Using 6036e & multiplexor to measure microvolts

[oavion]

I have Win98, Labview, a 6036e (16bit,200kS/s,16 analog inputs),a multiplexor connected to 16 buses collecting microvolt information from 8 sensors each. Where do I begin? How do I address each bus individually for the data? Do I begin in Labview treating the collection as a black box working my way down to the sensors or do I begin somewhere else and work up from the sensors. Any help is greatly appreciated.

[Russell]

Hello,

Is this multiplexer an NI product such as the AMUX-64T? If so, there is an addressing method you can use within the channel string to address each channel. See Single-Point Acquisition via AMUX-64T.

If this is a third-party or your own multiplexer, then things are much more complicated. The best thing to do is to have the multiplexer connected to channel 0. Use the SCANCLK timing signal from the DAQ board to increament the multiplexer. This signal pulses once after each A/D conversion has taken place. (See the DAQ board's user manual for more information.) In your progra
m you will only acquire from channel 0 and you will have to separate out the channels yourself in software.

I hope this helps!

Russell
Applications Engineer
National Instruments

[oavion]

Russell,

Hello & Thank you for your input! It is greatly appreciated! The multiplexer we're using is our own incorporating Texas Instruments CD74HCT541. I'm attempting to use all sixteen channels of the 6036E and address each of eight voltage sensors on each channel, A0-A8, simultaneously.

I've been using the voltage example provided with LabView, but the problem I'm up against right now is that only one channel is "active". In other words, when I run the VI, I only see data on the that one line although I can pick which channel to look at from the pull down menu. What do I need to change in order to see all the channels operating at once?

What I'd really like to do is have a chart for each channel & be able to monitor the voltages off of each
sensor. Don't really know if I'm putting the cart before the horse. That's where I am.

Thanks again for any help you may be able to provide.

[Russell]

Hello oavion,

Hmmm, I'm a little confused. You said you are connecting a CD74HCT541, but my investigation shows the CD74HCT541 to be an Octal Tri-State Buffer.

http://turtul.no-p.com/electro/datasheets/74HCT541.pdf

Perhaps you could give me more explanation about the external hardware you've connected?

If I understand you correctly, you want to have an 8x MUX on each of the 16 channels for a total of 128 channels. Now, if you're using your own MUX, you'll have to have a way to control each one, perhaps with a counter. The only resource you have from the DAQ board is a timing signal that pulses once for every sample it takes. (SCANCLK)

As far as your program is concerned, you'll have to sample each channel 8 times. For example, yo
ur channel string might look like this, "0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,2,2,..." After each sample, the SCANCLK signal will pulse. Then your hardware knows to advance the MUX to the next channel, we'll call it channel 0b and the next 0c and so on. Finally, after channel 0f, you'll start sampling the sensors connected to the MUX on channel 1 of the DAQ board.

This is done much more easily with our AMUX-64T board. You can cascade up to 4 of them for up to 256 channels if you'd like.

[oavion]

Hello Russell,

Yes. The correct component is CD74HC4051. Again, thank you very much for your input. I will try your suggestions. I apologize for the slow response. I assure you it is not due to a lack of interest or importance. I appreciate the offer of the AMUX-64t, but this is what I have to work with.

ps. Would a schematic help?

[Russell]

Hello oavion,

Okay, I found the datasheet for the CD74HC4051. This is still consistent with what I was assuming before. You see, this MUX is addressable. That is, each of it's 8 channels are enabled by a unique 3-bit address. So what you would like to do is increament through each address in order to sample all 8 channels on the MUX. The DAQ board provides you with a short pulse after each sample has been taken, SCANCLK. You will also need a binary counter to connect between the SCANCLK signal and the MUX. The SCANCLK will connect to the increament pin of the counter, and the counter's output register will connect to the address lines of the MUX. As the counter is increamented, it's output register will begin addressing
each of the MUX channels from 000 to 111.

Since you said you intend on connecting one 8-channel MUX to each DAQ board channel, your application will have to sample each channel 8 times in order to get all 8 channels from the multiplexer. Your DAQ channel string will look like how I said before, "0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,...,7,7,7,7,7,7,7,7". In your program, you'll have to be aware that the first 0 sample is MUX channel 0, the second 0 sample is MUX channel 1, third 0 is MUX channel 2 and so on.

I hope this helps!

-Russell