8/21/97 4:00 PM
OP297
–11–REV. D
In these circuits, I
REF
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 I
REF
. 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 I
REF
, 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
V
IN
33kΩ
1/2
OP-297
2
3
8
1
4
V–
V+
C
1
100pF
C
2
100pF
R
2
33kΩ
R
1
I
O
I
IN
I
REF
V
OUT
1/2
OP-297
6
5
7
MAT-04E
Q
1
1
3
2
Q
4
14
12
13
Q
2
7
5
6
Q
3
8
10
9
2kΩ
R
5
50kΩ
R
3
50kΩ
R
4
–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
V
OS
, I
OS
, I
B
, A
VO
, CMR, V
O
and I
SY
. 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.
