Isolated, Loop-Powered

Voltage-to-Current Converter

1B21

Rev. C

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FEATURES

Wide input range: 0 V to 1 V up to 0 V to 10 V

High CMV isolation: 1500 V rms

Programmable output ranges: 4mA to 20mA

0 to 20 mA

Load resistance range: 0 to 1.35 kΩ maximum

High accuracy

Low offset tempco: ±300 nA/°C

Low gain tempco: ±50 ppm/°C

Low nonlinearity: ±0.02%

High CMR: 90 dB minimum

Small package: 0.7” × 2.1” × 0.35”

Meets IEEE Standard 472: transient protection (SWC)

APPLICATIONS

Multichannel process control

DAC—current loop interface

Analog transmitters and controllers

Remote data acquisition systems

FUNCTIONAL BLOCK DIAGRAM

1

18

38

17

19

20

22

21

FB

INPUT

OUT

HIGH

OUT

LOW

REF

OUT

+15V

–15V

COM

T2

T1

SIGNAL

ISOLATION

POWER

ISOLATION

TIMING

1B21

TIMING

RECTIFIER

MODULATOR

DEMODULATOR

OSCILLATOR

REF

Figure 1.

GENERAL DESCRIPTION

The 1B21 is an isolated voltage-to-current converter that

incorporates a unique circuit design utilizing transformer-based

isolation and automated surface-mount manufacturing technology.

It provides an unbeatable combination of versatility and perfor-

mance in a compact plastic package. Designed for industrial

applications, it is especially suited for harsh environments with

extremely high common-mode interference.

Functionally, the V/I converter consists of four basic sections:

input conditioning, modulator, demodulator and current source

(see Figure 1). The input is a resistor-programmable gain stage

that accepts a 0 V to 1 V up to 0 V to 10 V voltage input. This

maps into a 0 mA to 20 mA output or can be offset by 20%

using the internal reference for 4 mA to 20 mA operation. The

high level signal is modulated and passed across the barrier

which provides complete input to output galvanic isolation of

1500 V rms continuous by the use of transformer-coupling

techniques. Nonlinearity is an excellent ±0.05% maximum.

Designed for multichannel applications, the 1B21 requires an

external loop supply and can accept up to 30 V maximum. This

provides a loop supply compliance of 27V, which is sufficient to

drive a 1.35 kΩ load resistance.

The 1B21 is fully specified over −25°C to +85°C and operates

over the industrial (−40°C to +85°C) temperature range.

DESIGN FEATURES AND USER BENEFITS

High CMV Isolation. The 1B21 features high input to output

galvanic isolation to eliminate ground loops and offer protection

against damage from transients and fault voltages. The isolation

barrier will withstand continuous CMV of 1500V rms and

meets the IEEE Standard for Transient Voltage Protection

(Std. 472-SWC).

Small Size. The 1B21 package size (0.7” × 2.1” DIP) makes it an

excellent choice in multichannel systems for maximum channel

density. The 0.35” height also facilitates applications with

limited board clearance.

Ease of Use. Complete isolated voltage-to-current conversion

with minimum external parts required to get a conditioned

current signal. No external buffers or drivers are required.