RF 1MΩ
100kΩ
9
2
10
D
M
6
14
D
R1
+
–
15
VOUT
A2
+
VIN
–
13
2MΩ
11
A1
17
R1
16
12
7
4
1
+
–
3
15V
RF
1MΩ
O/P PWR
0.47µF
19
+
Pulse GEN
20
15V
–
I/P PWR
VOUT = –VIN (RF/R1) RF = R1
FIGURE 11. Differential Input, Floating Source.
the input stage is split into two halves, RF and RF–. The
demodulator load (seen by pin 10 with respect to pin 3) is
still 2MΩ for the floating load as shown. Notice pin 19 is
common in Figure 11 whereas pin 20 is common in previous
figures.
in Figure 13. Here the instrumentation grade OPA177 is
supplied from the isolated power of the input stage. The
3656 is configured as a unity-gain buffer. The gain of the
OPA177 stage must be chosen to limit its full scale output
voltage to 5V and avoid overdriving the 3656’s demodula-
tors. Since the 3656 draws a significant amount of supply
current, extra filtering or the input supply is required as
shown (2 x 0.47µF).
SERIES STRING SOURCE
Figure 12 shows a situation where a small voltage, which is
part of a series string of other voltages, must be measured.
The basic problem is that the small voltage to be measured
is 500V above the system ground (i.e., a system common-
mode voltage of 500V exists). The circuit converts this
system CMV to an amplifier isolation mode voltage. Thus,
the isolation voltage ratings and isolation-mode rejection
specifications apply.
ELECTROMAGNETIC RADIATION
The transformer coupling used in 3656 for isolation makes
the 3656 a source of electromagnetic radiation unless it is
properly shielded. Physical separation between the 3656 and
sensitive components may not give sufficient attenuation by
itself. In these applications, the use of an electromagnetic
shield is a must. A shield, Burr-Brown 100MS, is specially
designed for use with the 3656 package. Note that the offset
voltage appearing at pin 15 may change by 4mV to 12mV
with use of the shield; however, this can be trimmed (see
Offset Trimming section).
IMPROVED INPUT CHARACTERISTICS
In situations where it is desired to have better DC input
amplifier characteristics than the 3656 normally provides, it
is possible to add a precision operational amplifier as shown
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN
assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject
to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not
authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.
®
15
3656
BB [ BURR-BROWN CORPORATION ]
