The AD640 uses five cascaded limiting amplifiers to approximate
a logarithmic response to an input signal of wide dynamic
range and wide bandwidth. This type of logarithmic amplifier
has traditionally been assembled from several small scale ICs
and numerous external components. The performance of these
semidiscrete circuits is often unsatisfactory. In particular, the
logarithmic slope and intercept (see FUNDAMENTALS OF
LOGARITHMIC CONVERSION) are usually not very stable
in the presence of supply and temperature variations even after
laborious and expensive individual calibration. The AD640
employs high precision analog circuit techniques to ensure stability
of scaling over wide variations in supply voltage and temperature.
Laser trimming, using ac stimuli and operating
conditions similar to those encountered in practice, provides fully
calibrated logarithmic conversion.