APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 SIMPLE OUTPUT FILTER ELIMINATES ISO AMP OUTPUT RIPPLE AND KEEPS FULL BANDWIDTH By Mark Stitt (602) 746-7445 The ISO120/121/122 isolation amplifiers (ISO amps) have a small (10-20mVp-p typ) 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. 0 –3 Gain (dB) The ISO120/121/122 is designed to have a 50kHz singlepole (Butterworth) signal response. By cascading the ISO amp with a simple 50kHz, Q = 1, two-pole, low-pass filter, the overall signal response becomes three-pole Butterworth. The result is a maximally flat 50kHz magnitude response and the output ripple reduced below the noise level. GAIN vs FREQUENCY +3 –9 2 –15 –21 Figure 1 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 = 50kHz. Since 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. Figure 2 compares the magnitude response of the standard and filtered ISO amp. Figures 3 and 4 show the output ripple improvement. Figures 5 and 6 show the good step response of both the standard and filtered ISO amp. 1 –27 2k 5k 10k 20k 50k Frequency (Hz) 100k 200k 1) Standard ISO122 has 50kHz single-pole (Butterworth) response. 2) ISO122 with cascaded 50kHz, Q = 1, two-pole, low-pass filter has three-pole Butterworth response. FIGURE 2. Gain vs Frequency Plot. C2 1000pF +VS1 +VS2 1 VIN 15 In 9 7 ISO 122 16 Gnd 8 R1 R2 4.75kΩ 9.76kΩ A1 OPA602 10 C1 220pF –VS2 Gnd2 2 VOUT = VIN Gnd1 –VS1 NOTE: Cascading the 50kHz single-pole response ISO122 with a 50kHz, Q = 1, two-pole, low-pass filter produces an overall three-pole Butterworth response and eliminates output ripple. FIGURE 1. ISO122 with Output Filter for Improved Ripple. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. © 1991 Burr-Brown Corporation AB-023 Printed in U.S.A. January, 1991 FIGURE 3. Standard ISO122 (approximately 20mVp-p output ripple). FIGURE 4. Filtered ISO122 (no visible output ripple). FIGURE 5. Step Response of Standard ISO122. FIGURE 6. Step Response of ISO122 with Added Twopole Output Filter. NOTE: Ripple is eliminated from the output waveform by the filter. FIGURE 7. Large-signal, 10kHz Sine-wave Response of ISO122 with and without Output Filter. 2