Target response tester

[Skippy]

I recall the 'speed issue' with translinear log-amps is that their speed varies depending on what conditions you run them at. Low currents mean circuit 'resistances' are high, and hence stray/parasitic/whatever capacitances start to affect performance. In essence - if you had a 1k resistor with 5 pF across it, you've got a 30 MHz low-pass filter. Now make the resistor 10M, and it's a 3kHz LPF.
So you'd have to run the log-amp hard, to ensure you're not getting issues.

Unfortunately, the last time I used a log-amp (for a photodiode amp) it was a discrete design, in part due to some specific technical requirements, and partly due to the available IC's all being expensive, needing too many trimming pots, and usually in a big DIL package.

[green]

Thinking about updating my target response tester to include a log amplifier and other modifications to compare with the decay curves I've been doing. Tx rate (1pps), Tx shape(constant current .5A peak or constant rate 1A peak), Tx width(adjustable 50usec to 50msec). Preamp(gain 450),post amp(gain 9). log amplifier(after preamp).

Discussion: constant current peak 1/2 of constant rate peak for same average Tx current, the lower the constant current peak the shorter the ramp time to peak, preamp and post amplifier the same as I've been using, need to modify constant rate circuit to allow adjusting Tx width while keeping 1A peak, Tx rate 1pps for low average Tx current,

Questions: any reason to make Tx rate adjustable? any changes or additions?

Comments or thoughts appreciated, would be easier to include changes now than modify latter.

What I would like to get is a digital scope module to use with a Windows laptop and that has both linear and logarithmic Y and X axes. If only Y then that would be good, but an X Y would be great for soil magnetics too where I need a LogY LogX display. If anyone knows of one that does not cost the earth, please let me know.A statement Eric made in another thread.

The reason for wanting to add the log amplifier. Been wanting to add the Tx width adjustment for constant rate. Never used a log amplifier IC. Not sure what to look at in the data sheet. Been looking at log114 and MAX4207 log amplifiers. Anyone used either one or maybe a different one to look at? The log114 looks a little faster, has 7 instead of 5 decade capability. Think maybe 3 decades is all I'll need do to amplifier noise out. Don't know how fast it needs to be. Including some charts I made awhile back with a faster coil. Don't know if log114 or MAX4207 would be fast enough for the 5x5mm aluminum foil. Maybe someone familiar with log amplifiers could make a guess. Not eliminating present capability so if the log amplifier doesn't work with shorter TC targets it wouldn't matter.

[green]

Thinking about updating my target response tester to include a log amplifier and other modifications to compare with the decay curves I've been doing. Tx rate (1pps), Tx shape(constant current .5A peak or constant rate 1A peak), Tx width(adjustable 50usec to 50msec). Preamp(gain 450),post amp(gain 9). log amplifier(after preamp).

Discussion: constant current peak 1/2 of constant rate peak for same average Tx current, the lower the constant current peak the shorter the ramp time to peak, preamp and post amplifier the same as I've been using, need to modify constant rate circuit to allow adjusting Tx width while keeping 1A peak, Tx rate 1pps for low average Tx current,

Questions: any reason to make Tx rate adjustable? any changes or additions?

Comments or thoughts appreciated, would be easier to include changes now than modify latter.