Application Bulletin «Input Overload Protection For The RCV420 4-20 mA Current-Loop Receiver» (Texas Instruments)

APPLICATION BULLETIN
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Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (520) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (520) 889-1510 • Immediate Product Info: (800) 548-6132 INPUT OVERLOAD PROTECTION FOR THE RCV420 4–20MA CURRENT-LOOP RECEIVER
By R. Mark Stitt and David Kunst (602) 746-7445 Because of its immunity to noise, voltage drops, and line An LM193 dual voltage comparator is used to detect under- resistance, the 4–20mA current loop has become the standard range and overrange conditions at the output of the RCV420. for analog signal transmission in the process control indus- The LM193 is designed to operate from a single power try. The RCV420 is the first one-chip solution for converting supply with an input common-mode range to ground. In this a 4–20mA signal into a precision 0–5V output. Although the application the LM193 is operated from a single +15V power on-chip current sensing resistor is designed to tolerate mod- supply for input common-mode range compatibility with the erate overloads, it can be damaged by large overloads which RCV420 output. The open-collector LM193 comparator can result from short circuits in the current loop. Complete outputs are connected through 10kΩ pull-up resistors to a input protection from short circuits to voltages of 50V or +5V supply for compatibility with TTL and similar logic more can be afforded with the addition of the relatively families. simple circuitry shown in this bulletin. The RCV420 has a gain of 0.3125V/mA and a 4mA span The complete protected 4–20mA current loop receiver cir- offset (a 4mA–20mA input produces a 0V–5V output). cuit is shown in Figure 1. The RCV420 is connected for a 0– Under input open circuit conditions (0mA input), the output 5V output with a 4–20mA current sink input. For a 4–20mA of the RCV420 goes to –1.25V. To level-shift the output up, current source input, connect the input to pin 3 instead of pin for a minimum of 0V at the comparator inputs, it is summed 1. An on-board precision +10.0V buried zener voltage refer- with the 10.0V voltage reference through the 10kΩ, 1.27kΩ ence in the RCV420 is used to offset the span (for 0V out resistor network. The table below shows selected operating with 4mA in) via pin 12. It can also be used for powering points for the RCV420 input/output and the comparator external circuitry such as the voltage dividers used to set the input. underrange/overrange comparator thresholds. +15V –15V 16 4
RCV420
1 300kΩ 99kΩ 92kΩ 12 15 V 75Ω OUT 0–5V 2 11.5kΩ 14 10.0V 10 75Ω 1.01kΩ 10.0V 11 Reference 3 1.27kΩ 300kΩ 100kΩ +5V 13 5 10k +15V Ω 8 8 4 1MΩ 10kΩ 10kΩ 7 2 10kΩ 10kΩ AT&T 555 LH1191 6 5 6.95V Timer Solid-State Relay LM193 1µF 3 1 0.01µF 0.57V 47kΩ 4–20mA Input 604Ω 22.9kΩ 470Ω 4 2N3904 1µF Overrange Underrange Output Output FIGURE 1. Input Protected RCV420. © 1 1990 Burr-Brown Corporation AB-014 Printed in U.S.A. July, 1990
SBVA003