1B21

Rev. C | Page 5 of 8

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.

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

range. Resistor values for typical input and output ranges are

shown in Table 2.

18

1

38

17

19

20

22

21

COM

T2

1B21

FB

INPUT

OUT

HIGH

OUT

LOW

REF

OUT

+15V

–15V

T1

SIGNAL

ISOLATION

POWER

ISOLATION

TIMING

TIMING

RECTIFIER

OSCILLATOR

MODULATOR

DEMODULATOR

REF

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

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

and no offset resistor is needed. For 4 mA to 20 mA operation

one gets

1

38

RECTIFIER

OSCILLATOR

REF

+15V

COM

–15V

18

17

19

20

4.7µF/20V

+

+

22

21

FB

INPUT

1B21

Figure 4. Basic Interconnections

Table 2. Resistor Values for Typical Ranges

Input Volts

Output mA

0 to 5

0-20

25

25

Open

0 to 10

0-20

50

25

Open

0 to 5

4-20

25

20

124.3

0 to 10

4-20

50

20

124.3

1 to 5

4-20

25

25

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.

1

38

RECTIFIER

OSCILLATOR

REF

+15V

COM

–15V

18

17

1k

Ω

50k

Ω

19.6k

Ω

20k

Ω

FB

INPUT

20k

Ω

274k

Ω

19

20

21

22

1k

Ω

+15V

–15V

1B21

Figure 5. Offset and Span Adjustment