THS4552 Datasheet, PDF (38/71 Pages) Texas Instruments – Dual-Channel, Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4552
SBOS831 – DECEMBER 2016
www.ti.com
Device Functional Modes (continued)
The input pin voltages move in a common-mode manner with the input signal, as described in the I/O Headroom
Considerations section. Confirm that the V_OCM voltage plus the input V_PP common-mode swing also stays in
range for the input pins.
9.4.1.2 DC-Coupled Input Signal Path Considerations for Single-Ended to Differential Conversions
The output considerations remain the same as for the ac-coupled design. Again, the input can be dc-coupled
when the output is ac coupled. A dc-coupled input with an ac-coupled output can have some advantages to
move the input V_ICM down by adjusting the V_OCM down if the source is ground referenced. When the source is dc-
coupled into the THS4552 (see Figure 69), both sides of the input circuit must be dc-coupled to retain differential
balance. Normally, the non-signal input side has an R_G element biased to whatever the source midrange is
expected to be, provided that 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 provides a
balanced differential output if the source swings 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 VOCM (when the
input is at ground) to one polarity of the swing. Biasing R_G2 to an expected midpoint for the input signal creates a
differential output swing around VOCM.
One significant consideration for a dc-coupled input is that VOCM 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 THS4552 is in range for applications using a single positive supply and a positive output V_OCM setting
because this dc common-mode 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). TINA-TI™ simulations of the
intended circuit offer a good check for input and output pin voltage swings (see Figure 72).
9.4.2 Operation from a Differential Input to a Differential Output
In many ways, this method is a much simpler way to operate the FDA from a design equations perspective.
Again, assuming that the two sides of the circuit are balanced with equal R_F and R_G elements, the differential
input impedance is now just the sum of the two R_G elements to a differential inverting summing junction. In these
designs, the input common-mode voltage at the summing junctions does not move with the signal but must be dc
biased in the design range for the input pins and must take into account the voltage headroom required to each
supply. Slightly different considerations apply to ac- or dc-coupled differential input to differential output designs,
as described in the following sections.
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