op497fy Analog Devices, Inc., op497fy Datasheet - Page 9

• Manufacturer Part Number: op497fy
• Description: Precision Picoampere Input Current Quad Operational Amplifier
• Manufacturer: Analog Devices, Inc.

NONLINEAR CIRCUITS
Due to its low input bias currents, the OP497 is an ideal log
amplifier in nonlinear circuits such as the square and square
root circuits shown in Figures 10 and 11. Using the squaring
circuit of Figure 10 as an example, the analysis begins by writing a
voltage-loop equation across transistors Q
All the transistors of the MAT04 are precisely matched and at
the same temperature, so the I
Exponentiating both sides of thick equation leads to:
Op amp A
V
equation for I
V In
OUT
T
1
= R2 × I



I
REF43 6
I
2InI
IN
2
+5V
S
1
forms a current-to-voltage converter which gives
2
4

 +

O
IN
O
, yields:
. Substituting (V
V
2.5 V
T
=
V
R
R
2
6
5
In
InI
OUT
OP497
V
REF



1/4
–5V
O
+5V
I
I
8
IN
S
I
4
=
+
O
R + R
2
R




 =

InI
=
I
7
S
R2
REF
( )
V In I
and V
REF
I
I
IN
REF
T
IN
/R1) for I
V
3



OUT
2
3



=
2
V
OP497
T



R1
= V
In I
1/4
IN
terms cancel, giving:
R
1
REF
I
F
I
(
, Q
S
O



3
O
IN
2
(

 +

2
R + R
1
, Q
×
and the above
R
V
I
3
REF
T
, and Q
4
)
V
In
OUT
R
R
)
F



I
I
REF
4
S
.
4



A similar analysis made for the square-root circuit of Figure 11
leads to its transfer function:
In these circuits, I
ply. To maintain accuracy, the negative supply should be well
regulated. For applications where very high accuracy is required,
a voltage reference may be used to set I
sideration 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
or Rl 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%.
V
V
IN
IN
133k
33k
R1
R1
I
IN
100pF
C1
V
REF
2
3
2
3
OUT
OP497
OP497
1/4
is a function of the negative power sup-
V+
V–
1/4
2
I
V+
V–
IN
8
4
Q1
4
8
=
1
3
R2
100pF
1
C1
1
6
Q2
( )(
2k
V
R5
7
5
IN
10
2
6
Q1
8
Q3
Q2
R1
I
1
O
3
7
5
REF
I
I
9
50k
O
100pF
REF
R3
10
6
5
MAT-04E
C2
MAT-04E
33k
8
6
5
Q3
. An important con-
OP497
R2
–15V
OP497
50k
1/4
)
R3
9
1/4
13
33k
R4
50k
100pF
R2
I
REF
–15V
C2
Q4
OP497
14
12
14
12
R4
50k
7
Q4
7
I
REF
13
V
OUT
V
OUT
REF
,