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
AN ERROR ANALYSIS OF THE ISO102
IN A SMALL SIGNAL MEASURING APPLICATION
High accuracy measurements of low-level signals in the
presence of high isolation mode voltages can be difficult due
to the errors of the isolation amplifiers themselves.
This error analysis shows that when a low drift operational
amplifier is used to preamplify the low-level source signal,
a low cost, simple and accurate solution is possible.
In the circuit shown in Figure 1, a 50mV shunt is used to
measure the current in a 500VDC motor. The OPA27
amplifies the 50mV by 200X to 10V full scale. The output
of the OPA27 is fed to the input of the ISO102, which is a
unity-gain isolation amplifier. The 5k鈩?and 1k鈩?potentiom-
eters connected to the ISO102 are used to adjust the gain and
offset errors to zero as described in the ISO102 data sheet.
SOME OBSERVATIONS
The total errors of the op amp and the iso amp combined are
approximately 0.6% of full-scale range. If the op amp had
not been used to preamplify the signal, the errors would have
been 74.4% of FSR. Clearly, the small cost of adding the op
amp buys a large performance improvement.
After gain and offset nulling, the dominant errors of the iso
amp are gain nonlinearity and power supply rejection. Thus,
well regulated supplies will reduce the errors even further.
The rms noise of the ISO102 with a 120Hz bandwidth is
only 0.18mVrms, which is only 0.0018% of the 10V full-
scale output. Therefore, even though the 16碌V/鈭欻z noise
spectral density specification may appear large compared to
other isolation amplifiers, it does not turn out to be a
significant error term. It is worth noting that even if the
bandwidth is increased to 10kHz, the noise of the iso amp
would only contribute 0.016% FSR error.
Input
Power Supply
+V
CC1
R
F
200k鈩?/div>
+500VDC
+15V
+V
CC1
Offset
Adjust
R
1
1k鈩?/div>
7
2
3
V
D
10k鈩?/div>
1
8
OPA27
4
6
Gain
Adjust
5k鈩?/div>
3
Offset 1k鈩?/div>
Adjust
21
23
22
V
IN
2
ISO102
Gain Adjust
Reference
1
Offset Adjust
Offset
4
Input Common
V
D
= 50mVDC (FS)
500VDC
DC Motor
C
1
0.022碌F
V
ISO
10
9
鈥揤
CC1
Output
Power Supply
+V
CC2
鈥揤
CC2
+15V
鈥?5V
+15V
鈥?5V
0.1碌F
24
0.1碌F
+
15V
1
鈥?5V
12
+15V
13
鈥?5V
14
V
OUT
16
15 C
2
0.04碌F
Bandwidth
Control
0.1碌F
0.1碌F
鈥揤
CC1
鈥?5V
Output Common
FIGURE 1. 50mV Shunt Measures Current in A 500VDC Motor.
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
1994 Burr-Brown Corporation
AB-161
Printed in U.S.A. January, 1994
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