Re: A-D front end - robust and for high voltage

From: Frank Miles (fpm_at_u.washington.edu)
Date: 02/02/05 

Date: Wed, 2 Feb 2005 17:58:52 +0000 (UTC)

In article <pan.2005.02.02.03.12.41.766557@example.net>,
Rich Grise <richgrise@example.net> wrote:
>On Mon, 31 Jan 2005 10:33:17 -0800, John Larkin wrote:
>
>> On 27 Jan 2005 09:51:28 -0800, gorilla_nerfball@hotmail.com (Gorilla
>> Nerfball) wrote:
>>
>>>Can anyone shed some light on how multimeters, scopes and other
>>>similar devices can measure anything from microvolts to 300V or more
>>>without any moving parts (relays, etc.). Specifically, how do I design
>>>the front end for an A-D capable of measuring up to 100V without
>>>sacrificing performance at lower signal levels too much?
>>
>>
>> DVMs have big manually-operated rotary switches, as do analog scopes.
>> Most digital scopes do in fact have relays; you can hear them click as
>> you change vertical ranges.
>
>I've heard relays click as an autoranging DVM autoranged.
>
>But about scopes and knobs, if you have your scope set at 1 mV/div, and
>accidentally hit the mains with it, what do they use for proteciton there?
>Just a high resistance, and a couple of diodes?
>
>In the old days, they used the fact that if the grid didn't arc over, the
>tube would be OK. ;-)

In at least the not-quite-so-old-days, when useful schematics were still
part of a decent 'scope manual, input current was limited by a parallel R-C
between attenuator and input JFet. The JFet input voltage was limited by
a low-leakage low-capacitance diode (or two).

This is why the maximum allowed input voltage declines as frequency increases.

Not 100% sure how the latest 'scopes do it, but (perhaps due to insufficient
imagination) it should still be the same now, this is so simple.

        -frank 

--

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