廬
APPLICATION BULLETIN
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
PRECISION IA SWINGS RAIL-TO-RAIL ON SINGLE 5V SUPPLY
By R. Mark Stitt (602) 746-7445
You can combine a precision instrumentation amplifier (IA)
and an inexpensive CMOS op amp to get the best of two
worlds鈥攖he precision of a bipolar IA and 5V single-supply
operation. Using the INA131 gives 50碌V max voltage
offset, 0.25碌V/擄C max offset drift, and 110dB min common-
mode rejection. How close the output swings to the power
supply rail depends on which 鈥渞ail-to-rail鈥?CMOS op amp
you use. With a TI TL2272 you can expect the output to
swing within 100mV of the rails.
The circuit is shown in Figure 1. INA131 is fixed Gain = 100
IA specified to operate on power supplies as low as
鹵2.25V.
Its output can swing
鹵1.25V
on
鹵2.5V
supplies. Adding a
gain-of-2 鈥渞ail-to-rail鈥?CMOS op amp, A
4
, inside the feed-
back loop boosts output swing to
鹵2.5V
(0 to 5V on a single
5V supply). Because the CMOS op amp is inside the
feedback loop, its errors are divided-down by the loop gain
of A
3
in the INA131 so they contribute negligible error to the
composite amplifier.
The gain of the CMOS amplifier is set by R
1
, R
2
, and R
3
.
Gain
=
1
+
R
1
鈥?R
2
+
R
2
鈥?R
3
R
1
鈥?R
3
With the values shown, Gain
鈮?/div>
2V/V. Remember, since A
4
is in the INA131 feedback loop, its exact gain is unimpor-
tant. The INA131 will still have a precise gain of 100V/V
鹵0.024%.
Using the R
1
, R
3
divider to set the gain forces the INA131
output swing to be centered midway between the +5V
supply and ground for rail-to-rail output swing.
Compensation capacitor, C
1
, provides high-frequency feed-
back around A
4
to assure loop stability. It is important to
choose A
4
with at least 2MHz small-signal bandwidth for
good loop stability.
+5V
+V
S
INA131
鈥揑n
Over-Voltage
Protection
R
1
100k鈩?/div>
R
2
47k鈩?/div>
5k鈩?/div>
A
1
25k鈩?/div>
25k鈩?/div>
Bridge
Sensor
R
9
C
1
100pF
A
4
V
OUT
2.63k鈩?/div>
25k鈩?/div>
A
3
V
O
External Gain-Set
Resistor Optional
Over-Voltage
Protection
A
2
5k鈩?/div>
25k鈩?/div>
R
3
100k鈩?/div>
REF
+In
鈥揤
S
FIGURE 1. Adding a 鈥淩ail-to-Rail鈥?CMOS Op Amp in its Feedback Loop Allows the INA131 Output to Swing Rail-to-Rail
on a Single 5V Supply.
漏
1993 Burr-Brown Corporation
AB-056A
Printed in U.S.A. July, 1993
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描述
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