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1B21AN
Isolated, Loop-Powered Voltage-to-Current Converter
ANALOG
DEVICES
Isolated, Loop-Powered
1loltage-to-Currtmt Converter
FEATURES
Wide Input Range: O-IV to 0-10V
High CMV Isolation: 1500V rms
Programmable Output Ranges: 4mA to 20mA
0 to 20mA
Load Resistance Range: 0 to 1.35kn max
High Accuracy
Low Offset Tempco: t300nAf'C
Low Gain Tempco: ce50ppm/''C
Low Nonlinearity: 10.02%
High CMR: 90dB min
Small Package: 0.7"x 2.1”x 0.35"
Meets IEEE Std. 472: Transient Protection (SWC)
APPLICATIONS
Multichannel Process Control
D/A Converter - Current Loop Interface
Analog Transmitters and Controllers
Remote Data Acquisition Systems
GENERAL DESCRIPTION
The 1B21 is an isolated voltage-to-current converter that incor-
porates a unique circuit design utilizing transformer based isolation
and automated surface mount manufacturing technology. It
provides an unbeatable combination of versatility and performance
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
(1B21 Functional Block Diagram). The input is a resistor pro-
grammable gain stage that accepts a O-lV to 0-10V voltage input.
This maps into a 0 to 20mA output or can be offset by 20%
using the internal reference for 4mA to 20mA operation. The
high level signal is modulated and passed across the barrier
which provides complete input to output galvanic isolation of
1500V rms continuous by the use of transformer coupling tech-
niques. Nonlinearity is an excellent :0.05% max.
REV. B
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 which may result from its use. No license is granted by implica-
tion or otherwise under any patent or patent rights of Analog Devices.
FUNCTIONAL BLOCK DIAGRAM
SIGNAL HI
ISOLATION
MODu LATOR ll DEMODULATOR l
POWER ITIMING our
htwrm ISOLATION LO
OSCILLATOR ll RECTIFIER
T2 IB21
Designed for multichannel applications, the 1B21 requires an
external loop supply and can accept up to 30V max. This would
provide a loop compliance of 27V, which is sufficient to drive a
1.35kQ load resistance.
The 1B21 is fully specified over -25T to + 85°C and operates
over the industrial (-400C 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-
Small Size: The IB21 package size (0.7" X 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.
One Technology Way, P.0. Box 9106, Norwood, MA 02062-9106, USA.
Tel: 617/329-4700 Fax: 617/326-8703 Twx: 710/394-6577
Telex: 924491 Cable: ANALOG NORWOODMASS
1 821 -$PEtlFltyrrltlhl (typical at +25%: and Vs = t15ll unless otherwise noted)
V VModelr
IB21AN OUTLINE DIMENSIONS
INPUT SPECIFICATIONS Dimensions shown in inches and (mm).
Input Range 0 to + 10V 0 mm
Full-Scale Input + 1V min to + 10V max 0 t5 MIN
Input Bias Current : 30pA ( : 400pA max) _ l P'"--------)]",', -------/ l. ‘1 F-
OUTPUT SPECIFICATIONS _ " IB21 , 3 3 Tm,
Current Output Range 4mA to 20mA, 0 to 20mA [35%) “3123” MAX)
Load Compliance at VLoop = 30V 27V min 'd g il ' (i,
Max Output Current a Input Overload 25mA ( on ' P3395”
Output Noise, 100Hz Bandwidth luA p-p F 123') (2 a-i l-- V Yd lf MAX
NONLINEARITY(% ofSpan) :0.02%(: 0.05% max) MAX
ISOLATION AC1060 MATING SOCKET
CMV, Input to Output Continuous 1500V rms _ - mo m, (,5;
CMR, @ 60Hz 90dB min (25:5) (65,trr 4. ,iMAxl,
TransientProtection IEEE-STD 472 (SWC) . . _ - , ,
ACCURACY l
Warm-Up Time to Rated Performance
Total Output Error @ + 25°C (Untrimmed)
Offset(V1N = 0V)1
. - - - CO- 0.5.no n.7eo
5 mm m 5) (115)
+4...) ——— o-o-l,, J. i
Lt.- 100WA \o.w(z.59oIA
EOTHENDS 0.125 l3m
Span(V1N = + 10V) t 0.6% FSR TYP
vs. Temperature( - 25°C to + 85°C)
Offset2 t 300nA/°C
Span t 50ppm/°C PIN DESIGNATIONS
REFERENCE OUTPUT PIN FU NCTION
Voltage + 6.4V dc 1 OUT HI
Output Error -_+- 1 .5% max 17 IN
Temperature Coefficient t 20ppm/°C max 18 FB
DYNAMIC RESPONSE 19 REF.
Settling Time to 0.1%ofF.S. for 10V Step 9ms 20 “W
SmallSignal Bandwidth 100Hz 3; 'lf'dl,
POWER SUPPLY 38 OUT LOW
Input Side
Operating Voltage : 15V t 5%
Quiescent Current
+ 15V Supply 10mA
- 15V Supply 5mA
Power Supply Rejection f: 0.01%/V
Loop Side
Operating Voltage + 15V to + 30V
Maximum Current 25mA
ENVIRONMENTAL
Temperature Range
Rated Performance - 25°C to + 85°C
Operating - 40°C to + 85°C
Storage - 40°C to + 85°C
Relative Humidity, Noncondensing
0 to 95% @ + 60°C
CASE SIZE
0.7"x2.1"y: 0.35"
(17.8 x 53.3 M 8.9)mm
lFor 0-20mA mode. For 4-20mA mode an additional 60WA is contributed by the t 1.5% reference error on the 4mA output.
