COMMON-MODE AND
ISOLATION-MODE REJECTION
APPLICATIONS
Figure 8 shows a system where isolation amplifiers (3650)
are used to measure the armature current and the armature
voltage of a motor.
The expression for the output error due to common-mode
and isolation mode voltage is:
VCM
VISO
The armature current of the motor is converted to a voltage
by the calibrated shunt RS and then amplifier (adjustable
gain) and isolated by the 3650.
VOUT = G
+
CMRR
IMRR
The armature voltage is sensed by the voltage divider (ad-
justable) shown and then amplified and isolated by the 3650.
GUARDING AND PROTECTION
To preserve the excellent inherent isolation characteristics of
these amplifiers, the following recommended practice should
be noted.
The 3650 provides the advantage of accurate current mea-
surement in the presence of high common-mode voltage.
Both 3650s provide the advantage of isolating the motor
ground from the control system ground. Isolated power is
provided by an isolated DC/DC converter (BB Model 722 or
equivalent).
1. Use shielded twisted pair of cable at the input as with
any instrumentation amplifier.
2. Care should be taken to minimize external capacitance.
A symmetrical layout of external components to achieve
balanced capacitance from the input terminals to output
common will preserve high IMR.
The 3652 is ideally suited for patient monitoring applica-
tions as shown in Figure 9. The fact that it is a true balanced
input instrumentation amplifier with very high differential
and common-mode impedance means that it can greatly
reduce the common-mode noise pick up due to imbalance in
lead impedances that often appear in patient monitoring
situations. The 3kV and 6kV shown in Figure 9 are the 10ms
pulse ratings of the +IR and –IR inputs for the common-mode
and differential input voltages with respect to input com-
mon. The rating of the isolation barrier is 2000Vpk continu-
3. External components and conductor patterns should be
at a distance equal to or greater than the distance
between the input and output terminals to prevent HV
breakdown.
4. Though not an absolute requirement, the use of lamin-
ated or conformally coated printed circuit boards is
recommended.
G = 1V/V
1MΩ
4.99kΩ
11
+
9.76kΩ
23
17
3650HG
To
500Ω
4.99kΩ
VA/100
Voltage
Sense
20
O/P Com
VA
(500V)
10
26
–
–VCC
13
–V
+VCC
14
12
+V
+VO
V–
E
O/P Com
I/O Com
C
V+
722
1.3kΩ
–VO
P+
VOUT
+V
+15VDC
–15VDC
–V
12
14
4.75kΩ
13
11
+VCC
26
+
–VCC
20
500Ω
23
VS
(100mV)
3650JG
RS
To
100VS
Current
Sense
17
O/P Com
4.99kΩ
10
–
G = 100V/V
FIGURE 8. Isolated Armature Current and Voltage Sensor.
®
10
3650/52
BB [ BURR-BROWN CORPORATION ]
