Re: [LAD] Milivoltmeter - Was FOSS Ethernet Soundcard

From: Folderol <folderol@email-addr-hidden>
Date: Fri Dec 04 2009 - 21:41:56 EET

First an apology. There is an error in the drawing (I knew I should
have waited a couple of days before posting it).

The bias for the first OpAmp (pin 8) is shown going to +V. It should go
to Gnd. As shown it is in the low current - low bandwidtch mode, which
is fine for the second one of course.

On Fri, 4 Dec 2009
00:51:12 +0100 fons@email-addr-hidden wrote:

> On Thu, Dec 03, 2009 at 11:13:05PM +0000, Folderol wrote:
>
> > I use the old fashioned method of wiring resistors directly on
> > standard glass loaded wafer switches. Initial tests suggest the
> > bandwidth well exceeds 20kHz - as opposed to less than 1kHz for many
> > quite expensive commercial units.
>
> They are not so 'standard' today, and probably difficult
> to find.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0352250

Cheap modern equivalent, although a bit smaller.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0327585

Very expensive 'full size' switch. Unfortunately only available as BBM
- I prefer MBB for sensitive input stuff.

> Bandwidth will not be the problem. What you get is a filter
> that will boost HF. 1 pf = +3dB at 17.7 Khz on all but the
> 20 mV range. 10 pf means +3dB at 1.77 kHz, and rising.

I decided to do some more detailed investigations, because while I
totally accept your statements, the actual measured results were quite
different. I was getting an essentially flat response right the way up
to 99.9kHz, a little wobble between ranges but no more that +2dB, and
about +1dB at 40kHz.

Taking the cover off I noticed and immediate jump to about +3dB at
99(etc)kHz and taking the unit out of the box completely while being
careful to avoid moving anything resulted in a further increase to
+4 to 5dB, also becoming quite variable. Clearly stray capacitance
to the box shielding was affecting the response.

Because I was unable to get 9M 0.1% resistors I used three lower values
in series shaped as a 'C' across the tags, so I wondered if the
'spread' of effective resistor body was having an effect too. As a test
I temporarily replaced this with a standard 10M resistor. Response was
now +2 to 3dB at 40kHz and about +8dB at the top end.

While I don't know the whole story of what capacitances there are to
where, even the worst of these variants still gave a just about usable
performance, so I simply put everything back as it was :)

Incidentally total current consumption is a sniff over 1.3mA.

-- 
Will J Godfrey
http://www.musically.me.uk
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.
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Received on Sat Dec 5 00:15:02 2009

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