THS4521 Datasheet, PDF (25/43 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521
THS4522
THS4524
www.ti.com.................................................................................................................................................... SBOS458B – DECEMBER 2008 – REVISED MAY 2009
APPLICATION INFORMATION
The following circuits show application information for
the THS4521, THS4522, and THS4524 family. 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
The THS4521, THS4522, and THS4524 family are
fully-differential operational amplifiers that can be
used to amplify differential input signals to differential
output signals. Figure 60 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.
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 60. Differential Input to Differential Output
Amplifier
Single-Ended Input to Differential Output
Amplifier
The THS4521, THS4522, and THS4524 family can
also amplify and convert single-ended input signals to
differential output signals. Figure 61 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.
V_S-
R_F
Figure 61. Single-Ended 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:
R
V_OUT+ ´ _R
G
+R
G
F
+
R
V_IN- ´ _R
F
+R
G
F
(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.
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Product Folder Link(s): THS4521 THS4522 THS4524