Re: Protecting ADC Inputs
From: John Larkin (jjlarkin_at_highSNIPlandTHIStechPLEASEnology.com)
Date: 12/28/04
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Date: Mon, 27 Dec 2004 17:38:41 -0800
On Mon, 27 Dec 2004 18:31:07 -0600, "Richard" <rwskinner ATawesomenet
Dot net> wrote:
>I'm built a circuit board for my Analog card. My normal inputs will be 0-5
>vdc but the max allowed on the ADC is 10 vdc.
>I was thinking about putting some zeners across the input circuit so the
>voltage will never go above 10 volts. I left a space from each input to
>ground for a surface mount 1/2 watt resistor. These were there so we could
>do a 4-20ma conversion.
>
>So, First, will zeners work without affecting my normal input voltage?
>Second, are zeners available close to the 10 volt range?
>
>Any better simplier solutions?
>
>Thanks,
>Richard
>
>
Zeners are good. 6.8 volts, 1 watt maybe, should absorb some pretty
good zaps. A 6-volt or better zener will conduct tiny currents at 5
volts. A series polyfuse might be nice, too, in case somebody decides
to connect +48 volts to your input or something.
If your ADC input is high impedance, a series RC ahead of it is
prudent too. That gives additional zap/latchup protection and filters
noise, too. It's a generally good policy to avoid connecting any IC
pin directly to the outside world.
John
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