OPA837IDBVT datasheet(39/64 Pages) TI2

39 / 64 page

OPA837

5.6 nF

147 W

255 W

12 nF

OPA837

430 pF

1.4 kW

1.91 kW

2.2 nF

OPA837, OPA2837

www.ti.com

SBOS673D – SEPTEMBER 2017 – REVISED DECEMBER 2018

Product Folder Links: OPA837 OPA2837

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8.2 Typical Applications

8.2.1 Active Filters

The OPAx837 is a good choice for active filters. Figure 83 and Figure 84 show MFB and Sallen-Key circuits

designed implementing second-order, low-pass Butterworth filter circuits. Figure 85 illustrates the frequency

response.

The main difference is that the MFB active filter provides an inverting amplifier in the pass band and the Sallen-

Key active filter is noninverting. The primary advantage for each active filter is that the Sallen-Key filter in unity

gain has no resistor gain error term or feedback resistor noise contribution. The MFB active filter has better

attenuation properties beyond the bandwidth of the op amp. The example circuits are assuming a split-supply

operation but single-supply operation is possible with midscale biasing.

Figure 83. MFB Active Filter, 100-kHz, Second-Order, Low-Pass Butterworth Filter Circuit

Figure 84. Sallen-Key Active Filter, 100-kHz, Second-Order, Low-Pass Butterworth Filter Circuit

8.2.1.1 Design Requirements

For both designs, target the following filter shape characteristic:

•

Gain of 1 V/V

•

100-kHz Butterworth response

•

Q = 0.707 gives a flat Butterworth design

Scale the resistors down to reduce their noise contribution. In the MFB design, the input resistor is the in-band

load to the prior stage. Use values slightly below the gain of –1 V/V in Table 3. The Sallen-Key filter shows a

high impedance input in-band, so scale those resistors down further to improve noise.

The output DC error and drift can be improved by adding bias current cancellation resistors. For the MFB filter

that is a resistor (and a noise filter capacitor) on the noninverting input to ground equal to the resistor inside the

loop times the noise gain. For the Sallen-Key design, add a feedback resistor equal to the sum of the two input

resistors.

8.2.1.2 Detailed Design Procedure

The filter designs shown in this section used an improved design flow that reduces the resistor noise and noise

gain peaking. For the MFB filter, the design was based on the information in the Design Methodology for MFB

Filters in ADC Interface Applications application note.

For the Sallen-Key design, the solution is based on the information in the Component Pre-Distortion for Sallen

Key Filters application note.

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