OP297 Analog Devices, OP297 Datasheet - Page 12

• Manufacturer Part Number: OP297
• Description: Dual Low Bias Current Precision Operational Amplifier
• Manufacturer: Analog Devices

Specifications of OP297

• -3db Bandwidth: 500kHz
• Slew Rate: 150mV/µs
• Vos: 80µV
• Ib: 50pA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 17nV/rtHz
• Vcc-vee: 4V to 40V
• Isy Per Amplifier: 625µA
• Packages: DIP,SOIC

OP297
NONLINEAR CIRCUITS
Due to its low input bias currents, the OP297 is an ideal log
amplifier in nonlinear circuits such as the square and square
root circuits shown in Figure 35 and Figure 36. Using the
squaring circuit of Figure 35 as an example, the analysis begins
by writing a voltage loop equation across Transistor Q1,
Transistor Q2, Transistor Q3, and Transistor Q4.
All the transistors of the
the same temperature, so the I
Exponentiating both sides of the equation leads to
Op Amp A2 forms a current-to-voltage converter, which gives
V
equation for I
A similar analysis made for the square root circuit of Figure 36
leads to its transfer function
V
IN
OUT
2ln I
V
V
V
I
= R2 × I
33kΩ
R1
OUT
T1
OUT
OUT
IN
ln
=
⎛
⎜
⎜
⎝
= ln I
=
=
I
I
( )
⎛
⎜
⎜
⎝
R2
I
I
IN
OUT
S1
OUT
100pF
I
IN
REF
2
3
R2
C1
REF
OUT
⎞
⎟
⎟
⎠
2
. Substituting (V
–
OP297
+
yields
2
(
+
V
Q1
1/2
⎞
⎟
⎟
⎠
V
+ ln I
V+
V–
⎛
⎜
⎝
IN
1
3
Figure 35. Squaring Amplifier
T2
8
4
V
R1
R1
)(
IN
ln
I
REF
6
MAT04
REF
⎛
⎜
⎜
⎝
1
⎞
⎟
⎠
Q2
I
2
I
= ln( I
IN
S
)
7
5
2
S
⎞
⎟
⎟
⎠
and V
10
=
8
Q3
OUT
are precisely matched and at
IN
V
/R1) for I
I
OUT
T3
9
MAT04E
50kΩ
× I
R3
T
ln
100pF
6
5
terms cancel, where
REF
C2
⎛
⎜
⎜
⎝
–
OP297
+
–15V
I
)
I
1/2
OUT
S3
R4
50kΩ
IN
33kΩ
R2
I
REF
and the previous
⎞
⎟
⎟
⎠
+
14
12
7
V
Q4
T4
13
ln
V
⎛
⎜
⎜
⎝
OUT
I
I
REF
S4
Rev. G | Page 12 of 16
⎞
⎟
⎟
⎠
V
In these circuits, I
To maintain accuracy, the negative supply should be well regu-
lated. For applications where very high accuracy is required, a
voltage reference can be used to set I
An important consideration 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
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%.
IN
33kΩ
R1
100pF
2
3
C1
–
OP297
+
REF
REF
Figure 36. Square Root Amplifier
1/2
V+
V–
or R1; R2 can be varied to keep the output
is a function of the negative power supply.
8
4
1
Q1
6
Q2
1
3
7
5
I
OUT
REF
33kΩ
.
10
R2
8
6
5
Q3
MAT04E
–
OP297
+
1/2
100pF
9
50kΩ
C2
R3
13
–15V
7
Q4
14
12
R4
50kΩ
I
REF
V
OUT