廬
APPLICATION BULLETIN
0 TO 20mA RECEIVER USING RCV420
By David Kunst and R. Mark Stitt (602) 746-7445
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
Many industrial current-loop applications call for conver-
sion of a 0 to 20mA input current into 0 to 5V output. The
RCV420 is intended primarily as a complete solution for
precise 4 to 20mA to 0 to 5V conversion. But, with the
addition of one or two external 1% resistors, the RCV420
can also accurately convert a 0 to 20mA input into a 0 to 5V
output.
The recommended hook-up for 0-20mA/0-5V conversion is
shown in Figure 1. To reduce the gain from 5V/16mA to 5V/
20mA, the internal 75鈩?sense resistor is paralleled with a
301鈩? 1% external resistor connected between pins 1 and 2.
Even though the external paralleling resistor has a 1%
tolerance, the worst-case gain error of the current-to-voltage
conversion will be only 0.5%. This is because the parallel
combination of an external 301鈩?resistor and the internal
75鈩?resistor is dominated by the internal resistor.
A tighter tolerance on the external paralleling resistor would
not significantly improve the gain accuracy. This is because
the internal 75鈩?sense resistor also has a tolerance of 1%.
The high gain accuracy of the RCV420 transfer function
comes from a fine laser trim of the internal amplifier鈥檚 gain
which compensates for any error in the 75鈩?internal sense
resistor. So even if the sense resistor were replaced by a
resistor of exact value, the gain error could be as much as
1%.
For best common-mode rejection performance, a second
301鈩?external resistor should be connected between pins 2
and 3 in parallel with the other internal 75鈩?sense resistor.
Without it, 86dB CMR would be degraded to about 80dB. If
high CMR is not needed, the second resistor shown can be
omitted.
To eliminate the offset, used for 4-20mA/0-5V conversion,
the 鈥淩ef In鈥?(pin 12) must be connected to ground instead of
to the 10V reference. The 鈥淩ef Out鈥?and 鈥淩ef Feedback鈥?/div>
(pins 10 and 11) should still be connected together to prevent
the reference circuitry from locking-up. Even though the
10.0V reference is not used for span offsetting, it is a
precision reference which may be useful for other circuitry.
+15V
16
1
301鈩?/div>
0-20mA
Input
301鈩?/div>
3
300k鈩?/div>
2
300k鈩?/div>
75鈩?/div>
鈥?5V
4
RCV420
99k鈩?/div>
92k鈩?/div>
15
11.5k鈩?/div>
14
10
75鈩?/div>
1.01k鈩?/div>
10.0V
Ref
11
100k鈩?/div>
13
5
V
O
0-5V
12
10.0V
OUT
FIGURE 1. 0-20mA/0-5V Receiver Using RCV420.
漏
1990 Burr-Brown Corporation
AB-018A
Printed in U.S.A. February, 1991
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