OP297_03 Datasheet, PDF (8/12 Pages) Analog Devices – Dual Low Bias Current Precision Operational Amplifier

OP297
PRECISION POSITIVE PEAK DETECTOR
In Figure 11, the C_H must be of polystyrene, Teflon^®, or poly-
ethylene to minimize dielectric absorption and leakage. The
droop rate is determined by the size of C_H and the bias current
of the OP297.
All the transistors of the MAT04 are precisely matched and at
the same temperature, so the I_S and V_T terms cancel, giving
( ) 2 ln IIN = ln IO + ln IREF = ln IO × IREF
Exponentiating both sides of the equation leads to
2
8
1/2
V_IN
1k⍀
OP297
3
RESET
1k⍀
1N4148
+15V
0.1␮F
1
1k⍀
6
1/2
7
1k⍀
OP297
5
C_H
0.1␮F
2N930
–15V
V_OUT
Figure 11. Precision Positive Peak Detector
SIMPLE BRIDGE CONDITIONING AMPLIFIER
Figure 12 shows a simple bridge conditioning amplifier using the
OP297. The transfer function is
V_OUT
=
V_REF


R
∆R
+ ∆R


R_F
R
The REF43 provides an accurate and stable reference voltage for
the bridge. To maintain the highest circuit accuracy, R_F should
be 0.1% or better with a low temperature coefficient.
15V
REF43
4
V_REF
R + ⌬R
R_F
2
1/2
1
3 ^OP297
V_OUT
( ) _{IO =}
I_IN ²
I_REF
Op amp A2 forms a current-to-voltage converter, which gives
V_OUT = R2 × I_O. Substituting (V_IN/R1) for I_IN and the above
equation for I_O yields
V_OUT
=


R2
I_REF




V_IN
R1


2
A similar analysis made for the square-root circuit of Figure 14
leads to its transfer function
( )( ) _{VOUT = R2}
V_IN I_REF
R1
R1
33k⍀
V_IN
C2
100pF
R2
33k⍀
2
1
Q1
3
C1
100pF
V+
2
8
1/2
OP297
3
4
6
7
Q2
5
1
6
1/2
7
I_O
OP297
5
V_OUT
MAT04E
8
Q3 ⁹
10
14
I_{REF Q4} ¹³
12
R3
50k⍀
R4
50k⍀
6
8
1/2
7
OP297
5
4
⌬R
R_F
V_OUT = V_{REF R + ⌬R}
R
Figure 12. A Simple Bridge Conditioning Amplifier
Using the OP297
V–
–15V
Figure 13. Squaring Amplifier
R2
33k⍀
C2
100pF
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 Figures 13 and 14. Using the squaring
circuit of Figure 13 as an example, the analysis begins by writing a
voltage loop equation across transistors Q1, Q2, Q3, and Q4.
V_T1 ln


I_IN
I_S1


+ V_T2
ln


I_IN
I_S2


=
V_T
3
ln


I_O
I_S3


+V_{T 4} ln


I_REF
I_{S 4}


R1
33k⍀
V_IN
6
1/2
7
I_O
OP297
5
V_OUT
I_REF
C1
100pF
V+
Q1 ¹
3
7
6 _Q2
5
MAT04E
8
Q3 ⁹
10
13
14
Q4
12
2
8
1/2
1
OP297
3
4
R3
50k⍀
R4
50k⍀
V–
–15V
Figure 14. Square-Root Amplifier
–8–
REV. E