THS4521 datasheet(21/53 Pages) TI2

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Starting from the test circuit of Figure 8-1, various elements are modified to show the effect of these elements

over a range of design targets, specifically:

• The gain setting is changed by adjusting the two RT and the RG resistors to provide a 50-Ω input match and

setting the feedback resistors to 1.5 kΩ.

• Resistive and capacitive output load testing. Changing to lower resistive loads is accomplished by adding

parallel resistors across the output pins in Figure 8-1. Changing to capacitive loads adds series output

resistors to a differential capacitance before the 1-kΩ sense path of Figure 8-1.

• Power-supply settings. Most often, balanced bipolar supplies are used; a 12-V tests use ±6-V supplies, 5-V

tests use ±2.5-V supplies, and 3-V tests use ±1.5-V supplies with the VOCM input control grounded.

• The disable control pin (PD) is tied to the positive supply (VS+) for any active channel test.

8.2 Output Interface Circuit for DC-Coupled Differential Testing

The pulse response plots are measured using the output circuit of Figure 8-4. The two sides of this circuit

present a 500-Ω load to ground (for a differential 1-kΩ load) with a 50-Ω source to the two scope inputs. Trace

math function of the scope combines the two sides to generate the step response plots of Figure 7-13, Figure

7-27, and Figure 7-44. Use balanced bipolar supplies for this test so that the THS4561 outputs deliver a ground-

centered differential swing. This setup produces no DC load currents using the circuit of Figure 8-4.

475

56.2

THS4561

Output

56.2

50-

Scope

Input

50-

Scope

Input

Figure 8-4. Output Interface for DC-Coupled Differential Outputs

8.3 Output Common-Mode Measurements

The circuit of Figure 8-5 is a typical setup for common-mode measurements.

RF1

1.5 k

100

RG1

1.5 k

RG2

1.5 k

50-

Measurement

Equipment

THS4561 Wideband,

Fully Differential Amplifier

VOCM

Input

RF2

1.5 k

VS+

Signal

Source

VS+

FDA

VS±

100

49.9

VOCM

Figure 8-5. Output Common-Mode Measurements

In Figure 8-5, the differential path is terminated back to ground with the two 1.5-kΩ input resistors and the VOCM

control input is driven from a 50-Ω matched source for the frequency response and step response curves of

Figure 7-47, Figure 7-49, and Figure 7-50. The outputs are summed to a center point (to obtain the average, or

common-mode output, VOCM) through two 100-Ω resistors. These 100-Ω resistors form an equivalent 50-Ω

source to the common-mode output for measurements. Figure 7-48 illustrates the common-mode output noise

measurements with a ground on the VOCM input pin or with the VOCM input pin floating. The higher noise in

Figure 7-48 for a floated input can be reduced by including a capacitor to ground at the VOCM control input pin.

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THS4561

SBOS874D – AUGUST 2017 – REVISED FEBRUARY 2021

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