EDNE June 2012 - (Page 53)

designideas readerS SOLVe deSIGN PrOBLeMS DIs Inside 54 Linearize optical distance sensors with a voltage-tofrequency converter 56 RC-timed shutoff function uses op amp and momentary switch 57 Use an integrator instead of coupling capacitors ▶To see all of EDN’s Design Ideas, visit www.edn.com/designideas Convert 1 to 5V signal to 4- to 20-mA output Thomas Mosteller, Linear Technology Corp ↘ Despite the long-predicted demise of the 4- to 20-mA current loop, this analog interface is still the most common method of connecting current-loop sources to a sensing circuit. This interface requires the conversion of a voltage signal— typically, 1 to 5V—to a 4- to 20-mA output. Stringent accuracy requirements dictate the use of either expensive precision resistors or a trimming potentiometer to calibrate out the initial error of less precise devices to meet the design goals. Neither technique is optimal in today’s surface-mounted, automatic-test-equipment-driven production environment. It’s difficult to get precise resistors in surface-mount packages, and trimming potentiometers require human intervention, a requirement that is incompatible with production goals. The Linear Technology LT5400 quad matched resistor network helps to solve these issues in a simple circuit that requires no trim adjustments but achieves a total error of less than 0.2% (Figure 1). The circuit uses two amplifier stages to exploit the unique matching characteristics of the LT5400. The first stage applies a 1 to 5V output—typically, from a DAC—to the noninverting input of op amp IC 1A . This voltage sets the current through R 1 to exactly VIN /R 1 through FET Q 2 . The same current is pulled down through R 2, so the voltage at the bottom of R2 is the 24V loop supply minus the input voltage. R3 250 This portion of the circuit has three main error sources: the matching of R1 and R2, IC1A’s offset voltage, and Q2’s leakage. The exact values of R 1 and R 2 are not critical, but they must exactly match each other. The LT5400A grade achieves this goal with ±0.01% error. The LT1490A has less-than-700-μV offset voltage over 0 to 70°C. This voltage contributes 0.07% error at an input 24V LOOP SUPPLY R2 10k ¼LT5400 − C1 220 pF 1 F IC1B ½LT1490A + R4 100 IRF9Z24S Q1 D1 IN4148 24V LOOP SUPPLY − 24V LOOP V+ IC1A V− ½LT1490A C2 220 pF R5 100 Q2 D2 IN4148 R6 22 1 TO 5V INPUT + NDS7002A R1 10k ¼LT5400 4- TO 20-mA OUT Figure 1 Precision matched resistors provide accurate voltage-to-current conversion. www.edn-europe.com JUNE 2012 | EDN EuropE 53 http://www.edn.com/designideas http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE June 2012

Agilent Technologies
International Rectifier
RS Components
International Rectifier
Analog Devices
Test & Measurement
Silicon Labs
Test-driven development for embedded C: why debug?
Baker’s best
Cover story
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Pico-projector design uses color LEDs
Digital isolation in smart energy metering applications
Mechatronics in design
Design Idea
Product Roundup
Tales from the cube

EDNE June 2012