ADS7812PB datasheet(13/28 Pages) TI2

13 / 28 page

ADS7812

SBAS042A

www.ti.com

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%.

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. 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.

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.

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 ADS7812 is 0.3125V to 2.8125V and

the input impedance is greater than 10M

Ω. 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, 10

µs of acquisition time will approximately double

sensor impedance for the same ILE/DLE performance.

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 specified temperature range

of the ADS7812. If the sensor impedance varies signifi-

cantly with temperature, the worst-case impedance should

be used.

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

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

sinking 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.

Texas Instruments manufactures a wide variety of opera-

tional and instrumentation amplifiers that can be used to

drive the input of the ADS7812. These include the OPA627,

OPA134, OPA132, and INA110.

REFERENCE

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.

REF

The REF pin is the output of the internal 2.5V reference or

the input for an external reference. A 1

µF 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.

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 4k

Ω

resistor that is in series with the internal reference. Even a

Ω external load to ground will cause a decrease in the

full-scale range of the converter by 4 LSBs.

FIGURE 10. Linearity Error vs Source Impedance in the High

Impedance Mode (R1IN = R2IN = R3IN = VIN).

LINEARITY ERROR vs SOURCE IMPEDANCE

External Source Impedance (k

Ω)

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

9 10 11 12

14 15

A = +25°C

Acquisition Time = 5µs

ILE

DLE

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