where VO is the output voltage of A3, and VCM is the
common-mode voltage between the inputs LA and RA and
the input common at pin 3 of the 3656.
be used to provide 15V for the pulse generator (pins 19 and
20). The input stage is configured as a unity gain buffer,
although other configurations such as current input could be
used. The circuit uses the isolation feature between the
output stage and the primary power supply to generate the
output current configuration that can work into a grounded
load. Note that the output transistors can only drive positive
current into the load. Bipolar current output would require a
second transistor and dual supply.
This circuit has the added benefit of having higher common-
mode rejection than the circuit in Figure 6 (approximately
10dB improvement).
BIPOLAR CURRENT OUTPUT
The three-port capability of the 3656 can be used to imple-
ment a current output isolation amplifier function—usually
difficult to implement when grounded loads are involved.
The circuit is shown in Figure 8 and the following equations
apply:
ISOLATED 4mA TO 20mA OUTPUT
Figure 10 shows the circuit of an expanded version of the
isolated current output function. It allows any input voltage
range to generate the 4mA to 20mA output excursion and is
also capable of zero suppression. The “span” (gain) is
adjusted by R2 and the “zero” (4mA output for minimum
input) is set by the 200kΩ pot in the output stage. A three-
terminal 5V reference is used to provide a stable 4mA
operating point. The reference is connected to insert an
adjustable bias between the demodulator output and the
noninverting input of the output stage.
RF
RA
R2
G = IOUT/VIN = 1 +
X
(R1 + R2) • RS
IOUT ≤ ±2.5mA
V1 ≤ ±4V (compliance)
RL ≤ 1.6kΩ
RF + RA = R1 + R2 ≤ 2MΩ
DIFFERENTIAL INPUT
Figure 11 shows the proper connections for differential input
configuration. The 3656 is capable of operating in this input
configuration only for floating loads (i.e., the source VIN
has no connection to the ground reference established at
pin 3). For this configuration the usual 2MΩ resistor used in
CURRENT OUTPUT—
LARGER UNIPOLAR CURRENTS
A more practical version of the current output function is
shown in Figure 9. If the circuit is powered from a source
greater than 15V as shown, a three-terminal regulator should
RF
9
2
10
D
M
6
14
D
IOUT
RA
+
15
A2
RL
RS
VL
–
13
11
R2
A1
17
16
12
R1
0.47µF
0.47µF
7
4
1
RC
3
O/P PWR
+
+
–
19
0.47µF
VIN
–
Pulse GEN
20
15V
I/P PWR
FIGURE 8. Bipolar Current Output.
®
13
3656
BB [ BURR-BROWN CORPORATION ]
