OP297_03 Datasheet, PDF (7/12 Pages) Analog Devices – Dual Low Bias Current Precision Operational Amplifier

OP297
UNITY-GAIN FOLLOWER
1/2
OP297
INVERTING AMPLIFIER
1/2
OP297
NONINVERTING AMPLIFIER
1/2
OP297
MINI-DIP
BOTTOM VIEW
8
1
A
B
APPLICATIONS
PRECISION ABSOLUTE VALUE AMPLIFIER
The circuit of Figure 9 is a precision absolute value amplifier with
an input impedance of 30 MΩ. The high gain and low TCV_OS
of the OP297 ensure 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
0.1␮F
R1
1k⍀
R3
1k⍀
C1
30pF
2
8
1/2
1
OP297
V_IN
3
C3
0.1␮F
4
D1
1N4148
D2
1N4148
5
1/2
7
OP297
6
0V V_OUT 10V
R2
2k⍀
Figure 7. Guard Ring Layout and Connections
GUARDING AND SHIELDING
To maintain the extremely high input impedances of the OP297,
care must be taken in circuit board layout and manufacturing.
Board surfaces must be kept scrupulously clean and free of mois-
ture. Conformal coating is recommended to provide 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 voltage close to
that on the inputs, as shown in Figure 7, so that leakage currents
become minimal. In noninverting applications, the guard ring
should be connected to the common-mode voltage 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 precision
of the OP297 and provides for very high accuracy in high closed
loop gain applications. Figure 8 illustrates the typical open-loop
gain linearity of the OP297 over the military temperature range.
R_L = 10k⍀
V_S = ؎15V
V_CM = 0V
T_A = +125؇C
–15V
Figure 9. Precision Absolute Value Amplifier
PRECISION CURRENT PUMP
Maximum output current of the precision current pump shown
in Figure 10 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.
R3
10k⍀
R1
10k⍀
V_IN
R2
10k⍀
2
1/2
OP297
3
1
+15V
R6
10k⍀
R4
85
10k⍀ 7 _1/2
OP297
6
I_OUT
10mA
I_OUT =
V_IN
R5
= ^VIN = 10mA/V
100⍀
–15V
Figure 10. Precision Current Pump
T_A = +25؇C
0
T_A = –55؇C
–15 –10 –5 0
5
10 15
OUTPUT VOLTAGE (V)
Figure 8. Open-Loop Linearity of the OP297
REV. E
–7–