THS4531A_16 Datasheet, PDF (42/59 Pages) Texas Instruments – Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

THS4531A
SLOS823C – DECEMBER 2012 – REVISED JANUARY 2016
www.ti.com
50-Input Match,
Gain of 2 V/V from Rt,
Single-Ended Source to
Differential Output
Wideband,
Fully-Differential Amplifier
Rf1
1.02 kΩ
50-Ω
Source
C1
Rg1
100 nF _{499 Ω}
Rt
52.3 Ω
Vocm
Rg2
523 Ω
Vcc
–
+
FDA _–
+
PD
Vcc
Rload
500 Ω
Output
Measurement
Point
C2
100 nF
Rf2
1.02 kΩ
Figure 96. AC-Coupled, Single-Ended Source to a Differential Gain of 2 V/V Test Circuit
9.2.6.2 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
THS4531A family (see Figure 97), 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 THS4531A 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).
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