廬
APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (520) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Immediate Product Info: (800) 548-6132
INPUT OVERLOAD PROTECTION FOR THE RCV420
4鈥?0MA CURRENT-LOOP RECEIVER
By R. Mark Stitt and David Kunst (602) 746-7445
Because of its immunity to noise, voltage drops, and line
resistance, the 4鈥?0mA current loop has become the standard
for analog signal transmission in the process control indus-
try. The RCV420 is the first one-chip solution for converting
a 4鈥?0mA signal into a precision 0鈥?V output. Although the
on-chip current sensing resistor is designed to tolerate mod-
erate overloads, it can be damaged by large overloads which
can result from short circuits in the current loop. Complete
input protection from short circuits to voltages of 50V or
more can be afforded with the addition of the relatively
simple circuitry shown in this bulletin.
The complete protected 4鈥?0mA current loop receiver cir-
cuit is shown in Figure 1. The RCV420 is connected for a 0鈥?/div>
5V output with a 4鈥?0mA current sink input. For a 4鈥?0mA
current source input, connect the input to pin 3 instead of pin
1. An on-board precision +10.0V buried zener voltage refer-
ence in the RCV420 is used to offset the span (for 0V out
with 4mA in) via pin 12. It can also be used for powering
external circuitry such as the voltage dividers used to set the
underrange/overrange comparator thresholds.
+15V
16
鈥?5V
4
RCV420
1
300k鈩?/div>
99k鈩?/div>
92k鈩?/div>
12
15
11.5k鈩?/div>
14
10.0V
10
75鈩?/div>
3
300k鈩?/div>
+5V
100k鈩?/div>
13
5
+15V
8
8
1M鈩?/div>
7
AT&T
LH1191
Solid-State
Relay
6
555
Timer
2
5
LM193
1碌F
3
1
0.01碌F
4
10k鈩?/div>
10k鈩?/div>
10k鈩?/div>
10k鈩?/div>
6.95V
10k鈩?/div>
1.01k鈩?/div>
10.0V
Reference
11
1.27k鈩?/div>
V
OUT
0鈥?V
An LM193 dual voltage comparator is used to detect under-
range and overrange conditions at the output of the RCV420.
The LM193 is designed to operate from a single power
supply with an input common-mode range to ground. In this
application the LM193 is operated from a single +15V power
supply for input common-mode range compatibility with the
RCV420 output. The open-collector LM193 comparator
outputs are connected through 10k鈩?pull-up resistors to a
+5V supply for compatibility with TTL and similar logic
families.
The RCV420 has a gain of 0.3125V/mA and a 4mA span
offset (a 4mA鈥?0mA input produces a 0V鈥?V output).
Under input open circuit conditions (0mA input), the output
of the RCV420 goes to 鈥?.25V. To level-shift the output up,
for a minimum of 0V at the comparator inputs, it is summed
with the 10.0V voltage reference through the 10k鈩? 1.27k鈩?/div>
resistor network. The table below shows selected operating
points for the RCV420 input/output and the comparator
input.
75鈩?/div>
2
4鈥?0mA
Input
47k鈩?/div>
470鈩?/div>
1碌F
Overrange
Output
2N3904
Underrange
Output
4
0.57V
604鈩?/div>
22.9k鈩?/div>
FIGURE 1. Input Protected RCV420.
漏
1990 Burr-Brown Corporation
1
AB-014
Printed in U.S.A. July, 1990
AB-014相關(guān)型號PDF文件下載
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型號
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描述
廠商
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AB-002 - MAKE A PRECISION CURRENT SOURCE OR CURRENT SINK
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英文版
AB-003 - VOLTAGE-REFERENCE FILTERS
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AB-004 - MAKE A PRECISION -10V REFERENCE
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AB-005 - Make A Precision -10V Reference
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