THS4531
www.ti.com
SLOS358B –SEPTEMBER 2011–REVISED MARCH 2012
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:
æ
ç
è
ö
÷
ø
æ
ö
÷
ø
RG
RF
V
´
+ V
´
ç
OUT+
IN-
RG + RF
RG + RF
è
(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 76 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:
2VOCM - V - VS-
(
60kΩ
)
S+
IEXT
=
(2)
where VOCM is the voltage applied to the VOCM pin.
VS+
IEXT
Internal
60 kΩ
60 kΩ
VOCM Circuit
VOCM
VS–
Figure 76. Simplified VOCM Input Circuit
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.
Copyright © 2011–2012, Texas Instruments Incorporated
Submit Documentation Feedback
33
Product Folder Link(s): THS4531
TI [ TEXAS INSTRUMENTS ]
