op22002 Analog Devices, Inc., op22002 Datasheet - Page 9

• Manufacturer Part Number: op22002
• Description: Dual Micropower Operational Amplifier
• Manufacturer: Analog Devices, Inc.

In this instrumentation amplifier configuration, error due to
CMRR effect is directly proportional to the differential CMRR
of the op amps. For the OP220A/E, this combined CMRR is a
minimum of 98 dB. A combined CMRR value of 100 dB and
common-mode input range of ± 2.5 V indicates a peak input-
referred error of only ± 25 mV.
Resistor matching is the other factor affecting CMRR. Defining
Ad as the differential gain of the instrumentation amplifier and
assuming that R1, R2, R3 and R4 are approximately equal (R
will be the nominal value), then CMRR will be approximately
A
be 88 dB with resistor matching of 0.1%. Trimming R1 to make
the ratio R3/R4 equal to R2/R1 will directly raise the CMRR
until it is limited by linearity and resistor stability considerations.
The high open-loop gain of the OP220 is very important in
achieving high accuracy in the two-op-amp instrumentation
amplifier configuration. Gain error can be approximated by:
where A
A
equal values of R1, R2, R3, and R4. For example, consider an
OP220 with A
set to 700, the gain error would be 1/1.001 which is approxi-
mately 0.1%.
Another effect of finite op amp gain is undesired feedthrough of
common-mode input. Defining A
amp A1, then the common-mode error (CME) at the output
due to this effect will be approximately:
For AD/A01, < 1, this simplifies to (2 A
amp gain is 700 V/mV, V
the error at the output due to this effect will be approximately 5 mV.
The OP220 offers a unique combination of excellent dc perfor-
mance, wide input range, and low supply current drain that is
particularly attractive for instrumentation amplifier design.
REV. A
D
02
divided by 4DR/R
is the open-loop gain of op amp A2. This analysis assumes
D
is the instrumentation amplifier differential gain and
Gain Error =
02
of 700 V/mV. If the differential gain A
CME
N
. CMRR at differential gain of 100 would
CM
=
1
1
is 2.5 V, and A
2
+
+
A
1
A
A
A
A
D
01
01
D
02
D
as the open-loop gain of op
A
,
1
2
01
A A
D
V
A
01 02
/A
CM
D
01
D
)
is set to 700, then
<
1
V
CM
. If the op
D
were
N
–9–
THREE OP AMP CONFIGURATION
A three op amp instrumentation amplifier configuration using
the OP220 and OP777 is recommended for applications requiring
high accuracy over a wide gain range. This circuit provides
excellent CMR over a wide input range. As with the two op amp
instrumentation amplifier circuits, tight matching of the two op
amps provides a real boost in performance.
A simplified schematic is shown in Figure 2. The input stage
(A1 and A2) serves to amplify the differential input V
amplifying the common-mode voltage V
then rejects the common-mode input. With ideal op amps and
no resistor matching errors, the outputs of each amplifier will be:
The differential gain A
input V
This three op amp instrumentation amplifier configuration using an
OP220 at the input and an OP777 at the output provides excellent
performance over a wide gain range with very low power consump-
tion. A gain range of 1 to 2,000 is practical and CMR of over
120 dB is readily achievable.
Figure 5. Three Op Amp Instrumentation Amplifier Using
OP220 and OP777
V
V
CM
CM
– 1/2 V
+ 1/2 V
CM
is rejected.
D
D
V
D
–
+
R0
V
V
V
V
O
O
1
2
= -
=
=
=
1/2
OP220
1/2
OP220
V
D
Ê
Á
Ë
A V
1
Ê
Á
Ë
2
D D
is 1 + 2R1/RO and the common-mode
1
+
R1
R1
-
A1
A2
+
V
2 1
R
2 1
R
V+
V–
R
O
1
R
O
=
V1
V2
ˆ
˜
¯
ˆ
˜
¯
V
Ê
Á
Ë
V
1
2
D
2
R2
R2
D
+
+
V
2 1
+
R
O
V
R
V
= V
R2
CM
O
CM
CM
D
ˆ
˜
¯
. The output stage
V–
A3
V
1 +
D
R2
V+
OP777
2R1
R0
OP220
D
without
V
O