AD8476ARMZ-R7 datasheet(20/25 Pages) AD

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Data Sheet

AD8476

Rev. B | Page 19 of 24

APPLICATIONS INFORMATION

TYPICAL CONFIGURATION

The AD8476 is designed to facilitate single-ended-to-differential

conversion, common-mode level shifting, and precision processing

of signals so that they are compatible with low voltage ADCs.

Figure 54 shows a typical connection diagram of the AD8476.

SINGLE-ENDED-TO-DIFFERENTIAL CONVERSION

Many industrial systems have single-ended inputs from input

sensors; however, the signals are frequently processed by high

performance differential input ADCs for higher precision. The

AD8476 performs the critical function of precisely converting

single-ended signals to the differential inputs of precision

ADCs, and it does so with no need for external components.

To convert a single-ended signal to a differential signal, connect

one input to the signal source and the other input to ground (see

Figure 54). Note that either input can be driven by the source

with the only effect being that the outputs have reversed polarity.

The AD8476 also accepts truly differential input signals in

precision systems with differential signal paths.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM pin of the AD8476 is internally biased by a

precision voltage divider comprising of two 1 MΩ resistors

between the supplies. This divider level shifts the output to

midsupply. Relying on the internal bias results in an output

common-mode voltage that is within 0.05% of the expected

value.

10kΩ

AD8476

LOAD

INPUT

SIGNAL

SOURCE

+5V

–VOUT

+VOUT

10µF

0.1µF

10µF

0.1µF

–5V

Figure 54. Typical Configuration—8-Lead MSOP

In cases where control of the output common-mode level is

desired, an external source or resistor divider can be used to

drive the VOCM pin. If driven directly from a source, or with a

resistor divider of unequal resistor values, the resistance seen by

the VOCM pin should be less than 1 kΩ. If an external voltage

divider consisting of equal resistor values is used to set VOCM

to midsupply, higher values can be used because the external

resistors are placed in parallel with the internal resistors. The

output common-mode offset listed in the Specifications section

assumes that the VOCM input is driven by a low impedance

voltage source.

Because of the internal divider, the VOCM pin sources and sinks

current, depending on the externally applied voltage and its

associated source resistance.

It is also possible to connect the VOCM input to the common-

mode level output of an ADC; however, care must be taken to

ensure that the output has sufficient drive capability. The input

impedance of the VOCM pin is 500 kΩ. If multiple AD8476

devices share one ADC reference output, a buffer may be neces-

sary to drive the parallel inputs.

Table 6. Differential Input ADCs1

ADC

Resolution Throughput Rate Power Dissipation

AD7674

16 Bits

100 kSPS

25 mW

AD7684

16 Bits

100 kSPS

6 mW

AD7687

16 Bits

250 kSPS

12.5 mW

AD7688

16 Bits

500 kSPS*

21.5 mW

Depending on measurement/application type, check that the AD8476 meets

settling time requirements.

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