AN-735
APPLICATION NOTE
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INTRODUCTION
The EVAL-PRAOPAMP-1RJ is an evaluation board which
accommodates single op amps in SOT-23 packages. It
is meant to provide the user with multiple choices and
extensive exibility for different applications circuits
and congurations. This board is not intended to be
used with high frequency components or high speed
ampliers. However, it provides the user with many
combinations for various circuit types including active
lters, differential ampliers, and external frequency
compensation circuits. A few examples of application
circuits are given in this application note.
Figure 1. Simple Low-Pass Filter
Universal Precision Op Amp Evaluation Board in SOT-23 Package
by Giampaolo Marino, Souane Bendaoud, and Steve Ranta
LOW-PASS FILTER
Figure 1 is a typical representation of a rst-order low-
pass lter. This circuit has a 6 dB per octave roll-off
after a close-loop –3 dB point dened by fC. Gain below
this frequency is dened as the magnitude of R7 to R2.
The circuit might be considered as an ac integrator for
frequencies well above fC; however, the time domain
response is that of a single RC, rather than an integral.
fC = 1/(2 R7 C7); –3 dB frequency
fL = 1/(2 R2 C7); unity gain frequency
Acl = –(R7/R2); close loop gain
R6 should be chosen equal to the parallel combination
between R7 and R2 in order to minimize errors due to
bias currents.
Figure 2. Difference Amplier
DIFFERENCE AMPLIFIER AND PERFORMANCE
OPTIMIZATION
Figure 2 shows an op amp congured as a difference
amplier. The difference amplier is the complement
of the summing amplier, and allows the subtraction
of two voltages or the cancellation of a signal common
to both inputs. The circuit shown in Figure 2 is useful
as a computational amplier in making a differential
to single-ended conversion or in rejecting a common-
mode signal. The output voltage VOUT is comprised of
two separate components:
1. A component VOUT1 due to VIN1 acting alone (VIN2
short circuited to ground.)
2. A component VOUT2 due to VIN2 acting alone (VIN1
short circuited to ground.)
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The algebraic sum of these two components should be
equal to VOUT. By applying the principles expressed in
the output voltage VOUT components, and by letting R4
= R2 and R7 = R6, then:
VOUT1 = VIN1 R7/R2
VOUT2 = –VIN2 R7/R2
VOUT = VOUT1 + VOUT2 = ( VIN1 – VIN2) R7/R1
Difference amplifiers are commonly used in high
accuracy circuits to improve the common-mode rejec-
tion ratio, typically known as CMRR.
For this type of application, CMRR depends upon how
tightly matched resistors are used; poorly matched resis-
tors result in a low value of CMRR.
To see how this works, consider a hypothetical source
of error for resistor R7 (1 – error). Using the superposi-
tion principle and letting R4 = R2 and R7 = R6, the output
voltage would be as follows:
V
R
R
R R
R R
error
VD R
R R error
OUT =
-+
+
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7
212 2 7
2 7 2
7
2 7
V V V
DD IN IN
= -2 1
From this equation, ACM and ADM can be dened as
follows:
ACM = R7/(R7 – R2) error
ADM = R7/R2 {1 – [(R2+2R7/R2+R7) error/2]}
These equations demonstrate that when there is not an
error in the resistor values, the ACM = 0 and the amplier
responds only to the differential voltage being applied to
its inputs; under these conditions, the CMRR of the circuit
becomes highly dependent on the CMRR of the amplier
selected for this job.
As mentioned above, errors introduced by resistor
mismatch can be a big drawback of discrete differential
ampliers, but there are different ways to optimize this
circuit conguration:
1. The differential gain is directly related to the ratio R7/
R2; therefore, one way to optimize the performance
of this circuit is to place the amplier in a high gain
conguration. When larger values for resistors R7
and R6 and smaller values for resistor R2 and R4 are
selected, the higher the gain, the higher the CMRR.
For example, when R7 = R6 = 10 k, and R2 = R4 = 1 k,
and error = 0.1%, CMRR improves to better than 80 dB.
