
shutdown between conversions, the LTC1286/LTC1298
achieves extremely low power consumption over a wide
range of sample rates (see Figure 4). The auto-shutdown
allows the supply curve to drop with reduced sample rate.
Several things must be taken into account to achieve such
a low power consumption.
Figure 4. Automatic Power Shutdown Between Conversions
Allows Power Consumption to Drop with Sample Rate.
1234
CS
CLK
DATA (D
IN
/D
OUT
)START SGL/DIFF ODD/SIGN MSBF B11 B10
•••
MSBF BIT LATCHED
BY LTC1298
LTC1298 CONTROLS DATA LINE AND SENDS
A/D RESULT BACK TO MPU
MPU CONTROLS DATA LINE AND SENDS
MUX ADDRESS TO LTC1298
PROCESSOR MUST RELEASE
DATA LINE AFTER 4TH RISING CLK
AND BEFORE THE 4TH FALLING CLK
LTC1298 TAKES CONTROL OF DATA LINE
ON 4TH FALLING CLK
LTC1286/98 F03
Figure 3. LTC1298 Operation with DIN and DOUT Tied Together
14
LTC1286/LTC1298
Figure 5. Shutdown current with CS high is 1nA typically,
regardless of the clock. Shutdown current with CS = ground
varies from 1µA at 1kHz to 35µA at 200kHz.
APPLICATION INFORMATION
WUUU
Clock Frequency
The maximum recommended clock frequency is 200kHz
for the LTC1286/LTC1298 running off a 5V supply. With
the supply voltage changing, the maximum clock fre-
quency for the devices also changes (see the typical curve
of Maximum Clock Rate vs Supply Voltage). If the maxi-
mum clock frequency is used, care must be taken to
ensure that the device converts correctly.
Mixed Supplies
It is possible to have a microprocessor running off a 5V
supply and communicate with the LTC1286 operating on
a 9V supply. The requirement to achieve this is that the
outputs of CS and CLK from the MPU have to be able to trip
the equivalent inputs of the LTC1286 and the output of
D
OUT
from the LTC1286 must be able to toggle the
equivalent input of the MPU (see typical curve of Digital
Input Logic Threshold vs Supply Voltage). With the
LTC1286 operating on a 9V supply, the output of D
OUT
may
go between 0V and 9V. The 9V output may damage the
MPU running off a 5V supply. The way to get around this
possibility is to have a resistor divider on D
OUT
(Figure 6)
and connect the center point to the MPU input. It should
be noted that to get full shutdown, the CS input of the
LTC1286 must be driven to the V
CC
voltage to keep the CS
input buffer from drawing current. An alternative is to
leave CS low after a conversion, clock data until D
OUT
outputs zeros, and then stop the clock low.
D
OUT
Loading
Capacitive loading on the digital output can increase power
consumption. A 100pF capacitor on the D
OUT
pin can add
more than 50µA to the supply current at a 200kHz clock
frequency. An extra 50µA or so of current goes into
charging and discharging the load capacitor. The same
goes for digital lines driven at a high frequency by any logic.
The C × V × f currents must be evaluated and the trouble-
some ones minimized.
OPERATING ON OTHER THAN 5V SUPPLIES (LTC1286)
The LTC1286 operates from 4.5V to 9V supplies and the
LTC1298 operates from a 5V supply. To operate the LTC1286
on other than 5V supplies a few things must be kept in
mind.
Input Logic Levels
The input logic levels of CS, CLK and D
IN
are made to meet
TTL on a 5V supply. When the supply voltage varies, the
input logic levels also change. For the LTC1286 to sample
and convert correctly, the digital inputs have to be in the
proper logical low and high levels relative to the operating
supply voltage (see typical curve of Digital Input Logic
Threshold vs Supply Voltage). If achieving micropower
consumption is desirable, the digital inputs must go rail-to-
rail between supply voltage and ground (see ACHIEVING
MICROPOWER PERFORMANCE section).
