THS4521 datasheet(33/51 Pages) TI

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10.2 Typical Application

One common application for the THS4561 is to take a single-ended, high VPP voltage swing (from a high-voltage

precision amplifier such as the OPA810 or OPA192) and deliver that swing to precision SAR ADC as a single-

ended to differential conversion with output common-mode control and implement an active 2nd-order multiple

feedback (MFB) filter design. Designing for a 16-VPP maximum input down to an 8-VPP differential swing requires

a gain of 0.5 V/V. Targeting a 170-kHz Butterworth response with the RC elements tilted towards low noise gives

the example design of Figure 10-7. The VCM control is set to half of a 4.096-V reference, which is typical for

5-V differential SAR applications. With the high voltage capabilities of the THS4561, the design can be easily

adopted for 20-VPP input swing to the FDA for a full 10-VPP swing into 5-V differential SAR ADC by simply using

wider power supplies for the THS4561 to allow for increased output swing headroom with minimal performance

degradation.

RG1

931

RG2

931

THS4561 Wideband,

Fully Differential Amplifier

VOCM

VS+

FDA

VS±

100 fF

RF1

1.5 k

470 pF

3.01 k

470 pF

RF2

1.5 k

680 pF

OPA810 or OPA192

Output

5 V

0 V

VS+

VS±

2.048 V

VOCM

VIN

2.2 nF

100 pF

8-VPP Differential

SAR ADC Input

To ADC

Figure 10-7. MFB Filter Driving an ADC Application:

Example 170-kHz Butterworth Response

10.2.1 Design Requirements

The requirements for this application are:

• Single-ended to differential conversion

• Attenuation by 0.5-V/V gain

• Active filter set to a Butterworth, 170-kHz response shape

• Output RC elements set by SAR input requirements (not part of the filter design)

• Filter element resistors and capacitors are set to limit added noise over the THS4561 and noise peaking

10.2.2 Detailed Design Procedure

The design proceeds using the techniques and tools suggested in the Design Methodology for MFB Filters in

ADC Interface Applications application note. The process includes:

• Scale the resistor values to not meaningfully contribute to the output noise produced by the THS4561 by itself

• Select the RC ratios to hit the filter targets when reducing the noise gain peaking within the filter design

• Set the output resistor to 10 Ω into a 2.2-nF differential capacitor

• Add 100-pF common-mode capacitors to the load capacitor to improve common noise filtering

• Inside the loop, add 20-Ω output resistors after the filter feedback capacitor to increase the isolation to the

load capacitor

• Include a place for a differential input capacitor (illustrated as 100 fF in Figure 10-7)

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THS4561

SBOS874C – AUGUST 2017 – REVISED DECEMBER 2020

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