1B21AN データシートの表示（PDF） - Analog Devices
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1B21AN Datasheet PDF : 8 Pages
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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.
FB 18
INPUT 17
REF
OUT
19
+15V 20
–15V 22
COM 21
REF
SIGNAL
ISOLATION
1
OUT
HIGH
MODULATOR
DEMODULATOR
TIMING
T1
POWER
ISOLATION
TIMING
OSCILLATOR
T2
RECTIFIER
38
OUT
LOW
1B21
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
with a peak-to-peak amplitude proportional to VIN. The signal
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
5/VIN = RF/RI
and no offset resistor is needed. For 4 mA to 20 mA operation
one gets
4/VIN = RF/RI
1B21
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
offset) is given by the equation: RO = 6.225 RF. For most
applications, it is recommended that RF be in the 25 kΩ ±20%
range. Resistor values for typical input and output ranges are
shown in Table 2.
FB
18
RI RF INPUT
VIN
17
1B21
VLOOP
1
RLOAD
RO
38
19
REF
+15V
COM
–15V
+ 20
21
+
22
4.7µF/20V
OSCILLATOR
RECTIFIER
Figure 4. Basic Interconnections
Table 2. Resistor Values for Typical Ranges
Input Volts
Output mA
RI kΩ RF kΩ
0 to 5
0-20
25
25
0 to 10
0-20
50
25
0 to 5
4-20
25
20
0 to 10
4-20
50
20
1 to 5
4-20
25
25
RO kΩ
Open
Open
124.3
124.3
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.
FB
1kΩ
18
19.6kΩ 20kΩ INPUT
VIN
17
+15V
274kΩ 20kΩ
50kΩ
–15V
1kΩ 19
REF
1B21
1
38
+15V 20
COM 21
–15V 22
OSCILLATOR
RECTIFIER
Figure 5. Offset and Span Adjustment
Rev. C | Page 5 of 8
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