V

IN

V

OUT = VIN

Isolation Barrier

ISO124

+V

S2

+V

S1

–V

S1

±V

S1

–V

S2

Gnd2

Gnd1

R

1

4.75kΩ

R

2

9.76kΩ

1µF

1µF

1µF

1µF

10µH

10µH

±V

S2

10µH

10µH

1µF

1µF

1µF 1µF

C

1

220pF

C

2

1000pF

OPA237

11

ISO124

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8.2 Typical Applications

8.2.1 Output Filters

For more information concerning output filters, see Simple Output Filter Elminiates ISO Amp Output Ripple and Keeps

Full Bandwidth and FilterPro™ MFB and Sallen-Key Low-Pass Filter Design Program User Guide.

Figure 10. ISO124 With Output Filter for Improved Ripple

8.2.1.1 Design Requirements

The ISO124 isolation amplifiers (ISO amps) have a small (10 to 20 mVp-p typical) residual demodulator ripple at

the output. A simple filter can be added to eliminate the output ripple without decreasing the 50kHz signal

bandwidth of the ISO amp.

8.2.1.2 Detailed Design Procedure

The ISO124 device is designed to have a 50-kHz single-pole (Butterworth) signal response. By cascading the

ISO amp with a simple 50-kHz, Q = 1, two-pole, low-pass filter, the overall signal response becomes three-pole

Butterworth. The result is a maximally flat 50-kHz magnitude response and the output ripple reduced below the

noise level. Figure 10 shows the complete circuit. The two-pole filter is a unity-gain Sallen-Key type consisting of

A1, R1, R2, C1, and C2. The values shown give Q = 1 and f–3dB bandwidth = 50 kHz. Because the op amp is

connected as a unity-gain follower, gain and gain accuracy of the ISO amp are unaffected. Using a precision op

amp such as the OPA602 also preserves the DC accuracy of the ISO amp.