Re: 10MHz peak measurement

From: Mark Jones (abuse_at_127.0.0.1)
Date: 12/07/04 

Date: Tue, 07 Dec 2004 00:13:43 -0500

jwodin@gmail.com wrote:
> I have the simple problem of trying build a circuit that turns a
> ~8-15Mhz, ~100mV-2V sine wave into a DC level (for read out by an ADC).
> I've tried just rectifying and putting an RC to hold the DC level, but
> I don't seem to get a flat response over the specified frequency
> range. The sine wave will change amplitude over seconds, but no faster
> than that. Any ideas, or am I just not good enough at making
> rectifiers with caps (which is very possible).
>
> Jesse
>

  Hi Jesse. What rectifier are you using? At these frequencies, the
diode switching speed may be an issue. And what value and type of cap?
Some caps are good for high frequencies and some aren't. Are you using
a resistor in series to the cap? If not, the instantaneous current is
probably limiting the effective "smoothing." It sounds like an RC
filter might be the key. Or even better an op-amp precision rectifier,
do a search for "TI Op Amps for Everybody" and you should find an
excellent TI website and resource. This would probably work better but
be more complex.

  Perhaps you could try a rectifier and 4th-order RC filter network,
like this:

  AC in D1 100k 100k 100k 100k DC out
  <-------|>|---\/\/--+--\/\/--+--\/\/--+--\/\/--+----->
         FAST |+ |+ |+ |+ (buffer with a
                     --- --- --- --- unity-gain op amp)
                     --- --- --- ---
                      | 10nF | 10nF | 10nF | 10nF
                      | | | |
                    ----- ----- ----- -----
                     --- --- --- ---
                      - - - -

  You could feed this a 1kHz 50% duty cycle squarewave signal and get
a nice, smooth DC voltage out. Exact values and methodology left to
experimentation.

-M

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