pci-6224

[bry0n]

Hi all

I am trying to create a DAQ assistant vi that has 18 analog channels and that will be used to monitor voltage levels.   Can this be done simultaneously or is the pci-6224 incapable of this?  I have tried to output the data into 2D arrays and they all will read similar voltage levels even when all but one are connected to a signal source (which for testing I am using a 5V signal).  

[Ali M]

Hello bry0n,

The PCI-6224 does not perform simultaneous sampling. M Series devices use AI Sample Clock (ai/SampleClock) and AI Convert Clock (ai/ConvertClock) to perform interval sampling. In this method, the Sample Clock is used to determine when to start acquiring samples and the Convert Clock is used to determine when each channel in the list is to be sampled. The time in between the sampling of two channels is called Interchannel delay and can be set in software. Detailed information on interval sampling has been included in link 1 below (Chapter 4/pg 4-26). If you are acquiring 5V DC, then the observed data is correct. For simultaneous sampling, please look into our S Series cards; these devices are built on a multiple analog-to-digital converter architecture, for increased throughput per channel and accuracy (all the channels are sampled exactly at the same time using sample-and-hold circuitry and have an independent ADC for each channel).

M Series User Manual

Regards,

Ali M

Applications Engineer

National Instruments

Message Edited by Ali M on 04-16-2010 01:02 PM

[John_P1]

Hi bryon,

The multiplexer on the 6224 allows you to sample multiple channels at approximately the same time.  Each channel is switched through the multiplexer one-at-a-time and converted by a single ADC on the device.  The end-user doesn't really need to worry about any of this, for most practical purposes the channels are sampled at the same time.  The minimum delay between channels on the 6224 is 4 microseconds (1/250kHz).

Measurements always need some time to settle from the previous channel.  This settling time is the amount of time it takes the charge to dissipate that was accumulated at the ADC during the previous measurement.  Settling time is dependent on the source impedance of what you have connected to the channel (think of the classic Discharging Capacitor type problem from an intro to circuits class).  The following graph from the 6224 Specifications shows the settling time for various source impedances:

If there is nothing connected to an input terminal, then you essentially have infinite source impedance (technically the input circuitry is providing the path to ground and has a very high impedance of >10 GΩ).  This means that the measurement will take a very long time to settle, and you will see the effects of one channel on the next.  We refer to this as "Ghosting" (a completely unconnected channel presents an extreme case), and you can read more information about it here:

How Do I Eliminate Ghosting From My Measurements

Having said that, to fix the behavior you can just ground any unused channels or make sure that all channels in your scan list are connected to a signal.

Best Regards,