More info on the bipolar sinh^-1 compression that John Lahr
describes can be found in:

"Nonlinear Circuits Handbook", edited by D.H.Sheingold of the
Analog Devices engineering staff, 1976; ISBN:0-916550-01-X.

The sinh^-1 circuit using matched positive and negative input log
amplifiers is shown in figure 10. There is also an excellent error
vs signal-to-noise analysis table. In general, the conformance errors
result in a SN ratio significantly greater than a simple linear
ratio for small signals (+,-0.1volt), but it is rather poor for
large amplitudes (10V). This is clearly listed as an "ideal" 
response for matched 752N and 752P log converters, which are 
necessary for dealing with temperature errors.. (These seem to
have been replaced with the AD640 and AD641 devices, at $65 ea.)

Later chapters describe the circuits, limitations, and transfer
functions of the devices, which (then) cost $49. THey were too
DC power hungry, at +,- 15V at +,- 7ma (for one converter) , to be 
considered for remote battery operated seismic stations. And, as
John mentioned, there was the problem of interpreting the non-linear
data response in the analog recording (microfilm). The possibility of an
inverse function in software to restore the linear response after the 
FM telemetry and the A to D conversion  is an interesting consideration.
But whether it is a better solution than a 24-bit ADC and digital
telemetry is unknown.

Regards,
Sean-Thomas


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