Does anyone know if the basic construction of a log amp (with a diode or BJT providing feedback from output to input) is in conflict with any ability to reach high slew rates?
I've been looking (octopart, digikey, mouser) for a logarithmic amplifier chip with a slew rate of at least 350 V/uS, preferably higher (2 kv/uS would be good). I'm seeing plenty of op-amps with slew rates in the Kv/uS range, and even a few instrumentation amps with respectable slew rates, but log amps with high slew rates don't seem to exist — what I'm finding is log amp slew rates down around a few dozen V/uS; too slow by an order of magnitude.
Am I on the right track if, instead of looking for off-the-shelf log amps, I instead start with one of the high-slew op amps and use some sort of fast diode? Is that the limiting factor, or is there something else I'm missing?
The application is single-ended, multi-channel, DC most of the time with infrequent and random input pulses ranging from a few mV to a few volts, rise times of a few nS, output same but log scaled. The output gets fed into an A/D converter; one goal of the log scaling is to bring the low-amplitude pulses up into a more useful area of the ADC's range; another goal is to attenuate large pulses to prevent clipping.
(Starting some notes as possible answers to my own question…)
- Breaking out my Art of Electronics; starting to think that what I'm really looking for is a log converter; since the diode's nonlinear curve is mainly what I'm after, a fast diode plus a high-speed op amp does seem to make sense. I'm worried about the noise from the diode, but don't remember how to quantify that. Will study.
- There is another SE question that looks related to the idea of the fast diode:
How to construct high speed logarithmic amplifiers