ADS7813 13
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The input impedance results from the various connections and
the internal resistor values (refer to the block diagram on the
front page of this data sheet). The internal resistor values are
typical and can change by ±30%, due to process variations.
However, the ratio matching of the resistors is considerably
better than this. Thus, the input range will vary only a few
tenths of a percent from part to part, while the input imped-
ance can vary up to ±30%.
The Specifications table contains the maximum limits for the
variation of the analog input range, but only for those ranges
where the comment field shows that the offset and gain are
specified (this includes all the ranges listed in Table I). For the
other ranges, the offset and gain are not tested and are not
specified.
Five of the input ranges in Table IV are not recommended for
general use. The upper-end of the –2.5V to +17.5V range and
+2.5V to +22.5V range exceed the absolute maximum analog
input voltage. These ranges can still be used as long as the
input voltage remains under the absolute maximum, but this
will moderately to significantly reduce the full-scale range of
the converter.
Likewise, three of the input ranges involve the connection at
R2IN being driven below GND. This input has a reverse-
biased ESD protection diode connection to ground. If R2IN is
taken below GND – 0.3V, this diode will be forward-biased
and will clamp the negative input at –0.4V to –0.7V, depend-
ing on the temperature. Since the negative full-scale value of
these input ranges exceed –0.4V, they are not recommended.
Note that Table IV assumes that the voltage at the REF pin is
+2.5V. This is true if the internal reference is being used or if
the external reference is +2.5V. Other reference voltages will
change the values in Table IV.
HIGH IMPEDANCE MODE
When R1IN, R2IN, and R3IN are connected to the analog input,
the input range of the ADS7813 is 0.3125V to 2.8125V and the
input impedance is greater than 10MΩ. This input range can be
used to connect the ADS7813 directly to a wide variety of
sensors. Figure 10 shows the impedance of the sensor versus
the change in ILE and DLE of the ADS7813. The performance
of the ADS7813 can be improved for higher sensor impedance
by allowing more time for acquisition. For example, 10µs of
acquisition time will approximately double sensor impedance
for the same ILE/DLE performance.
The input impedance and capacitance of the ADS7813 are
very stable with temperature. Assuming that this is true of the
sensor as well, the graph shown in Figure 10 will vary less
than a few percent over the ensured temperature range of the
ADS7813. If the sensor impedance varies significantly with
temperature, the worst-case impedance should be used.
DRIVING THE ADS7813 ANALOG INPUT
In general, any reasonably fast, high-quality operational or
instrumentation amplifier can be used to drive the ADS7813
input. When the converter enters the acquisition mode, there
is some charge injection from the converter input to the
amplifier output. This can result in inadequate settling time
with slower amplifiers. Be very careful with single-supply
amplifiers, particularly if their output will be required to
swing very close to the supply rails.
In addition, be careful in regards to the amplifier linearity. The
outputs of single-supply and rail-to-rail amplifiers can satu-
rate as they approach the supply rails. Rather than the ampli-
fier transfer function being a straight line, the curve can
become severely ‘S’ shaped. Also, watch for the point where
the amplifier switches from sourcing current to sinking cur-
rent. For some amplifiers, the transfer function can be notice-
ably discontinuous at this point, causing a significant change
in the output voltage for a much smaller change on the input.
Texas Instruments manufactures a wide variety of operational
and instrumentation amplifiers that can be used to drive the
input of the ADS7813. These include the OPA627, OPA132,
and INA110.
REFERENCE
The ADS7813 can be operated with its internal 2.5V refer-
ence or an external reference. By applying an external refer-
ence voltage to the REF pin, the internal reference voltage is
overdriven. The voltage at the REF input is internally buffered
by a unity gain buffer. The output of this buffer is present at
the BUF and CAP pins.
REF
The REF pin is the output of the internal 2.5V reference or the
input for an external reference. A 1µF to 2.2µF tantulum
capacitor should be connected between this pin and ground.
The capacitor should be placed as close to the ADS7813 as
possible.
When using the internal reference, the REF pin should not be
connected to any type of significant load. An external load
will cause a voltage drop across the internal 4kΩ resistor that
is in series with the internal reference. Even a 40MΩ external
load to ground will cause a decrease in the full-scale range of
the converter by 6 LSBs.
LINEARITY ERROR vs SOURCE IMPEDANCE
External Source Impedance (kΩ)
Change in Worst-Case
Linearity Error (LSBs)
10
9
8
7
6
5
4
3
2
1
00 1 2 3 4 5 6 7 8 9 10 11 12 14 1513
T
A
= +25°C
Acquisition Time = 5µs
ILE
DLE
FIGURE 10. Linearity Error vs Source Impedance in the High
Impedance Mode (R1IN = R2IN = R3IN = VIN).