THS4521 datasheet(27/53 Pages) TI2

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9.4.3 Differential Input to a Differential Output Mode

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 RF and RG elements, the differential

input impedance is now just the sum of the two RG 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-coupled or DC-coupled differential input to differential

output designs, as described in the following sections.

9.4.3.1 AC-Coupled, Differential-Input to Differential-Output Design Issues

The most common way to use the THS4561 with an AC-coupled differential source is to simply couple the input

into the RG resistors through the blocking capacitors. Figure 9-1 shows a typical blocking capacitor approach to

a differential input. An optional input differential termination resistor (RM) is included in this design. The RM

element allows the input RG resistors to be scaled up and still delivers lower differential input impedance to the

source. In this example, the RG elements sum to show a 1-kΩ differential impedance and the RM element

combines in parallel to provide a net 500-Ω ac differential impedance to the source. Again, the design ideally

proceeds by selecting the RF element values, then the RG to set the differential gain, and then an RM element (if

needed) to achieve a target input impedance. Alternatively, the RM element can be eliminated, with the 2 × RG

elements set to the desired input impedance and RF set to obtain the differential gain (equal to RF / RG).

RF1

1.5 k

1 k

RG1

499

RG2

499

THS4561 Wideband,

Fully Differential Amplifier

RF2

1.5 k

Differential I/O

with AC-Coupled

Input

VS+

VOCM

VS+

FDA

VS±

10 nF

1 µF

VIN

5 V

0 V

VS+

VS±

1 k

Figure 9-1. Example AC-Coupled Differential Input Design

The DC biasing for an AC-coupled differential input design is very simple. The output VOCM is set by the VOCM

input control voltage and, because there is no DC current path for the output common-mode voltage (as long as

RM is only differential and not split and connected to ground for instance), the VOCM DC bias also sets the

common-mode operating points for the input pins. For a purely differential input, the voltages on the input pins

remain fixed at the output VOCM setting and do not move with the input signal (unlike the single-ended input

configurations where the input pin common-mode voltages do move with the input signal).

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THS4561

SBOS874D – AUGUST 2017 – REVISED FEBRUARY 2021

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