THS4532_15 Datasheet, PDF (45/58 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
Systems Examples (continued)
9.3.3.2 DC-Coupled Input Signal Path Considerations for Single-Ended to Differential Conversion
The output considerations remain the same as for the AC-coupled design. Again, the input can be DC-coupled
while the output is AC-coupled. A DC-coupled input with an AC-coupled output might have some advantages to
move the input V_ICM down if the source is ground referenced. When the source is DC-coupled into the THS5432
family (see Figure 99), both sides of the input circuit must be DC-coupled to retain differential balance. Normally,
the nonsignal input side has an R_G element biased to whatever the source midrange is expected to be. Providing
this midscale reference gives a balanced differential swing around V_OCM at the outputs.
Often, R_G2 is simply grounded for DC-coupled, bipolar-input applications. This configuration gives a balanced
differential output if the source is swinging around ground. If the source swings from ground to some positive
voltage, grounding R_G2 gives a unipolar output differential swing from both outputs at V_OCM (when the input is at
ground) to one polarity of swing. Biasing R_G2 to an expected midpoint for the input signal creates a differential
output swing around V_OCM.
One significant consideration for a DC-coupled input is that V_OCM sets up a common-mode bias current from the
output back through R_F and R_G to the source on both sides of the feedback. Without input balancing networks,
the source must sink or source this DC current. After the input signal range and biasing on the other R_G element
is set, check that the voltage divider from V_OCM to V_IN through R_F and R_G (and possibly R_S) establishes an input
V_ICM at the device input pins that is in range.
If the average source is at ground, the negative rail input stage for the THS5432 family is in range for
applications using a single positive supply and a positive output V_OCM setting because this DC current lifts the
average FDA input summing junctions up off of ground to a positive voltage (the average of the V+ and V– input
pin voltages on the FDA).
50-Input Match,
Gain of 5 V/V from Rt,
Single-Ended Source to
Differential Step-Response Test
50-Ω
Source
Rg1
187 Ω
Rt
59 Ω
Wideband,
Fully-Differential Amplifier
Rf1
1 kΩ
Vcc
Vocm
–
+
FDA
–
+
PD
Rg2
Vcc
215 Ω
R1
500 Ω
Output
Measurement
Point
Rf2
1 kΩ
Figure 99. DC-Coupled, Single-Ended-to-Differential, Set for a Gain of 5 V/V
9.3.3.3 Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA
The design equations for setting the resistors around an FDA to convert from a single-ended input signal to
differential output can be approached from several directions. Here, several critical assumptions are made to
simplify the results:
• The feedback resistors are selected first and set equal on the two sides.
• The DC and AC impedances from the summing junctions back to the signal source and ground (or a bias
voltage on the nonsignal input side) are set equal to retain feedback divider balance on each side of the FDA.
Both of these assumptions are typical for delivering the best dynamic range through the FDA signal path.
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