Multiple analog inputs seem to alter one another (DAQ details: BNC 2120 and 6062E)

[JKLH]

I'm using the BNC 2120 DAQ board connected to the 6062E DAQ card to record two analog inputs. One input is connected to ai0 and the other to ai1. The example vi: 'Acq&Graph Voltage int clk' was used to measure both inputs with the vi set to read NChan and NSamp (channels being dev2/ai0:1). The output is the top graph in the picture. However this seemed a bit odd to me as one of them should be modulating with a different frequency. When I record both inputs individually (bottom two pictures) they are indeed both different from the inputs shown in the top graph.

 

Why would this be the case and how can I overcome this to measure the true signals?

Thanks!

Message Edited by JKLH on 05-20-2010 09:01 AM

[Rico P]

Hi JKLH,

The screenshots aren't very clear. Could you attach them please ensuring that the text is visible.

Regards,

[JKLH]

Hi Rico,

Here's the full size version of the picture, with readable text!

[JKLH]

Following on from the last message (I couldn't fit it all in one for some reason):

Thanks for your reply but thankfully I found out what the problem was last night. It's due to ghosting between the inputs, as described here for anyone else who's experiencing it:

http://digital.ni.com/public.nsf/websearch/73CB0FB296814E2286256FFD00028DDF?OpenDocument

The problem is diminished when the sample rate is lower. I found a sample rate of 100Hz and a (manually input) convert rate (inverse of interchannel delay) of 200Hz worked well. My next problem is working out how to get rid of this effect if I want the sample rate to be at least 50000Hz! I guess that's another issue though.

Regards, J

[Rico P]

Hi,

Good find! If you need to have a high sampling rate then use a card with simultaneous sampling. What are you using at the moment?

Kind regards,

[JKLH]

Hello,

Thanks for your prompt reply!

I'm using card 6062E. Do you know of a way of changing the sampling process so it performs it simultaneously or is the sampling process card-specific?

Best wishes,

J

[Rico P]

Hi J,

It's card specific so you won't be able to use simultaneous sampling on the 6062E as it has only 1 ADC so it multiplexes through the channels (as discribed in the KB you link).

I'll talk more through the Service Request then we can post the answer on the forum after.

Regards,

[Rico P · Accepted solution]

The E series card takes the samples as fast as possible. So for example, if you have 16 Analogue input channels but you're only reading from channel 0 and 1, the card will read channel 0 and channel 1 straight after and then wait 14 'ticks'. That tiny settling time is what's causing the ghosting.
I think you can get the card to wait a certain number of ticks with a property node. I've attached a screen shot. You can find the property node in the Functions Pallette >> Measurement I/O >> NI-DAQmx >> Timing Node. Expand the Property node so there is two inputs. The properties can be found by left clicking on the node and going to More >> AI Convert >> Delay from Sample Clock and its properties Delay units and Delay that you want.

If phase is important then the above isn't the best option as it causes a phase delay. So if you need true simultaneous sampling then you'll need different hardware. The S-Series is all simultaneous sampling.

Or, instead of the Delay and Delay Units property, try the Rate property found More >> AI Convert >> Rate.

If this doesn't work either you could move the second signal source to, say, AI8 and connect all others to ground. Take readings for these but just disregard the data. This way the ADC will settle at ground by the time it gets to the second relevant channel so you shouldn't see that frequency ghosting on to the other channel.

[P.Sanders]

Thanks for the info, helped solve my problem!