FREQUENCY (kHz)
1
0.002
SUPPLY CURRENT (µA)
5
1
0
15
20
25
35
20 100 140
LT1286/98 G01
10
30
80 180 200
40 60 120 160
CS = 0
(AFTER CONVERSION)
T
A
= 25°C
V
CC
= V
REF
= 5V
CS = V
CC
+IN
–IN
GND
V
CC
CLK
D
OUT
V
REF
50k
50k
5V
4.7µF
MPU
(e.g. 8051) 5V
P1.4
P1.3
P1.2
LTC1286/98 • F06
DIFFERENTIAL INPUTS
COMMON-MODE RANGE
0V TO 5V
9V
LTC1286
CS
Figure 6. Interfacing a 9V Powered LTC1286 to a 5V System
15
LTC1286/LTC1298
APPLICATION INFORMATION
WUUU
BOARD LAYOUT CONSIDERATIONS
Grounding and Bypassing
The LTC1286/LTC1298 are easy to use if some care is
taken. They should be used with an analog ground plane
and single point grounding techniques. The GND pin
should be tied directly to the ground plane.
The V
CC
pin should be bypassed to the ground plane with
a 10µF tantalum capacitor with leads as short as possible.
If the power supply is clean, the LTC1286/LTC1298 can
also operate with smaller 1µF or less surface mount or
ceramic bypass capacitors. All analog inputs should be
referenced directly to the single point ground. Digital
inputs and outputs should be shielded from and/or routed
away from the reference and analog circuitry.
SAMPLE-AND-HOLD
Both the LTC1286 and the LTC1298 provide a built-in
sample-and-hold (S&H) function to acquire signals. The
S&H of the LTC1286 acquires input signals from “+” input
relative to “–” input during the t
SMPL
time (see Figure 1).
However, the S&H of the LTC1298 can sample input
signals in the single-ended mode or in the differential
inputs during the t
SMPL
time (see Figure 7).
Single-Ended Inputs
The sample-and-hold of the LTC1298 allows conversion
of rapidly varying signals. The input voltage is sampled
during the t
SMPL
time as shown in Figure 7. The sampling
interval begins as the bit preceding the MSBF bit is shifted
in and continues until the falling CLK edge after the MSBF
bit is received. On this falling edge, the S&H goes into hold
mode and the conversion begins.
Figure 7. LTC1298 “+” and “–” Input Settling Windows
CLK
D
IN
D
OUT
"+" INPUT
"–" INPUT
SAMPLE HOLD
"+" INPUT MUST
SETTLE DURING
THIS TIME
t
SMPL
t
CONV
CS
SGL/DIFFSTART MSBF DON'T CARE
1ST BIT TEST "–" INPUT MUST
SETTLE DURING THIS TIME
B11
LTC1096/8 • F07
16
LTC1286/LTC1298
APPLICATION INFORMATION
WUUU
Differential Inputs
With differential inputs, the ADC no longer converts just a
single voltage but rather the difference between two volt-
ages. In this case, the voltage on the selected “+” input is
still sampled and held and therefore may be rapidly time
varying just as in single-ended mode. However, the volt-
age on the selected “–” input must remain constant and be
free of noise and ripple throughout the conversion time.
Otherwise, the differencing operation may not be per-
formed accurately. The conversion time is 12 CLK cycles.
Therefore, a change in the “–” input voltage during this
interval can cause conversion errors. For a sinusoidal
voltage on the “–” input this error would be:
V
ERROR (MAX)
= V
PEAK
× 2 × π × f(“–”) × 12/f
CLK
Where f(“–”) is the frequency of the “–” input voltage,
V
PEAK
is its peak amplitude and f
CLK
is the frequency of the
CLK. In most cases V
ERROR
will not be significant. For a
60Hz signal on the “–” input to generate a 1/4LSB error
(305µV) with the converter running at CLK = 200kHz, its
peak value would have to be 13.48mV.
ANALOG INPUTS
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1286/
LTC1298 have capacitive switching input current spikes.
These current spikes settle quickly and do not cause a
problem. However, if large source resistances are used or
if slow settling op amps drive the inputs, care must be
taken to insure that the transients caused by the current
spikes settle completely before the conversion begins.
