THS4532IPW datasheet(29/44 Pages) NSC

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60 kΩ

S+

S–

Internal

OCM Circuit

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OCM

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OCM

S+

S-

EXT

60kΩ

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OUT+

IN-

THS4532

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SLOS829 – FEBRUARY 2013

Input Common-Mode Voltage Range

The input common-model voltage of a fully differential op amp is the voltage at the “+ and –“ input pins of the op

amp.

It is important to not violate the input common-mode voltage range (VICR) of the op amp. Assuming the op amp is

in linear operation the voltage across the input pins is only a few millivolts at most. So finding the voltage at one

input pin will determine the input common-mode voltage of the op amp.

Treating the negative input as a summing node, the voltage is given by:

(1)

To determine the VICR of the op amp, the voltage at the negative input is evaluated at the extremes of VOUT+.

As the gain of the op amp increases, the input common-mode voltage becomes closer and closer to the input

common-mode voltage of the source.

Setting the Output Common-Mode Voltage

The output common-model voltage is set by the voltage at the VOCM pin and the internal circuit works to maintain

the output common-mode voltage as close as possible to this voltage. If left unconnected, the output common-

mode is set to mid-supply by internal circuitry, which may be over-driven from an external source. Figure 78 is

representative of the VOCM input. The internal VOCM circuit has about 24MHz of -3dB bandwidth, which is required

for best performance, but it is intended to be a DC bias input pin. Bypass capacitors are recommended on this

pin to reduce noise. The external current required to overdrive the internal resistor divider is given approximately

by the formula:

(2)

where VOCM is the voltage applied to the VOCM pin.

Figure 78. Simplified VOCM Input Circuit

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