For high gain conguration, select ampliers with
very low Ib and very high gain (such as the AD8551,
AD8571, AD8603, and AD8605) to reduce errors.
2. Select resistors that have much tighter tolerance and
accuracy. The more closely they are matched, the better
the CMRR. For example, if a CMRR of 90 dB is needed,
then match resistors to approximately 0.02%.
CURRENT-TO-VOLTAGE CONVERTER
Current may be measured in two ways with an opera-
tional amplier. Current can be converted to a voltage
with a resistor and then amplied or injected directly
into a summing node.
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Figure 3. Current-to-Voltage Converter
Figure 3 is a typical representation of a current-to-voltage
transducer. The input current is fed directly into the sum-
ming node and the amplier output voltage changes to
exactly the same current from the summing node through
R7. The scale factor of this circuit is R7 volts per amps.
The only conversion error in this circuit is IBIAS, which is
summed algebraically with IIN.
Figure 4. Bistable Multivibrator
Figure 5. Output Response
GENERATION OF SQUARE WAVEFORMS USING A
BISTABLE MULTIVIBRATOR
A square waveform can be simply generated by arrang-
ing the amplier for a bistable multivibrator to switch
states periodically as Figure 5 shows.
Once the output of the amplier reaches one of two pos-
sible levels, such as L+, capacitor C9 charges toward this
level through resistor R7. The voltage across C9, which
is applied to the negative input terminal of the ampli-
er denoted as V–, then rises exponentially toward L+
with a time constant = C9R7. Meanwhile, the voltage
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at the positive input terminal of the amplier, denoted as
V+ = BL+. This continues until the capacitor voltage
reaches the positive threshold VTH, at which point
the bi
stable multivibrator switches to the other stable
state in which VO = L– and V+ = BL–. This is shown in
Figure 5.
The capacitor then begins to discharge, and its voltage,
V–, decreases exponentially toward L–. This continues
until V– reaches the negative threshold VTL, at which time
the bistable multivibrator switches to the positive output
state, and the cycle repeats itself.
It is important to note that the frequency of the square
wave being generated, fO, depends only on the external
components being used. Any variation in L+ will cause
V+ to vary in proportion, ensuring the same transition
time and the same oscillation frequency. The maximum
operating frequency is determined by the amplifier
speed, which can be increased signicantly by using
faster devices.
The lowest operating frequency depends on the practical
upper limits set by R7 and C9.
Using the name convention outlined on the PRA OPAMP
evaluation board, the following circuit should be con-
nected as follows:
B = R4/(R4 + R9); feedback factor (noninverting input)
T = 2R7 C9 ln((1 + B)/(1 – B)); period of oscillation
fO = 1/T; oscillation frequency
Figure 6. Series Resistor Compensation
Figure 7. Capacitive Load Drive Without Resistor
Figure 8. Capacitive Load Drive with Resistor
EXTERNAL COMPENSATION TECHNIQUES
Series Resistor Compensation
The use of external compensation networks may be
required to optimize certain applications. Figure 6 is a
typical representation of a series resistor compensa-
tion
for stabilizing an op amp driving capacitive load. The
stabilizing effect of the series resistor isolates the op amp
output and the feedback network from the capacitive
load. The required amount of series resistance depends
on the part used, but values of 5 to 50 are usually
sufcient to prevent local resonance. The disadvantages
of this technique are a reduction in gain accuracy and
extra distortion when driving nonlinear loads.
Figure 9. Snubber Network
Figure 10. Capacitive Load Drive Without Snubber
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AN-735
Figure 11. Capacitive Load Drive with the Snubber
Snubber Network
Another way to stabilize an op amp driving a capacitive
load is with the use of a snubber, as shown in Figure 9.
This method presents the signicant advantage of not
reducing the output swing because there is not any
isolation resistor in the signal path. Also, the use of
the snubber does not degrade the gain accuracy or
cause extra distortion when driving a nonlinear load.
The exact RS and CS combinations can be determined
experimentally.
Figure 12. EVAL-PRAOPAMP-1RJ Electrical Schematic
Figure 13. EVAL-PRAOPAMP-1RJ Board Layout Patterns
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