“+” Input Settling
The input capacitor of the LTC1286 is switched onto “+”
input during the t
SMPL
time (see Figure 1) and samples the
input signal within that time. However, the input capacitor
of the LTC1298 is switched onto “+” input during the
sample phase (t
SMPL
, see Figure 7). The sample phase is
1 1/2 CLK cycles before conversion starts. The voltage on
the “+” input must settle completely within t
SMPLE
for the
LTC1286 and the LTC1298 respectively. Minimizing
R
SOURCE+
and C1 will improve the input settling time. If a
large “+” input source resistance must be used, the
sample time can be increased by using a slower CLK
frequency.
“–” Input Settling
At the end of the t
SMPL
, the input capacitor switches to the
“–” input and conversion starts (see Figures 1 and 7).
During the conversion, the “+” input voltage is effectively
“held” by the sample-and-hold and will not affect the
conversion result. However, it is critical that the “–” input
voltage settles completely during the first CLK cycle of the
conversion time and be free of noise. Minimizing R
SOURCE–
and C2 will improve settling time. If a large “–” input
source resistance must be used, the time allowed for
settling can be extended by using a slower CLK frequency.
Input Op Amps
When driving the analog inputs with an op amp it is
important that the op amp settle within the allowed time
(see Figure 7). Again, the“+” and “–” input sampling times
can be extended as described above to accommodate
slower op amps. Most op amps, including the LT1006 and
LT1413 single supply op amps, can be made to settle well
even with the minimum settling windows of 6µs (“+”
input) which occur at the maximum clock rate of 200kHz.
Source Resistance
The analog inputs of the LTC1286/LTC1298 look like a
20pF capacitor (C
IN
) in series with a 500Ω resistor (R
ON
)
as shown in Figure 8. C
IN
gets switched between the
selected “+” and “–” inputs once during each conversion
cycle. Large external source resistors and capacitances
Figure 8. Analog Input Equivalent Circuit
R
ON
= 500Ω
C
IN
= 20pF
LTC1286/98
“+”
INPUT
R
SOURCE
+
V
IN
+
C1
“–”
INPUT
R
SOURCE
–
V
IN
–
C2
LTC1286/98 • F08
17
LTC1286/LTC1298
converter, the reference input should be driven by a
reference with low R
OUT
(ex. LT1004, LT1019 and LT1021)
or a voltage source with low R
OUT
.
Reduced Reference Operation
The minimum reference voltage of the LTC1298 is limited
to 4.5V because the V
CC
supply and reference are inter-
nally tied together. However, the LTC1286 can operate
with reference voltages below 1V.
The effective resolution of the LTC1286 can be increased
by reducing the input span of the converter. The LTC1286
exhibits good linearity and gain over a wide range of
reference voltages (see typical curves of Change in Linear-
ity vs Reference Voltage and Change in Gain vs Reference
Voltage). However, care must be taken when operating at
low values of V
REF
because of the reduced LSB step size
and the resulting higher accuracy requirement placed on
the converter. The following factors must be considered
when operating at low V
REF
values:
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
Offset with Reduced V
REF
The offset of the LTC1286 has a larger effect on the output
code. When the ADC is operated with reduced reference
voltage. The offset (which is typically a fixed voltage)
becomes a larger fraction of an LSB as the size of the LSB
is reduced. The typical curve of Change in Offset vs
Reference Voltage shows how offset in LSBs is related to
reference voltage for a typical value of V
OS
. For example,
a V
OS
of 122µV which is 0.1LSB with a 5V reference
becomes 0.5LSB with a 1V reference and 2.5LSBs with a
will slow the settling of the inputs. It is important that the
overall RC time constants be short enough to allow the
analog inputs to completely settle within the allowed time.
RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 9. For large values of C
F
(e.g., 1µF), the
capacitive input switching currents are averaged into a net
DC current. Therefore, a filter should be chosen with a
small resistor and large capacitor to prevent DC drops
across the resistor. The magnitude of the DC current is
approximately I
DC
= 20pF × V
IN
/t
CYC
and is roughly
proportional to V
IN
. When running at the minimum cycle
time of 64µs, the input current equals 1.56µA at V
IN
= 5V.
In this case, a filter resistor of 75Ω will cause 0.1LSB of
full-scale error. If a larger filter resistor must be used,
errors can be eliminated by increasing the cycle time.
R
FILTER
V
IN
C
FILTER
LTC1286/98 • F09
LTC1286
“+”
“–”
I
DC
Figure 9. RC Input Filtering
LTC1286
REF
+
R
OUT
V
REF
1
4
GND
LTC1286/98 • F10
Figure 10. Reference Input Equivalent Circuit
Input Leakage Current
Input leakage currents can also create errors if the source
resistance gets too large. For instance, the maximum
input leakage specification of 1µA (at 125°C) flowing
through a source resistance of 240Ω will cause a voltage
drop of 240µV or 0.2LSB. This error will be much reduced
at lower temperatures because leakage drops rapidly (see
typical curve of Input Channel Leakage Current vs Tem-
perature).
REFERENCE INPUTS
The reference input of the LTC1286 is effectively a 50kΩ
resistor from the time CS goes low to the end of the
conversion. The reference input becomes a high impedence
node at any other time (see Figure 10). Since the voltage
on the reference input defines the voltage span of the A/D
APPLICATION INFORMATION
WUUU
18
LTC1286/LTC1298
0.2V reference. If this offset is unacceptable, it can be
corrected digitally by the receiving system or by offsetting
the “–” input of the LTC1286.
Noise with Reduced V
REF
The total input referred noise of the LTC1286 can be
reduced to approximately 400µV peak-to-peak using a
ground plane, good bypassing, good layout techniques
and minimizing noise on the reference inputs. This noise
is insignificant with a 5V reference but will become a larger
fraction of an LSB as the size of the LSB is reduced.
For operation with a 5V reference, the 400µV noise is
only 0.33LSB peak-to-peak. In this case, the LTC1286
noise will contribute virtually no uncertainty to the
output code. However, for reduced references the noise
may become a significant fraction of an LSB and cause
undesirable jitter in the output code. For example, with
a 2.5V reference this same 400µV noise is 0.66LSB
peak-to-peak. This will reduce the range of input volt-
ages over which a stable output code can be achieved by
1LSB. If the reference is further reduced to 1V, the 400µV
noise becomes equal to 1.65LSBs and a stable code may
be difficult to achieve. In this case averaging multiple
readings may be necessary.
This noise data was taken in a very clean setup. Any setup
induced noise (noise or ripple on V
CC
, V
REF
or V
IN
) will add
to the internal noise. The lower the reference voltage to be
used the more critical it becomes to have a clean, noise free
setup.
Conversion Speed with Reduced V
REF
With reduced reference voltages, the LSB step size is
reduced and the LTC1286 internal comparator over-
drive is reduced. Therefore, it may be necessary to
reduce the maximum CLK frequency when low values
of VREF are used.
DYNAMIC PERFORMANCE
The LTC1286/LTC1298 have exceptional sampling capa-
bility. Fast Fourier Transform (FFT) test techniques are
used to characterize the ADC’s frequency response, dis-
APPLICATION INFORMATION
WUUU
tortion and noise at the rated throughput. By applying a low
distortion sine wave and analyzing the digital output using
an FFT algorithm, the ADC’s spectral content can be
examined for frequencies outside the fundamental. Figure
11 shows a typical LTC1286 plot.
Signal-to-Noise Ratio
T
he Signal-to-Noise plus Distortion Ratio (S/N + D) is the
ratio between the RMS amplitude of the fundamental
input frequency to the RMS amplitude of all other fre-
quency components at the ADC’s output. The output is
band limited to frequencies above DC and below one half
the sampling frequency. Figure 12 shows a typical spec-
tral content with a 12.5kHz sampling rate.
