THS4532_15 Datasheet, PDF (41/58 Pages) Texas Instruments – THS4532 Ultra Low Power, Rail-to-Rail Output, Fully-Differential Amplifier

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THS4532
SLOS829A – FEBRUARY 2013 – REVISED JULY 2015
9.3 Systems Examples
9.3.1 Differential-Input to Differential-Output Amplifier
The THS5432 is a fully-differential amplifier and can be used to amplify differential input signals to differential
output signals. A basic block diagram of the circuit is shown in Figure 94 (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
THS4532
R_G
V_IN±
V_S±
R_F
V_OUT+
Figure 94. Differential Input to Differential Output Amplifier
9.3.1.1 AC-Coupled, Differential-Input to Differential-Output Design Issues
There are two typical ways to use the THS5432 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 95 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 95. Example Down-Converting Mixer Delivering an AC-Coupled Differential Signal to the THS5432
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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