THS4521-HT Datasheet, PDF (18/28 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521-HT
SBOS548D – APRIL 2011 – REVISED MAY 2012
www.ti.com
APPLICATION INFORMATION
The following circuits show application information for
the THS4521. For simplicity, power-supply
decoupling capacitors are not shown in these
diagrams; see the EVM and Layout
Recommendations section for suggested guidelines.
For more details on the use and operation of fully
differential op amps, refer to the Application Report
Fully-Differential Amplifiers (SLOA054), available for
download from the TI web site at www.ti.com.
Single-Ended
R_G
Input
R_G
R_F
V_S+
THS452x
Differential
Output
V_OUT-
V_OUT+
Differential Input to Differential Output
Amplifier
V_S-
The THS4521 is fully-differential operational
amplifiers that can be used to amplify differential
input signals to differential output signals. Figure 34
shows a basic block diagram of the circuit (V_OCM and
PD inputs not shown). The gain of the circuit is set by
R_F divided by R_G.
R_F
Figure 35. Single-Ended Input to Differential
Output Amplifier
Differential
Input
R_G
V_IN+
V_IN-
R_G
R_F
V_S+
THS452x
V_S-
Differential
Output
V_OUT-
V_OUT+
R_F
Figure 34. Differential Input to Differential Output
Amplifier
Input Common-Mode Voltage Range
The input common-mode voltage of a fully-differential
op amp is the voltage at the + and – input pins of the
device.
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. Therefore,
finding the voltage at one input pin determines the
input common-mode voltage of the op amp.
Treating the negative input as a summing node, the
voltage is given by Equation 1:
V
OUT+
´
R_G
R_G + R_F
+
V
IN-
´
R_F
R_G + R_F
(1)
Single-Ended Input to Differential Output
Amplifier
The THS4521 can also amplify and convert single-
ended input signals to differential output signals.
Figure 35 illustrates a basic block diagram of the
circuit (V_OCM and PD inputs not shown). The gain of
the circuit is again set by R_F divided by R_G.
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. The internal common-mode
control circuit maintains the output common-mode
voltage within 5-mV offset (typ) from the set voltage.
If left unconnected, the common-mode set point is set
to midsupply by internal circuitry, which may be
overdriven from an external source.
18
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