Effective Number of Bits
The Effective Number of Bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to S/(N+D)
by the equation:
ENOB = [S/(N + D) – 1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 12.5kHz with a 5V supply, the LTC1286
maintains above 11 ENOBs at 10kHz input frequency.
Above 10kHz the ENOBs gradually decline, as shown in
Figure 12, due to increasing second harmonic distortion.
The noise floor remains low.
FREQUENCY (kHz)
0
–60
–40
0
35
LTC 1286/98 G21
–80
–100
12 467
–120
–140
–20
MAGNITUDE (dB)
T
A
= 25°C
V
CC
= V
REF
= 5V
f
IN
= 5kHz
f
CLK
= 200kHz
f
SMPL
= 12.5kHz
Figure 11. LTC1286 Non-Averaged, 4096 Point FFT Plot
19
LTC1286/LTC1298
APPLICATION INFORMATION
WUUU
If two pure sine waves of frequencies f
a
and f
b
are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at sum and difference
frequencies of mf
a
± nf
b
, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (f
a
+ f
b
) and
(f
a
– f
b
) while 3rd order IMD terms include (2f
a
+ f
b
),
(2f
a
– f
b
), (f
a
+ 2f
b
), and (f
a
– 2f
b
). If the two input sine
waves are equal in magnitudes, the value (in dB) of the 2nd
order IMD products can be expressed by the following
formula:
IMD f f mplitude f f
ab ab
±
()
=±
()
20log a
amplitude at f
a
For input frequencies of 5kHz and 6kHz, the IMD of the
LTC1286/LTC1298 is 73dB with a 5V supply.
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec-
tral component excluding the input signal and DC. This
value is expressed in dBs relative to the RMS value of a full-
scale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is re-
duced by 3dB for a full-scale input.
The full-linear bandwidth is the input frequency at which
the effective bits rating of the ADC falls to 11 bits. Beyond
this frequency, distortion of the sampled input signal
increases. The LTC1286/LTC1298 have been designed to
optimize input bandwidth, allowing the ADCs to
undersample input signals with frequencies above the
converters’ Nyquist Frequency.
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamental
itself. The out-of-band harmonics alias into the frequency
band between DC and half of the sampling frequency. THD
is defined as:
THD =++++
20log VVV V
V
2
23
24
2N
2
1
...
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
second through the N
th
harmonics. The typical THD speci-
fication in the Dynamic Accuracy table includes the 2nd
through 5th harmonics. With a 7kHz input signal, the
LTC1286/LTC1298 have typical THD of 80dB with V
CC
= 5V.
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition
to THD. IMD is the change in one sinusoidal input
caused by the presence of another sinusoidal input at a
different frequency.
Figure 12. Effective Bits and S/(N + D) vs Input Frequency
INPUT FREQUENCY (kHz)
1
0
EFFECTIVE NUMBER OF BITS (ENOBs)
8
7
10
9
12
11
10 100 1000
LTC 1286/98 G20
6
50
44
62
56
74
68
38
5
4
3
2
1
T
A
= 25°C
V
CC
= 5V
f
CLK
= 200kHz
f
SMPL
= 12.5kHz
20
LTC1286/LTC1298
MICROPROCESSOR INTERFACES
The LTC1286/LTC1298 can interface directly without ex-
ternal hardware to most popular microprocessor (MPU)
synchronous serial formats (see Table 1). If an MPU
without a dedicated serial port is used, then 3 or 4 of the
MPU's parallel port lines can be programmed to form the
serial link to the LTC1286/LTC1298. Included here is one
serial interface example and one example showing a
parallel port programmed to form the serial interface.
Motorola SPI (MC68HC11)
The MC68HC11 has been chosen as an example of an MPU
with a dedicated serial port. This MPU transfers data MSB
-first and in 8-bit increments. The D
IN
word sent to the data
register starts with the SPI process. With three 8-bit
transfers, the A/D result is read into the MPU. The second
8-bit transfer clocks B11 through B8 of the A/D conversion
result into the processor. The third 8-bit transfer clocks
the remaining bits, B7 through B0, into the MPU. The data
is right justified into two memory locations. ANDing the
second byte with OF
HEX
clears the four most significant
bits. This operation was not included in the code. It can be
inserted in the data gathering loop or outside the loop
when the data is processed.
