ADS7813P datasheet(13/27 Pages) TI2

13 / 27 page

ADS7813

SBAS043C

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 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 10M

Ω. 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, 10

µs 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 1

µF 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 4k

Ω resistor that

is in series with the internal reference. Even a 40M

Ω 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 (k

Ω)

10 11 12

14 15

A = +25°C

Acquisition Time = 5µs

ILE

DLE

FIGURE 10. Linearity Error vs Source Impedance in the High

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

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