OP297FS datasheet(11/16 Pages) AD

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8/21/97 4:00 PM

OP297

–11–

REV. D

In these circuits, IREF is a function of the negative power supply.

To maintain accuracy, the negative supply should be well regu-

lated. For applications where very high accuracy is required, a

voltage reference may be used to set IREF. An important consid-

eration for the squaring circuit is that a sufficiently large input

voltage can force the output beyond the operating range of the

output op amp. Resistor R4 can be changed to scale IREF, or R1,

and R2 can be varied to keep the output voltage within the

usable range.

Unadjusted accuracy of the square-root circuit is better than

0.1% over an input voltage range of 100 mV to 10 V. For a

similar input voltage range, the accuracy of the squaring circuit

is better than 0.5%.

Figure 35. Square-Root Amplifier

VIN

33k

Ω

1/2

OP-297

V–

V+

100pF

33k

Ω

IIN

IREF

VOUT

1/2

OP-297

MAT-04E

Q1 1

Ω

50k

Ω

50k

Ω

–15V

–

OP297 SPICE MACRO-MODEL

Figures 36 and 37 show the node end net list for a SPICE

macro model of the OP297. The model is a simplified version of

the actual device and simulates important dc parameters such as

VOS, IOS, IB, AVO, CMR, VO and ISY. AC parameters such as

slew rate, gain and phase response and CMR change with fre-

quency are also simulated by the model.

The model uses typical parameters for the OP297. The poles

and zeros in the model were determined from the actual open

and closed-loop gain and phase response of the OP297. In this

way, the model presents an accurate ac representation of the

actual device. The model assumes an ambient temperature

of 25

°C.

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