MC68HC11 Code
In this example the D
IN
word configures the input MUX for
a single-ended input to be applied to CHO. The conversion
result is output MSB-first.
TYPICAL APPLICATIONS N
U
Table 1. Microprocessor with Hardware Serial Interfaces
Compatible with the LTC1286/LTC1298
PART NUMBER TYPE OF INTERFACE
Motorola
MC6805S2,S3 SPI
MC68HC11 SPI
MC68HC05 SPI
RCA
CDP68HC05 SPI
Hitachi
HD6305 SCI Synchronous
HD63705 SCI Synchronous
HD6301 SCI Synchronous
HD63701 SCI Synchronous
HD6303 SCI Synchronous
HD64180 CSI/O
National Semiconductor
COP400 Family MICROWIRE†
COP800 Family MICROWIRE/PLUS†
NS8050U MICROWIRE/PLUS†
HPC16000 Family MICROWIRE/PLUS†
Texas Instruments
TMS7002 Serial Port
TMS7042 Serial Port
TMS70C02 Serial Port
TMS70C42 Serial Port
TMS32011* Serial Port
TMS32020 Serial Port
Intel
8051 Bit Manipulation on Parallel Port
* Requires external hardware
†MICROWIRE and MICROWIRE/PLUS are trademarks of
National Semiconductor Corp.
21
LTC1286/LTC1298
LABEL MNEMONIC OPERAND COMMENTS
LDAA #$50 CONFIGURATION DATA FOR SPCR
STAA $1028 LOAD DATA INTO SPCR ($1028)
LDAA #$1B CONFIG. DATA FOR PORT D DDR
STAA $1009 LOAD DATA INTO PORT D DDR
LDAA #$01 LOAD DIN WORD INTO ACC A
STAA $50 LOAD DIN DATA INTO $50
LDAA #$A0 LOAD DIN WORD INTO ACC A
STAA $51 LOAD DIN DATA INTO $51
LDAA #$00 LOAD DUMMY DIN WORD INTO
ACC A
STAA $52 LOAD DUMMY DIN DATA INTO $52
LDX #$1000 LOAD INDEX REGISTER X WITH
$1000
LOOP BCLR $08,X,#$01 D0 GOES LOW (CS GOES LOW)
LDAA $50 LOAD DIN INTO ACC A FROM $50
STAA $102A LOAD DIN INTO SPI, START SCK
LDAA $1029 CHECK SPI STATUS REG
WAIT1 BPL WAIT1 CHECK IF TRANSFER IS DONE
LDAA $51 LOAD DIN INTO ACC A FROM $51
STAA $102A LOAD DIN INTO SPI, START SCK
WAIT2 LDAA $1029 CHECK SPI STATUS REG
BPL WAIT2 CHECK IF TRANSFER IS DONE
LDAA $102A LOAD LTC1291 MSBs INTO ACC A
STAA $62 STORE MSBs IN $62
LDAA $52 LOAD DUMMY INTO ACC A
FROM $52
STAA $102A LOAD DUMMY DIN INTO SPI,
START SCK
WAIT3 LDAA $1029 CHECK SPI STATUS REG
BPL WAIT3 CHECK IF TRANSFER IS DONE
BSET $08,X#$01 DO GOES HIGH (CS GOES HIGH)
LDAA $102A LOAD LTC1291 LSBs IN ACC
STAA $63 STORE LSBs IN $63
JMP LOOP START NEXT CONVERSION
LABEL MNEMONIC OPERAND COMMENTS
Timing Diagram for Interface to the MC68HC11
LTC1286/98 AI07
DOUT FROM LTC1298 STORED IN MC68HC11 RAM
B2 B1 B0
B3
B4
B6
B7 B5
0
0
LSB
MSB
#62
#63
00 B11 B10 B9 B8
CLK
D
OUT
CS
ANALOG
INPUTS
D0
SCK
MC68HC11
D
IN
MISO
LTC1298
CH0
CH1
BYTE 1
BYTE 2 MOSI
Hardware and Software Interface to the MC68HC11
CS
CLK
D
OUT
MPU
RECEIVED
WORD
LTC1286/98 AI06
SGL/
DIFF
START
B3B7 B6 B5 B4 B2 B0B1B11 B10 B9 B8
D
IN
MPU
TRANSMIT
WORD
BYTE 3 (DUMMY)
BYTE 2
0000
SGL/
DIFF
1
BYTE 1
X
ODD/
SIGN MSBF XX X
X
000 XXX
XX
XX
X
BYTE 3
BYTE 2
BYTE 1
B11
???0B10 B8
B9 B7 B6 B4
B5 B3 B2 B0
B1
DON'T CARE
ODD/
SIGN
???
