DAQ sampling rate

[Skydyvr]

The original topic of this thread was about using a DAQ (data acquisition system) specifically the analog portion. That means you have analog to digital converters and digital to analog converters. NI's non-simultaneous ADCs use 1 ADC and sample the multiple inputs sequentially. That is what this thread is about. I could be wrong, but I believe the NI-9411 since it is digital only probably is just a MUX. Therefore all inputs would  be sampled simultaneous, i.e. the inputs are parallel clocked in.

1) Yes, the sample frequency is 2MHz.

2) Nope, your sample rate for all channels is 2MHz.

3) If I'm correct, you could choose an encoder up to 2MHz.

4) The only limitation of the chassis would be if the interface between the chassis and your computer can handle the data rate for your data. But since NI specifies the sample rate of 500ns I'm sure the interface can handle that. You did not mention if you were running another module at the same time on the 9174 that would take up bandwidth on the interface.

[adrub]

Hey!

Thanks for an awesome answer 🙂

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@Skydyvr

4) The only limitation of the chassis would be if the interface between the chassis and your computer can handle the data rate for your data. But since NI specifies the sample rate of 500ns I'm sure the interface can handle that. You did not mention if you were running another module at the same time on the 9174 that would take up bandwidth on the interface.

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I'm using also Analog Output NI-9269 - one channel, Amplifier VF-90 (one channel IN one channel out) and Analog Input NI 9218 - 2 channels.Basically, I'm sending an impulse from AO NI-9269 through an amplifier and read the response and sent signal in AI NI 9218.

What encoder frequency should I choose to be sure that it works?

[Skydyvr]

I have no idea because again you didn't specify enough information. If you really want to know, here is what I would recommend.

1) Determine the maximum bits/sec the interface between the 9174 and computer can handle. Should be specified in the datasheet.

2) For the NI 9218 you need 2 CH * Bits/CH * sample rate = bits/sec for this device

3) For the 9269 is basically the same thing, # channels * # bits/ch * how many state changes you are doing per sec

4) I have no idea what the VF-90 would use as far as data to talk to it. So figure that out.

5) Add 2), 3), and 4) together. Multiply that by 1.25 for approximately 25% headroom.

6) Subtract 5 from 1 and that is your available bandwidth left on the interface for your encoder.

7) The encoder is probably 12 bits plus overhead so call it 20 bits.

😎 Divide 6) by 20 that should be your available sample rate.

So if you think I'm going to do all that for someone on a forum, you are surely mistaken. I don't even do that sh*t on my own projects. Just choose an encoder that is just fast enough (or throttle back a faster encoder) to get your desired data. Try it out and if you don't get errors it is probably good enough. If you get errors that make no sense, you may be dropping data and therefore something will have to slow down. Of course you could always do the math if you absolutely must know.

[adrub]

Hey!

Thanks for the answer. 

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@Skydyvr

So if you think I'm going to do all that for someone on a forum, you are surely mistaken. I don't even do that sh*t on my own projects. Just choose an encoder that is just fast enough (or throttle back a faster encoder) to get your desired data. Try it out and if you don't get errors it is probably good enough. If you get errors that make no sense, you may be dropping data and therefore something will have to slow down. Of course you could always do the math if you absolutely must know.

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Hahaha no. I don't want you to do this 🙂 Thanks for writing me steps. I think I will just try to get different encoders and check which one works 😉

Thanks again, you really helped me!

Cheers