Isolated, Loop-Powered
Voltage-to-Current Converter
1B21
Rev. C
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©1991–2009 Analog Devices, Inc. All rights reserved.
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
08530-001
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.
