廬
APPLICATION BULLETIN
鹵200V
DIFFERENCE AMPLIFIER WITH
COMMON-MODE VOLTAGE MONITOR
By Art Gass and R. Mark Stitt (602) 746-7445
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
The INA117 is a monolithic difference amplifier with the
unique ability to accept up to
鹵200V
common-mode input
signals while operating on standard
鹵15V
power supplies.
Using on-chip high-voltage resistor dividers, the INA117
rejects common-mode signals up to
鹵200V
and translates a
0V to
鹵10V
differential input signal to a 0V to
鹵10V
ground-referenced output signal.
In some applications it is also necessary to monitor the
common-mode level of the input signal. A common-mode
level monitor can be implemented with the addition of an
external op amp or two. Even though standard signal level
op amps are used, the circuit remains protected for momen-
tary common-mode or differential overloads up to
鹵500V.
If precision is not required, the circuit shown in Figure 1 can
be used to monitor the common-mode voltage with a maxi-
mum error of about
鹵5V.
This implementation actually
monitors the common-mode level of the INA117 nonin-
verting input (pin 3). The circuit works by measuring the
current in reference pins 1 and 5, which are normally
connected to ground. Amplifier A
1
forces the reference pins
to a virtual ground through feedback resistors R
6
+ R
7
. The
R
1
V
2
2
380k鈩?/div>
normal operation of the INA117 is unaffected since its
reference pins are connected to virtual ground. Resistors R
3
and R
4
in the INA117 form a voltage divider so that the top
of R
4
is at V
3
/20. Feedback of the op amp in the INA117
forces the voltage of its inverting input to be equal to its
noninverting input so that the top of resistor R
5
is also at V
3
/
20. The common mode level of V
3
is therefore related to the
current flowing out of pins 1 and 5.
I
1 + 5
= (V
3
/20)/(R
4
|| R
5
)
If
R
6
+ R
7
= R
4
|| R
5
then
A
1 OUT
= 鈥揤
3
/20.
Where
I
1 + 5
= total current flowing out of INA117 pins 1 and 5 [A]
R
4
|| R
5
= parallel combination of R
4
and R
5
[鈩
R
4
|| R
5
= (R
4
鈥
5
)/(R
4
+R
5
), nominally 10.27k鈩?/div>
A
1 OUT
= A
1
output voltage [V]
The signal is scaled by 1/20 so the output of A
1
does not
exceed its maximum of
鹵10V
with common-mode inputs of
鹵200V.
If smaller maximum common-mode voltages are to
be monitored, the value of R
6
+ R
7
can be increased for more
gain.
R
2
380k鈩?/div>
R
3
V
3
3
380k鈩?/div>
6
V
OUT
= V
3
鈥?V
2
R
5
21.1k鈩?/div>
R
4
20k鈩?/div>
INA117
8
1
5
100pF
R
6
5k鈩?/div>
Although the resistor ratios in the INA117 are accurately
laser trimmed, the absolute resistor values can vary by as
much as
鹵25%.
For better accuracy, the circuit must be
calibrated. To calibrate the gain, short pins 2 and 3 of the
INA117 to ground, offset adjust A
1
for 0V at its output,
connect pins 2 and 3 to a known V
REF
(such as +10V or
+100V), and adjust R
7
for an A
1
output of 鈥揤
REF
/20.
By definition, the true common-mode input voltage of the
INA117 is (V
2
+ V
3
)/2. The actual common-mode voltage
can be monitored with the addition of a second op amp as
shown in Figure 2. The second op amp is connected to sum
the 鈥揤
3
/20 output of A
1
at a gain of 鈥?V/V with the V
3
鈥?V
2
output of the INA117 at a gain of 鈥?/40V/V to produce an
output of V
CM
/20.
R
7
10k鈩?/div>
A
1
OPA27
鈥揤
3
/20
FIGURE 1. INA117 with V
3
Common-Mode Voltage
Monitor.
漏
1990 Burr-Brown Corporation
AB-010A
Printed in U.S.A. September, 1990
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