what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

allysemp · ‎01-03-2007

I am sitting an exam and the lituture for this subject is a littel fuzzy. I think the answer is 13 analogue channels because the singel-ended inputs use a common ground so that makes 5 and then because the differential inputs each use there own groung referende thats 8. is this the correct answer and resoning?

Also sorry to be a pain but "what is the maximum frequency of these signals , which can be measured using a DAQ board with the maximum sampling rate of 500kHz?" I have no idea how to begain to work this out as my notes on theis area confusing and any help how to begain to work this out would be most apreshiated.

thank you for your time

Marc_A · ‎01-03-2007

Wait, are you taking an exam right now and asking for an answer to one of the questions?

allysemp · ‎01-03-2007

hello

No the exam is in just under a week the questain is from last years paper theres to questains i cant do so i thought i would ask here.

Marc_A · ‎01-03-2007

Ok, in that case I'll help. You would need 12 channels on a board to get 4 differential and 4 RSE channels. You don't need a separate channel for ground. Take a look at some of the products NI sells and you'll see they are specified as 16 single ended/8 differential.

As far as the other question, it should be 500 / 8 = 62.5 KHz. Although the question says, "What is the maximum..." The maximum is 500 KHz if it's a simultaneously sampling board. Of course this will change if you mix RSE and differential channels. Maybe someone else can shed more light on this one.

Dennis_Knutson · ‎01-03-2007

The maximum sampling rate won't change if you have a mix of rse or differential channels so 62.5 khz is the fastest sampling rate you can have with a non-simultaneous sampling DAQ baord. You have to take into account Nyquist though, if you want to know the highest frequency you can measure. Nyquist says that the minimum sample rate is twice the frequency that you need to measure so with a max sample rate of 62.5 khz, the highest frequency you can measure is 31.25 khz.

Sarah_Coleman_NIUK · ‎01-03-2007

Hi,

The maximum signal frequency will be half the maximum sample rate of your board (according to nyquist's theorem).

The maximum signal frequency of each channel depends on the achitecture of your card. There are two possibilities:

Simultaneously sampled: If this is the architecture then each channel has it's own ADC so the maximum sample rate per channel would be the 500kS/s so the highest frequency you could measure accurately would be 250kHz per channel.
Multiplexed: There is only one ADC on board and each channel is switched into the ADC using a multiplexer. With this architecture, the maximum sample rate per channel depends on the number of channels you're using. For example, if you were reading 2 channels on a board with a maximum sample rate of 500kS/s, then the maximum sample rate per channel is 250kS/s. In which case, the highest frequency you could measure would be 250/2 = 125kHz (per channel)

I would guess the architecture the question is refferring to is the Multiplexed architecture.

Message Edited by SarahB on 01-03-2007 10:52 AM

Sarah

Applications Engineer | National Instruments | UK & Ireland

allysemp · ‎01-03-2007

hello

Thank you all for taking the time to reply it was really good of you and it helped a great deal. Should a similar questain apear next week in the exam i will now what to do.

For channels: single-ended inputs = channels (ie when you have one single-ended input you need one channel),

differential inputs = 2 channels ( ie when you have one differential input you need two channels).

Maximum frequency measured: Maximum sampling rate per channel =Maximum sampling rate / Number of channels

Maximum frequency which can be mesured = Maximum sampling rate per channel/2 (Due to nyquist's therorem)

(ie If a DAQ system was to acquire four analogue signals using differential inputs it would require 8 analogue channels. If it had a maximum sampling rate of 100kHz the maximum frequency rate you could messure would be 6.25kHz (from 100/8 = 12.5 6.25=12.5/2) Also if it was measureing a range +- 5V with a 12 card the resolution would be 5x10 -4 (from 10/5*2^12)

Thank you again for your help

LabVIEW

what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

Re: what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

Re: what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

Re: what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

Re: what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

Re: what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?

Re: what should the minimum number of channels be on a DAQ board being used to acquire eight analogue signals (four using differental inputs and four using single-ended inputs)? and why?