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1B22AN
Programmable, Isolated Voltage-to-Current Converter
REV.B
FUNCTIONAL BLOCK DIAGRAM
Programmable, Isolated
Voltage-to-Current Converter
FEATURES
Internal Isolated Loop Supply Drives 1000 � Load
Pin Programmable Inputs: 0 V to +5 V or 0 V to +10 V
Pin Programmable Outputs: 4 to 20 mA or 0 to 20 mA
High CMV Isolation: 1500 V RMS
Normal-Mode Output Protection: 240 V RMS
High Accuracy
Low Offset Tempco: �300 nA/�C
Low Gain Tempco: �50 ppm/�C
Low Nonlinearity: �0.02%
High CMR: 90 dB min
Small Package: 1.0" � 2.10" � 0.35"
Meets IEEE STD 472: CMV Transient Protection (SWC)
APPLICATIONS
Multichannel Process Control
D/A Converter—Current Loop Interface
Analog Transmitters and Controllers
Remote Data Acquisition Systems
GENERAL DESCRIPTIONThe 1B22 is an isolated voltage-to-current converter that
incorporates transformer isolation to achieve high performance
and automated surface mount manufacturing for low cost
and increased reliability. Designed for industrial applications,
it is especially suited for harsh environments with extremely
high common-mode interference. With programmable inputs
and outputs, the 1B22 provides an unbeatable combination of
versatility and performance in a compact plastic package.
Functionally, the V/I converter consists of four basic sections:
input conditioning, modulator/demodulator, isolated loop
supply and current source (Figure 1). The 1B22 is pin program-
mable for 0 V to +5 V or 0 V to +10 V inputs and 0 to 20 mA
or 4 to 20 mA outputs using an internal resistor network. It can
also be set by an external resistor to accept 0 V to +1 V to 0 V
to +10 V inputs. Transformer coupling provides 1500 V rms
galvanic isolation between the inputs and the current loop.
Nonlinearity is an excellent ±0.05% max.
Loop power is generated internally through a dc/dc converter
and is also isolated from the input side (1500 V rms). Loop
compliance voltage is dependent on the voltage supplied to the
1B22, and with VLOOP = 28 V, it is sufficient to drive a 1000 Ω
load.
The 1B22 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
Isolated Loop Power:Internal loop supply completely
isolates the loop from the input terminals (1500 V rms) and
provides the capability to drive 0 Ω to 1000 Ω loads. This elimi-
nates the need for an external dc/dc converter.
Ease of Use:The 1B22 offers complete isolated voltage-to-
current conversion with minimum external parts required to get
a conditioned current signal. No external buffers or drivers are
required.
High CMV Isolation:The 1B22 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 1500 V rms and meets
the IEEE Standard for Common-Mode Voltage Transient Pro-
tection (STD 472-SWC).
Small Size:The 1B22 package size (1.0" × 2.1" DIP) makes it
an excellent choice in multichannel systems for maximum chan-
nel density. The 0.35" height also facilitates applications with
limited board clearance.
1B22–SPECIFICATIONS(typical @ +25�C and VS = �15 V, VLOOP = +24 V, unless otherwise noted)DYNAMIC RESPONSE
Specifications subject to change without notice.
INSIDE THE 1B22The 1B22 produces an isolated 4 to 20 mA or 0 to 20 mA out-
put current which is proportional to the input voltage and
independent of the output load resistance (Figure 1). The input
stage is configured as an inverting amplifier with a resistor net-
work to provide pin-strappable input ranges of 0 V to +5 V and
0 V to +10 V and output ranges of 0 to 20 mA and 4 to 20 mA.
The conditioned signal is modulated to generate a square wave
that drives transformer T1. The peak-to-peak amplitude of the
signal is proportional to VIN. An internal, high stability reference
with a nominal output voltage of +6.4 V is used to develop a
4 mA offset for the 4 to 20 mA current loop output.
Figure 1.1B22 Functional Block Diagram
After passing through signal transformer T1, the amplitude
modulated signal is demodulated and filtered by a single pole
filter. This filtered output is the control signal for the voltage-
to-current converter stage. Timing information for the
demodulator is derived from the power transformer T2. The
1B22 outputs are protected from accidental shorts to ac line
voltages up to 240 V rms. Combined with 1500 V input to out-
put isolation, the 1B22 provides unbeatable protection against
transients, wiring errors and current loop short circuits to power
lines.
The dc-dc converter consists of a power driver, power trans-
former T2, a full wave rectifier and a filter. The dc-dc converter
provides the power for the output circuitry as well as the
isolated compliance voltage for the loop. This voltage is propor-
tional to VLOOP on the input side. The 1B22 requires ±15 V
supplies to power the input side circuitry and a +14 V to +30 V
supply for the dc-dc converter.
USING THE 1B22
Basic Interconnections: The 1B22 may be applied to achieverated performance as shown in Figure 2. For 0 V to 10 V signals
either IN1 or IN2 can be used for input; for 0 V to +5 V signals
jumper IN1 to IN2. Similarly, for 4 to 20 mA operation the
4 mA OFFSET node should be jumpered to the S. NODE,
while for 0 to 20 mA it should be tied to COM. Figure 3 shows
the functional diagram of the resistor network used in the 1B22.
For applications where a separate loop supply is not available,
the ±15 V supplies can be used by connecting +15 V to VLOOP
(Pin 24) and COM to P.COM (Pin 25). For additional compli-
Figure 2.Basic Interconnections
Figure 3.Internal Resistor Network
Optional Trim Adjustments:Figure 4 is an example of using
potentiometers for trimming gain and offset for a 0 V to +10 V
input and 4 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. The value of R1
should be 5 MΩ for the 0 V to +10 V range (for 0 V to +5 V
configurations, a 1.2 MΩ resistor should be used). For more
adjustment range, resistors smaller than 274 kΩ can be used.
Figure 4.Optional Offset and Span Adjustment
1B22C1193a
3/00 (rev. B)
PRINTED IN U.S.A.
Synchronizing Multiple 1B22s: In applications where mul-tiple 1B22s are used in close proximity, radiated individual
oscillator frequencies may cause “beat frequency” related out-
put errors. These errors can be eliminated by driving the SYNC
pins of all the units with a 40 kHz clock circuit at 50% duty
cycle (Figure 5). The SYNC input typically has an input imped-
ance of 150 k�180 pF.
Figure 5.Multiple 1B22s’ Synchronization
Loop Supply Requirements: The 1B22 design allows flexibleloop supply options. The loop supply voltage required for any
value of load resistance can be calculated from the following
equation:
This value allows for approximately 10% overrange capability.
The graph in Figure 6 shows the relationship between supply
voltage and load resistance.
Figure 6.Loop Supply vs. Load
APPLICATIONS
Isolated D/A Converter: The 1B22 offers total ground isola-tion and protection from high voltage transients in interfacing
D/A converters to standard 4 to 20 mA current loops. The D/A
converter, such as the Analog Devices’ 12-bit AD7245
DACPORT®, should be connected for operation on the unipo-
lar 0 V to +10 V output range. This is shown in Figure 7.
Figure 7.D/A Converter–Isolated 4-20 mA Interface
Pressure Transmitter: In Figure 8, the 1B22 is used in apressure transmitter application to provide complete input-
output isolation and avoid signal errors due to ground loop cur-
rents. The process pressure is monitored with a strain gage type
pressure transducer interfaced by the Analog Devices’ 1B32
transducer signal conditioner. The high level voltage output of
the 1B32 is converted to the isolated 4 to 20 mA current for
transmission to a remote recorder or indicator.
Figure 8.Isolated Pressure Transmitter
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