Protecting ADC Inputs

[Richard]

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




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[Bob Masta]

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?

My choice would be to use a series resistor with
reverse-biased clamp diodes to each rail. (This
is a popular method used for example on the
inputs to CMOS logic gates.) This assumes that
the input impedance of the ADC is high, and the
small RC with the effective diode capacitance is
compatible with your signals.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

 

[John Larkin]

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