improve analog input channel accuracy

[j_a]

Hello,

I am using a PXI-6259 to read multiple analog voltages and I cant seem to get the same accuracy when reading multiple channels versus a single channel using the test panel on MAX. How do I add delays to the scanning of the MUX to improve accuracy? Or are other means to improve accuracy?

Thanks! 

[jyang72211]

What kind of single are you measuring?  With mutiple channels, your sampling rate is lower due to multiplexing, but your resolution shouldn't change.  Depending on what you want to do, you can put some averaging algorithm in place to get rid of unwanted noise. 

[JÞB]

In this reply John Passiak explains this phenomina, known as "ghosting" and presents methods to mediate the effect.

It boils down to either lower the sample rate or use a buffer amp to reduce the source impedance.

[j_a]

Thanks Guys.

Since I already have a low impedance buffer, I think settling time is not an issue. I did change the sample rate and that improved it wuite a bit. Will do some optimization.

[Jonathan_S1]

Hi,

You can also change the convert clock rate, which is the clock that controls when the multiplexed samples are each converted.  Normally, this is set automatically.  If you are sampling well below the multiplexed maximum sampling rate, you can try and mess around with this too.

Jon S

[Jonathan_S1]

Check this out about the convert clock.

Jon

[MarkMoss · Accepted solution]

In addition to the other posters' comments about minimizing the source impedance and conversion clock rate, there are a couple of other things you can do to minimize the error:

1. If you can't decrease the conversion clock rate due to a high sample rate, you might try adding additional "dummy" channels where you have large jumps in the measured voltage.  For example if you need to measure three channels at about 5 V and three channels at about 2.5 V, you might read the first 5 V channel and the first 2.5 V channel an extra time (and discard the measurement).  This has the same effect as decreasing the conversion clock rate, but only for the problem channels.

2. Re-order your channels by the expected voltage level.  Using the example above, read all of the 5 V channels, then all of the 2.5 V channels.  Don't alternate between 5 V and 2.5 V channels.

3. Add capacitance across the input terminals.  Since the capacitor is always across your input signal, it provides a reservoir to quickly charge/discharge the ADC input capacitance.  If your input signal changes quickly, though, make sure that the added capacitance won't filter out the signal you're interested in.

Mark Moss

Electrical Validation Engineer

GHSP