2For a complete discussion of the temperature effects of the offset resistor and reference refer to "Using the IB21" section.
Specifications subject to change without notice.
-2- REV. B
INSIDE THE IB21
Referring to the functional block diagram, the : 15V power
inputs provide power to both the input side circuitry and the
power oscillator. The 25kHz 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.
SIGNAL 1 S.”
ISOLATION
INPUT "
MODULATOR I I DEMODULATOR
POWER TIMING OUT
TIMING ISOLATION LO
- 15V RECTIFIER
OSCILLATOR l
COM T2
IB21 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. This signal drives
the signal transformer TI. An internal reference with a nominal
output voltage of +6.4V and tempco of ae 20ppm/°C is provided
to develop a 4mA offset for 4mA to 20mA 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 IB21
Input Configurations: The IB21 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 l. The output of the internal amplifier is constrained to
0 to - 5V, which maps into 0 t0 20mA across the isolation
barrier. Thus to create a 4mA offset at the output, the input
amplifier has to be offset by IV.
For example, for 0 to 20mA operation the transfer function for
the input stage is:
5/VIN = RF/RI
and no offset resistor is needed. For 4mA to 20mA operation we
4/VIN = RF/RI
which maps the input voltage into a 4V span. To create a 1V
offset at the output of the internal amplifier (4mA at the output
of the IB21) a current derived from the reference can be fed
into the summing node. The offset resistor (for a 1V output
offset) will be given by the equation: Ro = 6.4RF. For most
applications it is recommended that R, be in the 25kn t20%
range. Resistor values for typical input and output ranges are
shown in Table I.
REV. B
RECTIFIER
4.7[LF/20V
Figure 1. Basic Interconnections
Input Output R, RF Ro
Volts mA M) kn m
0-5 0-20 25 25 Open
0-10 0-20 50 25 Open
0-5 4-20 25 20 128
0-10 4-20 50 20 128
1-5 4-20 25 25 Open
Table l, Resistor Values for Typical Ranges
Adjustments: Figure 2 is an example of using potentiometer:
for trimming gain and offset for a 0-5V input and 0 to 20mA
output. The network for offset adjustment keeps the resistors
relatively small to minimize noise effects while giving a sensitivity
of t 1% of span. For more adjustment range, resistors smaller
than 274k can be used. Resistor values from Table I can be
substituted for other input and output ranges.
In general, any bipolar voltage can be input to the IB21 as long
as it is offset to meet the 0 to - 5V constraint of the modulator
and the input signal range is 1V minimum.
IF6k INPUT
F 15V 20k .
19 REF
-15V 1k
+15V 20
COM 21 OSC RECTIFIER
- 15V 22 -
Figure 2. Offset and Span Adjustment
TC Considerations of External Resistors: The specifications
for gain and offset temperature coefficient (TC) for thc 1B21
exclude the effects of external components. The total gain TC
for the circuit in Figure 1 is:
Gain TC = IB21 Gain TC + (Tracking TC of IV and RO
The offset TC is also affected by the thermal stability of the
internal voltage reference and its contribution is:
Ref TC = (VRmr)(Rrc/Ro)(4mA/V)(TC of VREF +
Tracking TC of R1: and Ro)/1 M 106
Total Offset T C = IB21 Offset TC + Ref TC
Specifically using RF, R, and Ro from Case 3 in Table I, with
absolute TCs of , 25ppm/°C we get:
Gain TC = 50 + (25+25) = 100ppm/°C
Offset TC = 300+(6.4V)(20k/128k)(4mA/V)(20+25+25)/
1 M 106
= :580nA/0C
Similarly, when using a resistor network with a tracking spec of
, Sppm/°C, the total gain TC would be t55ppm/oC and the
total offset TC would be i400nA/°C.
APPLICATIONS
Output Protection: In many industrial applications it may be
necessary to protect the current output from accidental shorts to
ac line voltages in addition to high common-mode voltages and
short circuits to ground. The circuit shown in Figure 3 can be
used for this purpose. The maximum permissible load resistance
will be lowered by the fuse resistance (typically Mt) when
protection circuitry is utilized.
Fa IS IB21 1
INPUT 17 - _ I
' FUSE
IN4002 mm (S.ikl
38 - -
REF our 19 REF GE.
- f v'. VARISTOR Rum
nsv 20 manoz V27ZA1
COM 21 1 osc RECTiFIER
- 15v 22 _ e-
Figure 3. Output Protection Circuitry
Low Drift Input Network: Figure 4 shows a configuration
suitable for applications where errors have to be minimized over
a wide temperature range. A temperature tracking network such
as a 50k Beckman (PN 698-3R50KD) can be used to implement
both offset and gain for either 0 to 20mA or 4mA t0 20mA
current loops. For 0-10V signals either INl or 1N2 can be used
for input. For 0-5V signals, jumper INI to 1N2. Similarly, for
4mA to 20mA operation the 4mA node should be jumpered to
OFFSET, while for 0 t0 20mA it should be tied to COM.
BECKMAN 50k Rm
t" --.- "I r' -- -
IN2 C '.'.r A.
INI % "".
OFFSET tb-
4mA ['
MODES LABELLED FOR RECTIFIER
ILLUSTRATION ONLY.
Figure 4. Low Tempco Resistor Network Configuration
Mu1tiloop Isolation: Multiple IB21s can be connected to a
single loop supply in parallel as shown in Figure 5. The amperage
of the loop supply should be sufficient to drive all the loops at
full-scale output.
Figure 5. Multiple IB21s with Single Loop Supply
REV. B
C1170a—5-6/91
PRINTED IN USA.
