AD8475

Rev. B | Page 18 of 24

INPUT VOLTAGE RANGE

The AD8475 can measure input voltages that are larger than the

supply rails. The internal gain and feedback resistors form a

divider, which reduces the input voltage seen by the internal

input nodes of the amplifier. The largest voltage that can be

measured is constrained by the capability of the amplifier’s

internal summing nodes. This voltage is defined by the input

voltage and the ratio between the feedback and the gain resistors.

Figure 51 shows the voltage at the internal summing nodes of

the amplifier, defined by the input voltage and internal resistor

reduces to

⎟

⎠

⎞

⎜

⎝

⎛

+

+

=

=

P

MINUS

PLUS

V

RG

RF

VOCM

RG

RF

RG

V

V

2

1

The internal amplifier of the AD8475 has rail-to-rail inputs. To

obtain accurate measurements with minimal distortion, the

voltage at the internal inputs of the amplifier must stay below

AD8475 can measure an input as high as ±12.5 V and maintain

its excellent distortion performance.

The AD8475 provides overvoltage protection for excessive input

voltages beyond the supply rails. Integrated ESD protection diodes

DRIVING THE AD8475

Care should be taken to drive the AD8475 with a low

impedance source: for example, another amplifier. Source

resistance can unbalance the resistor ratios and, therefore,

significantly degrade the gain accuracy and common-mode

rejection of the AD8475. For the best performance, source

impedance to the AD8475 input terminals should be kept

below 0.1 Ω. Refer to the DC Precision section for details on

the critical role of resistor ratios in the precision of the AD8475.

POWER SUPPLIES

The AD8475 operates over a wide range of supply voltages. It

can be powered on a single supply as low as 3 V and as high as

10 V. The AD8475 can also operate on dual supplies from

±1.5 V up to ±5 V

A stable dc voltage should be used to power the AD8475. Note

that noise on the supply pins can adversely affect performance.

For more information, see the PSRR performance curve in

Figure 13.

Place a bypass capacitor of 0.1 μF between each supply pin and

ground, as close as possible to each supply pin. Use a tantalum

capacitor of 10 μF between each supply and ground. It can be

farther away from the supply pins and, typically, it can be

shared by other precision integrated circuits.

RF

RF

RG

RG

VOCM

RF + RG

RF

RG

RF

VOCM

RF + RG

RG

+

+

2

1

Figure 51. Voltages at the Internal Op Amp Inputs of the AD8475