Data Sheet

AD8475

Rev. D | Page 19 of 24

APPLICATIONS INFORMATION

TYPICAL CONFIGURATION

The AD8475 is designed to facilitate single-ended-to-differential

conversion, common-mode level shifting, and precision attenuation

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

Figure 53 shows a typical connection diagram of the AD8475

in a gain of 0.4. To use the AD8475 in a gain of 0.8, drive the

±IN 0.8x inputs with a low impedance source.

SINGLE-ENDED TO DIFFERENTIAL CONVERSION

Many industrial systems use single-ended voltages in the signal

path; however, the signals are frequently processed by high

performance differential input ADCs for higher precision. The

AD8475 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 55). Note that either input can be driven by the source

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

The AD8475 also accepts truly differential input signals in

precision systems with differential signal paths.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM pin of the AD8475 is internally biased with a

precision voltage divider comprising two 200 kΩ 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 0.01% of the expected value.

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

desired, an external source with output resistance less than

100 Ω can be used to drive the VOCM pin. 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 gain error 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 voltage

reference of an ADC via a resistor divider as shown in Figure 55.

Connecting the VOCM input in this manner reduces power

supply noise and optimizes the output common mode voltage

of the AD8475 to utilize the entire differential input voltage

range of the ADC. If AD8475 is used with a single supply that

is the same voltage as the voltage reference, two 10 kΩ resistors

connected to the VOCM pin is sufficient to override the inter-

nal resistors. Otherwise, a voltage follower should be used to

drive VOCM.

0.1µF

REF

+

0.1µF

10µF

LOW

IMPEDANCE

INPUT SOURCE

0.1µF

10µF

1kΩ

1kΩ

1.25kΩ

1.25kΩ

AD8475

–IN 0.8x

–IN 0.4x

VOCM

+OUT

+IN 0.8x

+IN 0.4x

NC

–OUT

1.25kΩ

1.25kΩ

Figure 52. Typical Configuration—10-Lead MSOP