ni 6453 multiplex delay offset issue

[Jimtemcon]

Hi all,

Having a setup with the usb 6453 and 16 analog mems microphones. Using single ended RSE input.

Using python daqmx to read out data. Using a task, finite acquisition RSE.

I'm sampling at 10k for max 2s.

When I sample 4 channels that aren't multiplexed they seem all right:

correct

But when I measure channels that are multiplexed I see a offset between the channels.

incorrect

I fail to understand what is happening. Suppose it is very obvious?!

I've read the docs, and I tried adjusting the AIConv.Rate property. But I can't find a setup that influences the results.

What am I missing here?

[santo_13]

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@Jimtemcon wrote:

Hi all,

Having a setup with the usb 6453 and 16 analog mems microphones. Using single ended RSE input.

Using python daqmx to read out data. Using a task, finite acquisition RSE.

I'm sampling at 10k for max 2s.

When I sample 4 channels that aren't multiplexed they seem all right:

correct

But when I measure channels that are multiplexed I see a offset between the channels.

incorrect

I fail to understand what is happening. Suppose it is very obvious?!

I've read the docs, and I tried adjusting the AIConv.Rate property. But I can't find a setup that influences the results.

What am I missing here?

---

You need a DSA card to sample Microphone, 645x is a general purpose DAQ and not suited for sound capture.

Please review - Troubleshooting Unexpected Analog Measurements - NI

[Jimtemcon]

Thanks for your reply.

Not sure what you what you with sound capture. It's just voltage I'm measuring right? 

[santo_13]

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@Jimtemcon wrote:

Thanks for your reply.

Not sure what you what you with sound capture. It's just voltage I'm measuring right? 

---

You're measuring a voltage that represents sound. Sound signals require frequency accuracy, amplitude accuracy and resolution.

DSA cards use delta-sigma ADC that allow higher resolution 24-bit and very good frequency accuracy so that the signal's frequency content is well preserved.

[cbaxter]

The documentation is undergoing maintenance, so I can not dig too deep into this, but from the 6451 documentation that I have, there are some caveats with RSE and NRSE listed.

Only use NRSE input connections if the input signal meets the following conditions:
• The input signal is high-level (greater than 1 V).
• The leads connecting the signal to the USB-6451 are less than 3 m (10 ft).

Only use RSE input connections if the input signal meets the following conditions:
• The input signal can share a common reference point, AI GND, with other signals that use RSE.
• The input signal is high-level (greater than 1 V).
• The leads connecting the signal to the USB-6451 are less than 3 m (10 ft).

I would check since you appear to be using small signals, that other considerations are being take care of as well. Mainly if you need bias resistors anywhere (depends on your output impedance), and are all of your signal wires firmly attached.

I agree the signals are "just voltage", but you may end up needing to keep the microphones differential.

[Jimtemcon]

Thanks for this input!

I'm using small signals indeed, lower than 1 V. So that could be a thing.

I'm not seeing these requirements in the 6453 manual, but the devices are very similar, worth a shot to investigate.