OP497 Datasheet, PDF (9/12 Pages) Analog Devices – Precision Picoampere Input Current Quad Operational Amplifier

+5V
2
REF43 ^{6 2.5 V VREF}
R
R
4
R
R + ⌬R
R_F
2
1/4
1
3 ^OP497
V_OUT
+5V
6
8
1/4
7
5 ^OP497
4
V_OUT = V_REF (
⌬R
R + ⌬R
)
R_F
R
–5V
Figure 9. A Simple Bridge Conditioning Amplifier Using
the OP497
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 Q₁, Q₂, Q₃, and Q₄.

V_{T 1}In _
I _IN
I_S1



+V_{T 2}In_
I _IN
I_{S 2}



=V_{T 3}In_ I
I_O
I_{S 3}



+V_{T 4}In_
I_REF
I_{S 4}


All the transistors of the MAT04 are precisely matched and at
the same temperature, so the I_S and V_T terms cancel, giving:
( ) ^2InI_IN ^{= InI}_O ^{+ InI}_REF ^{= In I}_O ^{× I}_REF
Exponentiating both sides of thick equation leads to:
( )²
I_O =
I _IN
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


VR_IN1
²

OP497
C2
100pF
R1
133k⍀ ^IIN
V_IN
1
2 _Q1
3
7
6
Q2
R2
6
33k⍀
1/4
7
I O ₅ OP497
V_OUT
V+ C1
100pF
2
8
1/4
1
OP497
3
4
5
MAT-04E 14
8
9
I_REF Q4 ¹³
Q3
12
10
R3
50k⍀
R4
50k⍀
V–
–15V
Figure 10. Squaring Amplifier
A similar analysis made for the square-root circuit of Figure 11
leads to its transfer function:
( )( ) _{VOUT = R2}
V_IN I_REF
R1
In these circuits, I_REF is a function of the negative power sup-
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_REF. An important con-
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_REF,
or Rl and R2 can be varied to keep the output voltage within
the usable range.
R2
33k⍀
C2
6
100pF
1/4
7
I_{O 5 OP497}
V_OUT
R1
33k⍀
V_IN
I_IN
C1
100pF
V+
2
8
1/4 1
3 ^OP497
4
Q1 1 _MAT-04E
2
I_REF
3
7
6 _Q2
14
13
8
Q4
12
Q3 ⁹
5 10
R5
2k⍀
R3
50k⍀
R4
50k⍀
V–
–15V
Figure 11. Square-Root Amplifier
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%.
REV. D
–9–