THS4531A_15 Datasheet, PDF (39/59 Pages) Texas Instruments – THS4531A Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

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THS4531A
SLOS823B – DECEMBER 2012 – REVISED JUNE 2015
9.2.4 Differential-Input to Differential-Output Amplifier
The THS4531A is a fully differential op amp and can be used to amplify differential input signals to differential
output signals. A basic block diagram of the circuit is shown in Figure 92 (V_OCM and PD inputs not shown). The
gain of the circuit is set by R_F divided by R_G.
R_G
R_F
V_IN+
V_OUT±
Differential
Input
V_S+
Differential
Output
+
THS4531A
±
R_G
V_IN±
V_S±
R_F
V_OUT+
Figure 92. Differential Input to Differential Output Amplifier
9.2.4.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
There are two typical ways to use the THS4531A family with an AC-coupled differential source. In the first
method, the source is differential and can be coupled in through two blocking capacitors. The second method
uses either a single-ended or a differential source and couples in through a transformer (or balun). Figure 93
shows a typical blocking capacitor approach to a differential input. An optional differential-input termination
resistor (R_M) is included in this design. This R_M element allows the input R_G resistors to be scaled up while still
delivering lower differential input impedance to the source. In this example, the R_G elements sum to show a 500-
Ω differential impedance, while the R_M element combines in parallel to give a net 100-Ω, AC-coupled, differential
impedance to the source. Again, the design proceeds ideally by selecting the R_F element values, then the R_G to
set the differential gain, then an R_M element (if needed) to achieve the target input impedance. Alternatively, the
R_M element can be eliminated, the R_G elements set to the desired input impedance, and R_F set to the get the
differential gain (R_F / R_G).
Wideband,
Fully-Differential Amplifier
Rf1
1 kΩ
C1
100 nF
Rg1
250 Ω
Downconverter
Differential
Output
C2
100 nF
Vocm
Rm
125 Ω
Rg2
250 Ω
Vcc
–
+
FDA
–
+
PD
Vcc
Rf2
1 kΩ
R1
500 Ω
Output
Measurement
Point
Figure 93. Example Down-Converting Mixer Delivering an AC-Coupled Differential Signal to the
THS4531A
The DC biasing here is very simple. The output V_OCM is set by the input control voltage; and because there is no
DC-current path for the output common-mode voltage, that DC bias also sets the input pins common-mode
operating points.
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