Datasheet

æ ö æ ö
´ + ´
ç ÷ ç ÷
+ +
è ø è ø
G
F
OUT+ IN-
G F G F
R
R
V V
R R R R
( )
- -
=
OCM S+ S-
EXT
2V V V
I
60kΩ
60 kΩ
60 kΩ
V
S+
V
S–
Internal
V
OCM
Circuit
I
EXT
V
OCM
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 (V
ICR
) 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 V
ICR
of the op amp, the voltage at the negative input is evaluated at the extremes of V
OUT+
.
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 V
OCM
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 V
OCM
input. The internal V
OCM
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 V
OCM
is the voltage applied to the V
OCM
pin.
Figure 76. Simplified V
OCM
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.
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