OP297
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 consider-
ation 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%.
OP297 SPICE MACRO MODEL
Figures 14 and 15 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 frequency
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.
99
V2
R3
C2
5
6
12
15
–IN
2
R
IN2
8
R1
C
IN
+IN
1
R
IN1
7
I
OS
3
R2
E
OS
D1
D2
Q1
10
R5
4
Q2
11
R6
98
D4
14
I1
50
C6
R11
C7
R13
V3
E
REF
G1
R7
C3
R4
13
D3
R8
16
R9
C4
E1
9
G1
98
R10
C5
E2
R12
E3
R14
G3
R15
C8
9
99
D7
R16
I
SYS
22
23
D6 27
D5 26
D8
V4
G6
R18
V5
25
L1
R17
D9
50
28 29
G7
G4
G5
D10
R19
Figure 15. Macro Model
REV. E
–9–
AD [ ANALOG DEVICES ]
