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10 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification, and TI

does not warrant its accuracy or completeness. TI’s customers are responsible for determining

suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

10.1 Application Information

Most applications for the THS4561 strive to deliver the best dynamic range in a design that delivers the desired

signal processing along with adequate phase margin for the amplifier itself. The following sections detail some of

the design issues with analysis and guidelines for improved performance.

10.1.1 Differential Open-Loop Gain and Output Impedance

The most important elements to the closed-loop performance are the open-loop gain and open-loop output

impedance. Figure 10-1 shows the simulated differential open-loop gain and phase from the differential inputs to

the differential outputs with no load and with a 100-Ω load. Operating with no load removes any effect introduced

by the open-loop output impedance to a finite load. Figure 10-2 shows the simulated differential open-loop output

impedance.

Frequency (Hz)

-40

-300

-25

-270

-10

-240

-210

-180

-150

-120

-90

-60

10k

100k

10M

100M

D103

Gain, no load

Gain, 100-: load

Phase, no load

Phase, 100-: load

Figure 10-1. No-Load and 100-Ω Loaded AOL Gain

and Phase

Frequency (Hz)

100

1000

10000

100

10k

100k

10M

100M

Figure 10-2. Differential Open-Loop Output

Impedance

This open-loop output impedance combines with the load to shift the apparent open-loop gain and phase to the

output pins when the load changes. The rail-to-rail output stage shows a very high impedance at low frequencies

that reduces with frequency to a lower midrange value and then peaks again at higher frequencies. The

maximum value at low frequencies is set by the common-mode sensing resistors to be a 6-kΩ dc value (see

Section 9.2.) This high impedance at a low frequency is significantly reduced in closed-loop operation by the

loop gain, as shown in the closed-loop output impedance of Figure 7-43. Figure 10-1 compares the no load AOL

gain to the AOL gain driving a 100-Ω load that shows the effect of the output impedance. The heavier loads pull

the AOL gain down faster to lower crossovers with more phase shift at the lower frequencies.

The much faster phase rolloff for the 100-Ω differential load explains the greater peaked response illustrated in

Figure 7-4 and Figure 7-18 when the load decreases. This same effect happens for the RC loads common with

converter interface designs. Use the TINA-TI™ model to verify loop phase margin in any design.

THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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