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Subject: Re: Bitscope schematics round 2
Date: Tue, 8 Oct 2002 08:21:48 +0000 (UTC)
NNTP-Posting-Date: Tue, 8 Oct 2002 08:21:48 +0000 (UTC)
User-Agent: tin/pre-1.4-19990805 ("Preacher Man") (UNIX) (Linux/2.4.19 (i686))
Ryan Gammon wrote:
> There are a couple of things about the bitscope (see
> http://www.bitscope.com/design) analog input buffer schematic at
> http://www.bitscope.com/design/hardware/analog/ that I do not understand.
> Q1: Notice how the dc-blocking capacitor is connected:
> I worry about the capacitor in (1) doing something weird with parasitics
> in the switch when it's in the dc position. If capacitors have thermal
> noise, I suppose that's a bit of a worry too.
It's largely shorted by the switch, any generated noise will be divided
by the potential divider of the capacitors ESR and the switch also.
> On the other hand, (2) disconnects the input from the output while
> switching, whereas there is always a connection through the capacitor in
> I know that when you use a relay, you're supposed to put a diode across
> it to prevent it from blowing back due to an inductance-induced voltage
> spike. Is the capacitor here performing a similar function?
No, though there is the (largely theoretical in this case I suspect)
of charge building up on the capacitor, and damaging something when
switched to AC coupling again.
> Q2: The 1 Mohm resistor in parallel with the jfet input gives the
> circuit its 1 Mohm input resistance. Leaving it out would give the
> circuit an even higher input resistance. What would be the disadvantage
> of leaving it out? I guess it acts somewhat like a pulldown resistor --
> without it, a disconnected input would float. The probes for this scope
> claim to be 10:1 10 Mohm/16 pF capacitance. Does the 10:1 attenuation
> come from a voltage divider between 10 Mohm's in the probe and 1 Mohm's
> in the scope?
Work out the capacitance of the cable.
It's not a really high frequency before the cable capacitance totally
> Why is this called a vertical amplifier?
On a 'conventional' scope, it's connected to the vertical deflection plates,
and makes the beam move up and down.
http://inquisitor.i.am/ | mailto:email@example.com | Ian Stirling.
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