Datasheet OP97 (Analog Devices) - 14

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OP97 GUARDING AND SHIELDING
To maintain the extremely high input impedances of the OP97, The OP97 is an excellent choice as an output amplifier for care must be taken in circuit board layout and manufacturing. higher resolution CMOS DACs. Its tightly trimmed offset Board surfaces must be kept scrupulously clean and free of voltage and minimal bias current result in virtually no moisture. Conformal coating is recommended to provide a degradation of linearity, even over wide temperature ranges. humidity barrier. Even a clean PCB can have 100 pA of leakage Figure 36 shows a versatile monitor circuit that can typically currents between adjacent traces; therefore, use guard rings sense current at any point between the ±15 V supplies. This around the inputs. Guard traces are operated at a voltage close makes it ideal for sensing current in applications such as full to that on the inputs, so that leakage currents are minimal. In bridge drivers where bidirectional current is associated with noninverting applications, connect the guard ring to the common- large common-mode voltage changes. The 114 dB CMRR of the mode voltage at the inverting input (Pin 2). In inverting appli- OP97 makes the contribution of the amplifier to common- cations, both inputs remain at ground, so that the guard trace mode error negligible, leaving only the error due to the resistor should be grounded. Make guard traces on both sides of the ratio inequality. Ideally, R2/R4 = R3/R5. circuit board. High impedance circuitry is extremely susceptible to RF pickup,
R1 IL 10kΩ
line frequency hum, and radiated noise from switching power
V1 R5
supplies. Enclosing sensitive analog sections within grounded
R 10kΩ L R2 R3
shields is generally necessary to prevent excessive noise pickup.
10kΩ 10kΩ +15V
Twisted-pair cable aid in rejection of line frequency hum.
2 7 6 V OP97 OUT 3 4 30pF
36
R4
0
RFB
9-
10kΩ I –15V O 2
029 0
6 V
Figure 36. Current Monitor
AD7548 OP97 OUT 3 IO
5 03 9-
DIGITAL
29
INPUTS
00 Figure 35. DAC Output Amplifier
UNITY-GAIN FOLLOWER NONINVERTING AMPLIFIER 2 2 6 6 OP97 OP97 3 3 PDIP INVERTING AMPLIFIER BOTTOM VIEW 8 1 2 6 OP97 3
37 0 9- 29 00 Figure 37. Guard Ring Layout and Connections Rev. G | Page 13 of 16 Document Outline FEATURES PIN CONNECTIONS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATION INFORMATION AC PERFORMANCE GUARDING AND SHIELDING OUTLINE DIMENSIONS ORDERING GUIDE