AD648 Analog Devices, AD648 Datasheet - Page 10

• Manufacturer Part Number: AD648
• Description: Dual Precision/ Low Power BiFET Op Amp
• Manufacturer: Analog Devices

AD648
LOG RATIO AMPLIFIER
Log ratio amplifiers are useful for a variety of signal condition-
ing applications, such as linearizing exponential transducer out-
puts and compressing analog signals having a wide dynamic
range. The AD648’s picoamp level input current and low input
offset voltage make it a good choice for the front-end amplifier
of the log ratio circuit shown in Figure 30. This circuit produces
an output voltage equal to the log base 10 of the ratio of the in-
put currents I
for voltage inputs.
Input currents I
a matched pair of logging transistors. Voltages at points A and B
are developed according to the following familiar diode equation:
In this equation, k is Boltzmann’s constant, T is absolute tem-
perature, q is an electron charge, and I
tion current of the logging transistors. The difference of these
two voltages is taken by the subtractor section and scaled by a
factor of approximately 16 by resistors R9, R10 and R8. Tem-
perature compensation is provided by resistors R8 and R15,
1
and I
1
and I
2
V
. Resistive inputs R1 and R2 are provided
BE
2
set the collector currents of Q1 and Q2,
= (kT/q) ln (I
C
ES
/I
ES
is the reverse satura-
)
Figure 30. Precision Log Ratio Amplifier
–10–
which have a positive 3500 ppm/ C temperature coefficient.
The transfer function for the output voltage is:
Frequency compensation is provided by R11, R12, C1, and C2.
Small signal bandwidth is approximately 300 kHz at input cur-
rents above 100 A and will proportionally decrease with lower
signal levels. D1, D2, R13, and R14 compensate for the effects
of the two logging transistors’ ohmic emitter resistance.
To trim this circuit, set the two input currents to 10 A and ad-
just V
set I
voltage is 1 V by trimming potentiometer R10. Offset adjust-
ment for A1 and A2 is provided to increase the accuracy of the
voltage inputs.
This circuit ensures a 1% log conformance error over an input
current range of 300 pA to l mA, with low level accuracy limited
by the AD648’s input current. The low level input voltage accu-
racy of this circuit is limited by the input offset voltage and drift
of the AD648.
2
to 1 A and adjust the scale factor such that the output
OUT
to zero by adjusting the potentiometer on A3. Then
V
OUT
= 1 V log
10
(I
2
/I
1
)
REV. C