1B22

REV. B

–3–

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

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-

±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 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.

For applications where a separate loop supply is not available,

the

(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.

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 M

Ω for the 0 V to +10 V range (for 0 V to +5 V

configurations, a 1.2 M

Ω resistor should be used). For more

adjustment range, resistors smaller than 274 k

Ω can be used.

IN2

4mA

OFF

50k

RESISTOR

NETWORK

REF

IN1

S. NODE

R1

POWER

DRIVER

RECTIFIER/

LOOP

SUPPLY

SYNC

+15V

COM

–15V

PROT

OUT

HI

OUT

LO

20k

1k

274k

50k

+15V

–15V

1k

1B22

Figure 4. Optional Offset and Span Adjustment