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From: "Bill sloman"
Subject: Re: R & C tempco repeatability
Date: Fri, 13 Sep 2002 00:34:28 +0200
Organization: Planet Internet
References: <email@example.com> <firstname.lastname@example.org>
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"fred bartoli" wrote in message
> Bill sloman a écrit dans le message :
> > "fred bartoli" wrote in message
> > news:email@example.com...
> > > Still for my 100kHz IF amplifier for which I still want really stable
> > > phase deviation.
> As I don't want to go the high price way (glass caps...) and also don't
> to control the temperature (to reach the target I'll need a 0.1°C control)
> it seems the best way is a time multiplexed calibration. (maybe 1 point
> of 100 or 200)
> I did rejected this at first because I was afraid of cal signal leakages :
> the input refered noise floor is 0.15uV (6kHz BW), but sync demodulation
> will easily lower this.
> Now considering all the implications it seems I have no other choice and
> after some thinking it may not be as difficult as I first thought.
> This will also rise the point of the demodulators clocks leakages : can I
> put the IF amp and the demodulator on the same board ? A (huge :-) ) 100nv
> leakage on a 100R impedance from a 5V source at 100kHz is a 0.3fF
> !!! All that not considering ground plane pollution.
> And maybe I should want 10nV or 1nV to exploit all the possible dynamic
> range of the detector.
> Do you have some experience about this (demodulator and IF on the same
> board) ?
No. The closest I've got was variable gain amplifier with a gain of up to
10,000 at up to about 17MHz, where the DC operating point was stabilised by
slow negative feedback through a second variable gain path whose gain went
down as the signal gain went up.
The 80dB of gain was from the output of a a Burr-Brown OPA-655 sets up as a
transimpedance amplifier converting a the output current from a photo-diode
into a voltage, so it probably comes fairly close to your levels.
With that little board I did spend time working out possible feed-back paths
and blocking them - very nearly every amplifier chip on the system had a
ferrite bead from its power rails to its decoupling capacitors, and most of
the decoupling networks included a nice lossy tantalum chip capacitor to
kill any filter resonances. I didn't have to go to a second printed circuit
layout to kill any leakage paths (but I did have to add an one extra
capacitor across the TI LTC2201 CMOS op amp, which had about 15pF of input
capacitance which TI didn't specify on the data sheet).
The job is definitely do-able - super-hetrodyne radios have been doing
exactly this for many years. They did tend to have little metal boxes around
the local oscillator, but those were mostly mounted onto single-sides
printed circuit boards. If you lay out your circuit onto a four or six layer
printed circuit board, with the inner layers largely devoted to solid ground
and power planes, you can almost always skip the little metal shielding
boxes. You may need to put occasional slots in the buried planes to to keep
clock return currents from flowing under sensitive bits of the front end,
but there isn't any black magic involved.
Bill Sloman, Nijmegen
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