Electronic – Logarithmic amplifier with high slew rate — mutually exclusive


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

Best Answer

Progressive-limiting Log amps might satisfy. Log conformance for many of these is +/- 0.5dB and accommodate a 60dB dynamic range.

Since you specify an input voltage source, most accept input voltage, and yield an output that is also voltage, in the ballpark of 20 mV/dB.
A big problem is output bandwidth. The fastest I see (AD8310) has rise time of 15 ns, delayed by 6 ns. However, bandwidth is not dependent on amplitude.

I take it that the occasional input pulse response above a somewhat noisy DC background is of most interest. A low-frequency offset compensation loop works in your favor here. To achieve a good output noise floor and good dynamic range, you might want to consider a high-pass filter at the input.

These log-converters respond equally to negative-going pulses as well as positive-going pulses. This non-linear response doesn't work in your favor - should your unipolar pulse overshoot, the negative-going overshoot is converted to a positive-going tail.