9.2.2 An MFB Filter Driving an ADC Application

A common application use case for fully-differential amplifiers is to easily convert a single-ended signal into a

differential signal to drive a differential input source, such as an ADC or class-D amplifier. Figure 9-8 shows an

example of the OPA1637 used to convert a single-ended, low-voltage signal audio source, such as a small

electret microphone, and deliver a low-noise differential signal that is common-mode shifted to the center of the

ADC input range. A multiple-feedback (MFB) configuration is used to provide a Butterworth filter response, giving

a 40-dB/decade rolloff with a –3-dB frequency of 30 kHz.

RG1

604

RG2

604

VOCM

+

±

+5 V

+

±

VS+

FDA

PD

±5 V

RO1

20

RO2

20

RF1

3.65 k

CF1

1 nF

RI1

732

RI2

732

CF2

1 nF

RF2

3.65 k

6.2 nF

VIN

RA1

100

RA2

100

CA2

1 nF

CA3

47 pF

CA1

47 pF

ADC

Figure 9-8. Example 30-kHz Butterworth Filter

9.2.2.1 Design Requirements

The requirements for this application are:

• Single-ended to differential conversion

• 5-V/V gain

• Active filter set to a Butterworth, 30-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 OPA1637 and noise peaking

9.2.2.2 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 OPA1637.

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

• Set the output resistor to 100 Ω into a 1-nF differential capacitor.

• Add 47-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.

OPA1637

SBOSA00B – DECEMBER 2019 – REVISED AUGUST 2020

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