?????
MSBF
TYPICAL APPLICATIONS N
U
22
LTC1286/LTC1298
TYPICAL APPLICATIONS N
U
Interfacing to the Parallel Port of the INTEL 8051
Family
The Intel 8051 has been chosen to demonstrate the
interface between the LTC1298 and parallel port micro-
processors. Normally the CS, CLK and D
IN
signals would
be generated on 3 port lines and the D
OUT
signal read on
a 4th port line. This works very well. However, we will
demonstrate here an interface with the D
IN
and D
OUT
of the
LTC1298 tied together as described in the SERIAL INTER-
FACE section. This saves one wire.
The 8051 first sends the start bit and MUX address to the
LTC1298 over the data line connected to P1.2. Then P1.2
is reconfigured as an input (by writing to it a one) and the
8051 reads back the 12-bit A/D result over the same data
line.
LABEL MNEMONIC OPERAND COMMENTS
MOV A, #FFH D
IN
word for LTC1298
SETB P1.4 Make sure CS is high
CLR P1.4 CS goes low
MOV R4, #04 Load counter
LOOP 1 RLC A Rotate D
IN
bit into Carry
CLR P1.3 SCLK goes low
MOV P1.2, C Output D
IN
bit to LTC1298
SETB P1.3 SCLK goes high
DJNZ R4, LOOP 1 Next bit
MOV P1, #04 Bit 2 becomes an input
CLR P1.3 SCLK goes low
MOV R4, #09 Load counter
LOOP 2 MOV C, P1.2 Read data bit into Carry
RLC A Rotate data bit into Acc.
SETB P1.3 SCLK goes high
CLR P1.3 SCLK goes low
DJNZ R4, LOOP 2 Next bit
MOV R2, A Store MSBs in R2
CLR A Clear Acc.
MOV R4, #04 Load counter
LOOP 3 MOV C, P1.2 Read data bit into Carry
RLC A Rotate data bit into Acc.
SETB P1.3 SCLK goes high
CLR P1.3 SCLK goes low
DJNZ R4, LOOP 3 Next bit
MOV R4, #04 Load counter
LOOP 4 RRC A Rotate right into Acc.
DJNZ R4, LOOP 4 Next Rotate
MOV R3, A Store LSBs in R3
SETB P1.4 CS goes high
DOUT FROM 1298 STORED IN 8501 RAM
MSB
R2 B11 B10 B9 B8 B7 B6 B5 B4
LSB
R3 B3 B2 B1 B0 0 0 0 0
CS
CLK
D
OUT
D
IN
LTC1298
ANALOG
INPUTS
P1.4
P1.3
P1.2 8051
MUX ADDRESS
A/D RESULT
LTC1286/98 TA01
CLK
MSBF BIT LATCHED
INTO LTC1298
8051 P1.2 OUTPUTS DATA
TO LTC1298 LTC1298 SENDS A/D RESULT
BACK TO 8051 P1.2
LTC1298 TAKES CONTROL OF DATA
LINE ON 4TH FALLING CLK
8051 P1.2 RECONFIGURED
AS IN INPUT AFTER THE 4TH RISING CLK
AND BEFORE THE 4TH FALLING CLK
MSBF B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
SGL/
DIFF
START
DATA
(
DIN
/D
OUT
)
LTC1286/98 TA02
CS
ODD/
SIGN
23
LTC1286/LTC1298
A “Quick Look” Circuit for the LTC1286
Users can get a quick look at the function and timing of the
LT1286 by using the following simple circuit (Figure 13).
