DAQ USB-6451 (OEM) : Analog Input (AI) peak speed

[Coherent_L]

I’m working on capturing 16 analog input (AI) channels.
The USB-6451 can acquire 8 AI channels simultaneously at 100 kS/s, so I configured it for 50 kS/s when using 16 channels.

According to the manual, channels share ADCs in pairs — for example:

• AI1 and AI9 share one ADC

• AI8 and AI16 share one ADC, and so on.

Issue

When the sample rate is greater than 200 kS/s (for example, 500 kS/s) and I record about 100 samples, I see a problem if paired channels are at different voltage levels.
For instance, if AI6 = 4 V and AI14 = 3 V, then during the first few samples:

• AI6 shows a voltage drop, and

• AI14 shows a voltage rise.

All 16 channels are configured as Reference Single-Ended (RSE).

Observation

• The problem disappears when the sample rate is below 200 kS/s.

• It also disappears when both channels are at the same voltage level.

[santo_13 · Accepted solution]

You may be experiencing Ghosting in multiplexed ADC architecture DAQ

Please read,

How Do I Eliminate Ghosting from My Measurements? - NI

[Coherent_L]

Thank you for your solution!

Just wanted to confirm my following understanding: 

NI explicitly notes that dummy channels count toward the device’s maximum aggregate sampling rate.
If I currently scan 16 real channels and then add 8 dummy channels (one per ADC pair), the scan list becomes 24 channels.

As a result, adding 8 dummy channels will reduce the maximum per-channel rate from 500 kS/s to approximately 330 kS/s, unless I increase the device’s allowed conversion time (interchannel delay).

[santo_13]

Or you can lower the output impedance of your signal to lower the settling time and minimize the ghosting effect.

Eliminate Ghosting on Adjacent Input Channels by Decreasing Source Impedance - NI

What signals are you connecting to the AI?

[Coherent_L]

we are working on customized Optical power meter, we’re connecting a photodiode (PD) PCBA to the NI USB-6451, the signal on each AI (analog input) channel is almost certainly a analog voltage

[santo_13]

So, the PhotoDiode doesn't have any amplifier or buffer in the path? if not, you must include one before connecting to the DAQ, as the output impedance of a PD will be typically high and be easily affected by ghosting.

In general, lower the output impedance of these Analog signals, the effect of ghosting will be minimal.

[Coherent_L]

Thank you for your reply, we already reduce it and small enough, I guess that is our limitation. But anyways we will not take those unstable samples as my solution.