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1B21 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer 1B21
Beschreibung Voltage-to-Current Converter
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 9 Seiten
1B21 Datasheet, Funktion
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
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
Isolated, Loop-Powered
Voltage-to-Current Converter
1B21
FB 18
INPUT 17
REF
OUT
19
+15V 20
–15V 22
COM 21
FUNCTIONAL BLOCK DIAGRAM
SIGNAL
ISOLATION
1
OUT
HIGH
REF
MODULATOR
DEMODULATOR
TIMING
T1
POWER
ISOLATION
TIMING
38
OUT
LOW
OSCILLATOR
T2
RECTIFIER
1B21
Figure 1.
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.
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.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
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.






1B21 Datasheet, Funktion
INSIDE THE 1B21
Referring to the functional block diagram (see Figure 3), the
±15 V power inputs provide power to both the input side
circuitry and the power oscillator. The 25 kHz power oscillator
provides both the timing information for the signal modulator
and drives transformer T2 for the output side power supplies.
The secondary winding of T2 is full wave rectified and filtered
to create the output side power.
FB 18
INPUT 17
REF
OUT
19
+15V 20
–15V 22
COM 21
REF
SIGNAL
ISOLATION
1
OUT
HIGH
MODULATOR
DEMODULATOR
TIMING
T1
POWER
ISOLATION
TIMING
OSCILLATOR
T2
RECTIFIER
38
OUT
LOW
1B21
Figure 3. 1B21 Functional Block Diagram
The input stage is configured as an inverting amplifier with
three user-supplied resistors for gain, offset, and feedback.
The conditioned signal is modulated to generate a square wave
with a peak-to-peak amplitude proportional to VIN. The signal
drives the signal transformer T1. An internal reference with a
nominal output voltage of +6.225 V and tempco of ±15 ppm/°C
is provided to develop a 4 mA offset for 4 mA to 20 mA current
loop applications.
After passing through signal transformer T1, the amplitude
modulated signal is demodulated and filtered by a single pole
filter. Timing information for the output side is derived from
the power transformer T2. The filtered output provides the
control signal for the voltage-to-current converter stage. An
external power supply is required in series with the load to
complete the current loop.
USING THE 1B21
Input Configurations
The 1B21 has been designed with a flexible input stage for a
variety of input and output ranges. The basic interconnection
for setting gain and offset is shown in Figure 4. The output of
the internal amplifier is constrained to 0 to −5 V, which maps
into 0 to 20 mA across the isolation barrier. Thus to create a
4 mA offset at the output, the input amplifier has to be offset
by 1 V.
For example, for 0 to 20 mA operation, the transfer function for
the input stage is
5/VIN = RF/RI
and no offset resistor is needed. For 4 mA to 20 mA operation
one gets
4/VIN = RF/RI
1B21
This maps the input voltage into a 4 V span. To create a 1 V
offset at the output of the internal amplifier (4 mA at the output
of the 1B21) a current derived from the reference can be fed
into the summing node. The offset resistor (for a 1 V output
offset) is given by the equation: RO = 6.225 RF. For most
applications, it is recommended that RF be in the 25 kΩ ±20%
range. Resistor values for typical input and output ranges are
shown in Table 2.
FB
18
RI RF INPUT
VIN 17
1B21
VLOOP
1
RLOAD
RO
19
REF
38
+15V
COM
–15V
+ 20
21
+
22
4.7µF/20V
OSCILLATOR
RECTIFIER
Figure 4. Basic Interconnections
Table 2. Resistor Values for Typical Ranges
Input Volts
Output mA
RI kΩ RF
0 to 5
0-20
25 25
0 to 10
0-20
50 25
0 to 5
4-20
25 20
0 to 10
4-20
50 20
1 to 5
4-20
25 25
RO
Open
Open
124.3
124.3
Open
Adjustments
Figure 5 is an example of using potentiometers for trimming
gain and offset for a 0 V to 5 V input and 0 V to 20 mA output.
The network for offset adjustment keeps the resistors relatively
small to minimize noise effects while giving a sensitivity of ±1%
of span. For more adjustment range, resistors smaller than 274 k
can be used. Resistor values from Table 2 can be substituted for
other input and output ranges.
In general, any bipolar voltage can be input to the 1B21 as
long as it is offset to meet the 0 V to −5 V constraint of the
modulator and the input signal range is 1 V minimum.
FB
1k
19.6k
18
20k
INPUT
VIN 17
+15V
274k20k
50k
–15V
1k19
REF
1B21
1
38
+15V 20
COM 21
–15V 22
OSCILLATOR
RECTIFIER
Figure 5. Offset and Span Adjustment
Rev. C | Page 5 of 8

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