Voltage Analog Input - Open/Short Circuit Behaviour

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Ed C's Avatar (by Gravatar)Ed C posted on
March 2, 2006, 3:00 pm
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Hi all,

Quick question about voltage analog inputs:

I'm using a 1746-NI8 Analog 8 Channel Input card for a SLC processor,
configured for 1-5V input corresponding to 3277-16384 proportional
value inputs.  I'm curious as to what value I would read at the card in
the event of either a short circuit or open circuit at the input
terminal.

I'm thinking that for a short circuit, I'd read something like 0 Volts
and hence 0 value.  But what would I read for a open circuit?  I would
test this out, but I don't have an input card at the moment.

Thanks in advance.
Ed


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Ursa Major's Avatar (by Gravatar)Ursa Major posted on
March 2, 2006, 3:52 pm
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Re: Voltage Analog Input - Open/Short Circuit Behaviour





The NI8 is capable of detecting a broken input. You choose where you want
the value to go when you config the card. Each channel is configured
independently. Choices are zero, upscale, or downscale.



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Jerry Avins's Avatar (by Gravatar)Jerry Avins posted on
March 2, 2006, 6:37 pm
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Re: Voltage Analog Input - Open/Short Circuit Behaviour



Ed C wrote:

National instruments has a web forum. Lacking access to that, I would
expect to read a noise 0V on an open.

Jerry
--
Engineering is the art of making what you want from things you can get.
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Scott Seidman's Avatar (by Gravatar)Scott Seidman posted on
March 3, 2006, 8:46 am
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Re: Voltage Analog Input - Open/Short Circuit Behaviour



OdnRxVT5ZWHJrZnZ2dnUVZ_tKdnZ2d@rcn.net:


A short is predictable, but an open is far from predictable.  Think about
how a sample and hold works on an individual card.  If there's no
discharge path, you might even get an artifactual copy of what was
sampled on the previous (valid) channel.

Generally, you should short analog in channels that you're not using.

--
Scott
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Jerry Avins's Avatar (by Gravatar)Jerry Avins posted on
March 3, 2006, 9:37 am
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Re: Voltage Analog Input - Open/Short Circuit Behaviour



Scott Seidman wrote:

A short is not always a good idea. A resistor to ground may be better.
The resistor can go to either rail as well, so +/- MAX is also possible.
These connections can sometimes be made internally.

Jerry
--
Engineering is the art of making what you want from things you can get.
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BFoelsch's Avatar (by Gravatar)BFoelsch posted on
March 5, 2006, 10:33 am
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Re: Voltage Analog Input - Open/Short Circuit Behaviour





Bits 6 & 7 in the channel configuration word specify the open circuit
behavior, the input value can be set to go upscale, downscale or to zero.

Bit 12 in the input status word goes high when an open input is detected on
that channel.

The module has no way to detect a shorted input.



Ed C's Avatar (by Gravatar)Ed C posted on
March 7, 2006, 9:32 am
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Re: Voltage Analog Input - Open/Short Circuit Behaviour



Thank you all for the input.  You have been most helpful.
Cheers.
Ed


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