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 varia-
tions. However, the ratio matching of the resistors is consid-
erably better than this. Thus, the input range will vary only
a few tenths of a percent from part to part, while the input
impedance may vary up to ±30%.
time with slower amplifiers. Be very careful with single-
supply amplifiers, particularly if their output will be re-
quired to swing very close to the supply rails.
In addition, be careful in regards to the amplifier’s linearity.
The outputs of single-supply and “rail-to-rail” amplifiers can
saturate as they approach the supply rails. Rather than the
amplifier’s 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 sink-
ing current. For some amplifiers, the transfer function can be
noticeably discontinuous at this point, causing a significant
change in the output voltage for a much smaller change on
the input.
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 guaranteed (this includes all the ranges listed in
Table I). For the other ranges, the offset and gain are not
tested and are not guaranteed.
Burr-Brown manufactures a wide variety of operational and
instrumentation amplifiers that can be used to drive the input
of the ADS7812. These include the OPA627, OPA134,
OPA132, and INA110.
Five of the input ranges in Table IV are not recommended
for general use. For two of the these, the input voltage
exceeds the absolute maximum. 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.
REFERENCE
The other three input ranges involve the connection at R2IN
being driven below GND – 0.3V. This input has a reverse-
biased ESD protection diode connection to ground. If R2IN
is taken below ground, this diode will be forward-biased and
will clamp the negative input at –0.4V to –0.7V, depending
on the temperature. Here again, these ranges can still be used
at the cost of the full-scale range of the converter.
The ADS7812 can be operated with its internal 2.5V refer-
ence or an external reference. By applying an external
reference 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.
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.
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 tantalum
capacitor should be connected between this pin and ground.
The capacitor should be placed as close as possible to the
ADS7812.
HIGH IMPEDANCE MODE
When R1IN, R2IN, and R3IN are connected to the analog input,
the input range of the ADS7812 is 0.3125V to 2.8125V and
the input impedance is greater than 10MΩ. This input range
can be used to connect the ADS7812 directly to a wide
variety of sensors. Figure 10 shows the impedance of the
sensor versus the change in ILE and DLE of the ADS7812.
The performance of the ADS7812 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.
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
4MΩ external load to ground will cause a decrease in the
full-scale range of the converter by 4 LSBs.
LINEARITY ERROR vs SOURCE IMPEDANCE
0.60
0.55
TA = +25°C
The input impedance and capacitance of the ADS7812 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 guaranteed temperature
range of the ADS7812. If the sensor impedance varies
significantly with temperature, the worst-case impedance
should be used.
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Acquisition Time = 5µs
DLE
ILE
DRIVING THE ADS7812 ANALOG INPUT
In general, any “reasonably fast”, high quality operational or
instrumentation amplifier can be used to drive the ADS7812
input. When the converter enters the acquisition mode, there
is some charge injection from the converter’s input to the
amplifier’s output. This can result in inadequate settling
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
External Source Impedance (kΩ)
FIGURE 10. Linearity Error vs Source Impedance in the High
Impedance Mode (R1IN = R2IN = R3IN = VIN).
®
13
ADS7812
BB [ BURR-BROWN CORPORATION ]
