THEORY OF OPERATION
Details of the 3656 are shown in Figure 1. The external
connections shown, place it in its simplest gain configura-
tion —unity gain, noninverting. Several other amplifier gain
configurations and power isolation configurations are
possible. See Installation and Operating Instructions and
Applications sections for details.
at pin 15 equal to the input voltage at pin 7 with no galvanic
connection between them.
Several amplifier and power connection variations are pos-
sible:
1. The input stage may be connected in various operational
amplifier gain configurations.
Isolation of both signal and power is accomplished with a
single miniature toroid transformer with multiple windings.
A pulse generator operating at approximately 750kHz pro-
vides a two-part voltage waveform to transformer, T1. One
part of the waveform is rectified by diodes D1 through D4 to
provide the isolated power to the input and output stages
(+V, –V and V+, V–). The other part of the waveform is
modulated with input signal information by the modulator
operating into the V2 winding of the transformer.
2. The output stage may be operated at gains above unity.
3. The internally generated isolated voltages which provide
power to A1 and A2 may be overridden and external
supply voltages used instead.
Versatility and its three independent isolated grounds allow
simple solutions to demanding analog signal conditioning
problems. See the Installation and Operating Instructions
and Applications sections for details.
The modulated signal is coupled by windings W6 and W7 to
two matched demodulators—one in the input stage and one
in the output stage—which generate identical voltages at
their outputs, pins 10 and 11 (Voltages identical with respect
to their respective commons, pins 3 and 17). In the input
stage the input amplifier, A1, the modulator and the input
demodulator are connected in a negative feedback loop. This
forces the voltage at pin 6 (connect as shown in Figure 1) to
equal the input signal voltage applied at pin 7. Since the
input and the output demodulators are matched and produce
identical output voltages, the voltage at pin 11 (referenced to
pin 17, the output common) is equal to the voltage at pin 10
(referenced to pin 3, the input common). In the output stage,
output amplifier A2 is connected as a unity gain buffer, thus
the output voltage at pin 15 equals the output demodulator
voltage at pin 11. The end result is an isolated output voltage
INSTALLATION AND
OPERATING INSTRUCTIONS
The 3656 is a very versatile device capable of being used in
a variety of isolation and amplification configurations. There
are several fundamental considerations that determine con-
figuration and component value constraints:
1. Consideration must be given to the load placed on the
resistance (pin 10 and pin 11) by external circuitry. Their
output resistance is 100kΩ and a load resistor of 2MΩ or
greater is recommended to prevent a voltage divider
loading effect in excess of 5%.
T1
P+
P–
19
20
+
–
0.47µF
Pulse
Generator
15VDC
W1
Input
Demodulator
10
6
5
8
BAL
BAL
W6
100kΩ
14
11 13
9
2
15
+
Output
Demodulator
A1
A2
Modulator
VOUT
100kΩ
W7
W3
W5
7
1
D2
V+
+V
D1
16
W2
0.47µF
0.47µF
+
–
VIN
D3
D4
4
V–
12
–V
W4
–
3
17
FIGURE 1. Block Diagram.
®
6
3656
ROCHESTER [ Rochester Electronics ]
