What is the real current rating of the PCI-6259 dio?

[Sean.C]

The spec of the PCI-6269 daq board suggests that the DIO should be able to generate around 20mA of current, but I am finding that if I feed it into a single DIO into a 1kohm lowpass RC filter (sample rate of 1MHz), the filter seems to be significantly loading down the signal.  This should only draw about 5mA.

Is the 20+mA a safety spec?  If so, what is the actual current source/sink capability of the DIO?

Thanks,

Sean

[Wallace_F]

Hi Sean,

The maximum current you can source/sink with your PCI-6259 is 24 mA. Can you please explain what you mean when you say that the filter seems to be significantly loading down the signal?  Are you seeing less than 5 Volts for DO high level? Are you using multiple lines in the port to source current to multiple external circuits?

Regards,

Wallace F.

[Sean.C]

Posting to be informative and give fair warning:

I meant to write up something more comprehensive, but life is busy...  😉

I am using the dio of an m-series daq card to generate additional "analog outputs" by generating delta-sigma bitstreams (followed by hardware filters and amplifiers).  I was having trouble because it seemed as though the signals were getting loaded down.  After investigating what was going on:

1) The daq dio was performing as specified.

2) The output of the filters were being loaded down by the inputs of the op-amps being used.

It turns out that the op-amps have diode-tied inputs (presumably as a protective measure?).  These were not described anywhere in the text of the spec sheet, but I found them in the transistor diagram.  In many op-amp circuits this would not be a problem, because the amplifier gain is used to drive the difference between the inputs to very small levels, but in my case, I was using them as integrators in a feedback loop (with the signal from the PC as a PC-controlled reference/set point value.

Since this seems to be true of more than a few op-amps, (I checked several other datasheets), I thought it might be useful to give warning to others who may be encountering similar problems.

Cheers!