OP297 Datasheet, PDF (8/16 Pages) Analog Devices – Dual Low Bias Current Precision Operational Amplifier

8/21/97 4:00 PM
OP297
UNITY-GAIN FOLLOWER
–
1/2
OP-297
+
NONINVERTING AMPLIFIER
–
1/2
OP-297
+
INVERTING AMPLIFIER
–
1/2
OP-297
+
MINI-DIP
BOTTOM VIEW
8
1
A
B
Figure 28. Guard Ring Layout and Connections
a humidity barrier. Even a clean PC board can have 100 pA of
leakage currents between adjacent traces, so guard rings should
be used around the inputs. Guard traces are operated at a volt-
age close to that on the inputs, as shown in Figure 28, so that
leakage currents become minimal. In noninverting applications,
the guard ring should be connected to the common-mode volt-
age at the inverting input. In inverting applications, both inputs
remain at ground, so the guard trace should be grounded. Guard
traces should be on both sides of the circuit board.
OPEN-LOOP GAIN LINEARITY
The OP297 has both an extremely high gain of 2000 V/mV
minimum and constant gain linearity. This enhances the preci-
sion of the OP297 and provides for very high accuracy in high
closed loop gain applications. Figure 29 illustrates the typical
open-loop gain linearity of the OP297 over the military tempera-
ture range.
R_L = 10kΩ
V_S = ±15V
V_CM = 0V
T_A = +125°C
T_A = +25°C
T_A = –55°C
–15 –10 –5 0
5
10 15
OUTPUT VOLTAGE (VOLTS)
Figure 29. Open-Loop Linearity of the OP297
APPLICATIONS
PRECISION ABSOLUTE VALUE AMPLIFIER
The circuit of Figure 30 is a precision absolute value amplifier
with an input impedance of 30 MΩ. The high gain and low
TCV_OS of the OP297 insure accurate operation with microvolt
input signals. In this circuit, the input always appears as a
common-mode signal to the op amps. The CMR of the OP297
exceeds 120 dB, yielding an error of less than 2 ppm.
+15V
C₂
0.1µF
R₁
R₃
1kΩ
1kΩ
2_– ⁸
1/2
1
OP-297
C₁
30pF
D₁
1N4148
D₂
6_–
1/2
OP-297
5₊
7
0V < V_OUT < 10V
V_IN
3₊
4 C3
1N4148
R₂
2kΩ
0.1µF
–15V
Figure 30. Precision Absolute Value Amplifier
PRECISION CURRENT PUMP
Maximum output current of the precision current pump shown
in Figure 31 is ± 10 mA. Voltage compliance is ± 10 V with
± 15 V supplies. Output impedance of the current transmitter
exceeds 3 MΩ with linearity better than 16 bits.
–8–
REV. D