OP220_15 Datasheet, PDF (8/12 Pages) Analog Devices – Dual Micropower Operational Amplifier

OP220
50mV
100
90
2␮s
R0
R1
R2
GAIN
ADJ
10
0%
20mV
V_CM – 1/2 V_{D –}
V_D
+
V_CM + 1/2 V_D
V1
A1
1/2
R3
OP220
R4
A2
V_O
1/2
OP220
INPUT
OP220
25k⍀
OUTPUT
100pF
E _{Figure 2. Small-Signal Transient Response}
2V
T 100
90
200␮s
E ₁₀
0%
L _5V
O ^INPUT
OP220
S _10k⍀
40k⍀
OUTPUT
R_L
25k⍀
C_L
100pF
Figure 3. Large-Signal Transient Response
B _{INSTRUMENTATION AMPLIFIER APPLICATIONS OF}
THE OP220
Two Op Amp Configuration
O ^{The excellent input characteristics of the OP220 make it ideal for}
V_O
=
R4
R3
ÈÎÍ_{1 +}
1
2
ÊËÁ
R2
R1
+
RR₄₃ˆ¯˜
+
R2 + R
R0
3
˘˙_VD
˚
+
R4
R3
ÊËÁ
R3
R4
-
R2
R1
ˆ¯˜
V_CM
If
R1 =
R2
=
R3
=
R4 , thenV_O
=
2^ÊËÁ1 +
R1
R0
ˆ¯˜
V_D
Figure 4. Two Op Amp Instrumentation Amplifier
Configuration
The input voltages are represented as a common-mode input
V_CM plus a differential input V_D. The ratio R3/R4 is made equal
to the ratio R2/R, to reject the common-mode input V_CM. The
differential signal V_D is then amplified according to:
V_O
=
R4
R3
Ê
ËÁ
1
+
R3
R4
+
R2 + R3ˆ
R_O ¯˜
R3
V_D , where _R4
=
R2
R1
Note that gain can be independently varied by adjusting R_O.
From considerations of dynamic range, resistor tempco match-
ing, and matching of amplifier response, it is generally best to
make RX, R2, R3, and R4 approximately equal. Designating
R1, R2, R3, and R4 as RN allows the output equation to be
further simplified:
V_O
= 2^ÊËÁ1 +
R_N
R_O
ˆ
¯˜
V_D , where R_N
= R1 = R2 = R3 = R4
Dynamic range is limited by A1 as well as A2; the output of A1 is:
V1
=
-ÊËÁ1 +
R_N
R_O
ˆ
¯˜
V_D
+
2 V_CM
If the instrumentation amplifier were designed for a gain of 10
and maximum V_D of ± 1 V, then R_N/R_O would need to be four
and V_O would be a maximum of ± 10 V. Amplifier A1 would
use in instrumentation amplifier configurations where low-level ^{have a maximum output of ± 5 V plus 2 V}CM^{, thus a limit of}
differential signals are to be amplified. The low-noise, low input ^{±10 V on the output of A1 would imply a limit of ±2.5 V on V}CM^.
offsets, low drift, and high gain combined with excellent CMRR
provide the characteristics needed for high-performance instru-
mentation amplifiers. In addition, the power supply current
drain is very low.
The circuit of Figure 4 is recommended for applications where
A nominal value of 100 kW for R_N is suitable for most applica-
tions. A range of 200 W to 25 kW for R_O will then provide a gain
range of 10 to 1,000. The current through R_O is V_D/R_O, so the
amplifiers must supply ± 10 mV/200 W when the gain is at the
maximum value of 1,000 and V_D is at ± 10 mV.
the common-mode input range is relatively low and differential
gain will be in the range of 10 to 1,000. This two op amp instrumen-
tation amplifier features independent adjustment of common-mode
rejection and differential gain. Input impedance is very high since
both inputs are applied to noninverting op amp inputs.
Rejecting common-mode inputs is most important in accurately
amplifying low-level differential signals. Two factors determine
the CMR of this instrumentation amplifier configuration (assuming
infinite gain):
1. CMRR of the op amps
2. Matching of the resistor network (R3/R4 = R2/R1)
–8–
REV. A