1B22
INSIDE THE 1B22
The 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 2. Basic Interconnections
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.
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.
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.
IN1
IN2
OUT
HI
1B22
V
IN
1k⍀
50k⍀
OUT
LO
RESISTOR
NETWORK
4mA
OFF
REF
R1
20k⍀
SYNC
PROT
+15V
S. NODE
274k⍀
1k⍀
50k⍀
–15V
+15V
COM
USING THE 1B22
RECTIFIER/
POWER
DRIVER
LOOP
SUPPLY
Basic Interconnections: The 1B22 may be applied to achieve
rated 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.
–15V
V
P. COM
LOOP
Figure 4. Optional Offset and Span Adjustment
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-
ance voltage, P.COM can be connected to –15 V to drive higher
loads.
REV. B
–3–
ADI [ ADI ]
