4 Dec
2009
4 Dec
'09
3:41 p.m.
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(a)kokkinizita.net wrote:
On Thu, Dec 03, 2009 at 11:13:05PM +0000, Folderol
wrote:
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduc…
Cheap modern equivalent, although a bit smaller.
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduc…
Very expensive 'full size' switch. Unfortunately only available as BBM
- I prefer MBB for sensitive input stuff.
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. 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.