AD648

REV. C

–10–

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-

for voltage inputs.

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-

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,

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

µA 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

µA and ad-

µA and adjust the scale factor such that the output

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.

Figure 30. Precision Log Ratio Amplifier