廬
APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?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.
The ISO120/121/122 is designed to have a 50kHz single-
pole (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.
Figure 1 shows the complete circuit. The two-pole filter is a
unity-gain Sallen-Key type consisting of A
1
, R
1
, R
2
, C
1
, and
C
2
. The values shown give Q = 1 and f
鈥?dB
bandwidth =
50kHz. Since the op amp is connected as a unity-gain
follower, gain and gain accuracy of the ISO amp are unaf-
fected. 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.
Gain (dB)
GAIN vs FREQUENCY
+3
0
鈥?
1
鈥?
2
鈥?5
鈥?1
鈥?7
2k
5k
10k
20k
50k
Frequency (Hz)
100k
200k
1) Standard ISO122 has 50kHz single-pole (Butterworth) re-
sponse.
2) ISO122 with cascaded 50kHz, Q = 1, two-pole, low-pass
filter has three-pole Butterworth response.
FIGURE 2. Gain vs Frequency Plot.
C
2
1000pF
+V
S1
+V
S2
1
V
IN
15
In
ISO
122
16
Gnd1
鈥揤
S1
鈥揤
S2
Gnd2
Gnd
2
10
9
7
8
R
1
4.75k鈩?/div>
R
2
9.76k鈩?/div>
C
1
220pF
A
1
OPA602
V
OUT
= V
IN
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鈥檚 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.
漏
1991 Burr-Brown Corporation
AB-023
Printed in U.S.A. January, 1991
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