V
REF
is tied to V
CC
. V
IN
is applied to the +IN input and the
–IN input is tied to the ground. CS is driven at 1/16 the
clock rate by the 74C161 and D
OUT
outputs the data. The
output data from the D
OUT
pin can be viewed on an
oscilloscope that is set up to trigger on the falling edge of
CS (Figure 14). Note the LSB data is partially clocked out
before CS goes high.
TYPICAL APPLICATIONS N
U
Micropower Battery Voltage Monitor
A common problem in battery systems is battery voltage
monitoring. This circuit monitors the 10 cell stack of NiCad
or NiMH batteries found in laptop computers. It draws only
67µA from the 5V supply at f
SMPL
= 0.1kHz and 25µA to
55µA from the battery. The 12-bits of resolution of the
LTC1286 are positioned over the desired range of 8V to
16V. This is easily accomplished by using the ADC’s
differential inputs. Tying the –input to the reference gives
an ADC input span of V
REF
to 2V
REF
(2.5V to 5V). The
resistor divider then scales the input voltage for 8V to 16V.
Figure 13. “Quick Look” Circuit for the LTC1286
CLR
CLK
A
B
C
D
P
GND
V
CC
RC
QA
QB
QC
QD
T
LOAD
74C161
V
IN
TO OSCILLOSCOPE
CLOCK IN 250kHz
LTC1286/98 F13
V
CC
CLK
D
OUT
LTC1286
+IN
–IN
GND
4.7µF5V
5V
V
REF
CS
Figure 14. Scope Trace the LTC1286 “Quick Look” Circuit
Showing A/D Output 101010101010 (AAAHEX)
Figure 15. Micropower Battery Voltage Monitor
39k
5V
LT1004-2.5
200k
91k
3Ω
BATTERY MONITOR
INPUT 8V TO 16V
1µF
0.1µF
CS
CLK
D
OUT
LTC1286
LTC1286/98 F15
–IN
V
CC
V
REF
GND
+IN
MSB
(B11)
VERTICAL: 5V/DIV
HORIZONTAL: 10µs/DIV
LSB
(B0)
NULL
BIT
LTC1286/98 F14
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
24
LTC1286/LTC1298
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
●
FAX
: (408) 434-0507
●
TELEX
: 499-3977
LINEAR TECHNOLOGY CORPORATION 1994
sn128698 128698fs LT/GP 0394 10K • PRINTED IN USA
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTION
U
N8 Package
8-Lead Plastic DIP
0.045 ± 0.015
(1.143 ± 0.381)
0.100 ± 0.010
(2.540 ± 0.254)
0.065
(1.651)
TYP
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.005
(3.302 ± 0.127)
0.020
(0.508)
MIN
0.018 ± 0.003
(0.457 ± 0.076)
0.125
(3.175)
MIN
12 34
8765
0.250 ± 0.010
(6.350 ± 0.254)
0.400
(10.160)
MAX
0.009 – 0.015
(0.229 – 0.381)
0.300 – 0.320
(7.620 – 8.128)
0.325 +0.025
–0.015
+0.635
–0.381
8.255
()
1234
0.150 – 0.157*
(3.810 – 3.988)
8765
0.189 – 0.197*
(4.801 – 5.004)
0.228 – 0.244
(5.791 – 6.197)
SO8 0294
0.016 – 0.050
0.406 – 1.270
0.010 – 0.020
(0.254 – 0.508)× 45°
0°– 8° TYP
0.008 – 0.010
(0.203 – 0.254)
0.053 – 0.069
(1.346 – 1.752)
0.014 – 0.019
(0.355 – 0.483)
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
S8 Package
8-Lead Plastic SOIC
