THS4521 datasheet(31/58 Pages) TI2

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625 kΩ

S+

–

Internal

OCM Circuit

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OCM

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OCM

S+

S-

EXT

1.25 MΩ

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OUT+

IN-

THS4531

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SLOS358C – SEPTEMBER 2011 – REVISED APRIL 2020

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Application Circuits (continued)

9.2.4 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.

9.2.5 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 76 is

representative of the VOCM input. The internal VOCM circuit has about 24 MHz of -3 dB 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 76. Simplified VOCM Input Circuit

9.2.6 Single-Supply Operation

To facilitate testing with common lab equipment, the THS4531 EVM is built to allow for split-supply operation and

most of the data presented in this data sheet was taken with split-supply power inputs. But the device is

designed for use with single-supply power operation and can easily be used with single-supply power without

degrading the performance. The only requirement is to bias the device properly and the specifications in this data

sheet are given for single supply operation.

THS4521 Datasheet(PDF) 31 Page - Texas Instruments

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