AD648
LO G RATIO AMP LIFIER
which have a positive 3500 ppm/°C temperature coefficient.
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. T he 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. T his circuit produces
an output voltage equal to the log base 10 of the ratio of the in-
put currents I1 and I2. Resistive inputs R1 and R2 are provided
for voltage inputs.
T he transfer function for the output voltage is:
VOUT = 1 V log10 (I2/I1)
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.
T o trim this circuit, set the two input currents to 10 µA and ad-
just VOUT to zero by adjusting the potentiometer on A3. T hen
set I2 to 1 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.
Input currents I1 and I2 set the collector currents of Q1 and Q2,
a matched pair of logging transistors. Voltages at points A and B
are developed according to the following familiar diode equation:
VBE = (kT/q) ln (IC/IES
)
In this equation, k is Boltzmann’s constant, T is absolute tem-
perature, q is an electron charge, and IES is the reverse satura-
tion current of the logging transistors. T he 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. T em-
perature compensation is provided by resistors R8 and R15,
T his 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. T he 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 Am plifier
–10–
REV. C
ADI [ ADI ]
