Need Guidance in retriggering at same point NI 6363 BNC

[Hasham622x]

Hi,

Apologies for the disturbance. I am using an NI 6363 BNC DAQ system, where I have connected 4 AI channels in differential mode. The sampling frequency is set to 500 kHz, with an analog trigger from APFI0 at a trigger level of 0.2 V using a 360 Hz signal, 0.04 V hysteresis, and falling-edge mode.

I observed that the data points I am receiving fall into three or four distinct steps within the hysteresis window level, although I expect all points to be at or near the same level. I have also attached a graph of the raw data points from AI0 for your reference.

Kindly please guide..

Kind regards

Hasham

[GerdW]

Hi Hasham,

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@Hasham622x wrote:

I am using an NI 6363 BNC DAQ system,

I observed that the data points I am receiving fall into three or four distinct steps within the hysteresis window level,

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The 6363 provides a 16bit ADC, so you get a resolution of 0.3mV in the default ±10V input range!

This will limit the values you can expect in your chart, especially when you set a very small range in the Y axis…

Keep in mind: "resolution" is not the same as "accuracy"!

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@Hasham622x wrote:

The sampling frequency is set to 500 kHz, with an analog trigger from APFI0 at a trigger level of 0.2 V using a 360 Hz signal, 0.04 V hysteresis, and falling-edge mode.

I observed that the data points I am receiving fall into three or four distinct steps within the hysteresis window level, although I expect all points to be at or near the same level.

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For me most of the samples are within the "same level"!

What do you get when you plot a histogram of those samples in your image? I would guess more then 90% fall in the bin 0.168-0.174…

How much noise is in your signal? We cannot comment on your "expectations" as long as you don't provide substantial information on your system!

[Hasham622x]

Thank you for your reply. I have set the input range to ±5 V, which still provides a resolution of 0.3 mV. I tried reducing the hysteresis window from 0.04 V to 0.02 V, but this caused a delay and increased the code execution time, which I want to avoid.

[Hasham622x]

Hi,

Sorry for the disturbance..

I have identified the cause of the transient behavior in my data points, which is due to the APFI0 pin (external trigger). It appears that the pin has a high capacitance, leading to the observed transients.

Could you please guide me on how to reduce this capacitance effect on the APFI0 pin? I have already connected a 3.3 kΩ resistor, but I have not observed any significant change.

Looking forward to your guidance. Thank you!

[Hasham622x]

Apologies for the disturbance. I am using an NI 6363 BNC DAQ system, where I have connected 4 AI channels in differential mode. The sampling frequency is set to 500 kHz, with an analog trigger from APFI0 at a trigger level of 0.2 V using a 360 Hz signal, 0.04 V hysteresis, and falling-edge mode.

I observed that the data points I am receiving fall into three or four distinct steps within the hysteresis window level, although I expect all points to be at or near the same level. Later 

I have identified the cause of the transient behavior in my data points, which is due to the APFI0 pin (external trigger). It appears that the pin has a high capacitance, leading to the observed transients.

Could you please guide me on how to reduce this capacitance effect on the APFI0 pin? I have already connected a 3.3 kΩ resistor, but I have not observed any significant change.

Looking forward to your guidance. Thank you!

[santo_13]

Add a unity gain buffer at the DAQ's input so your external signal has enough drive strength to drive the APFI0 capacitance.

Do note that the unity gain buffer will add an offset to your signal; it may be trivial based on the accuracy needed.

An alternate option is to add an external comparator circuit and feed the DAQ with a digital edge instead of the analog level trigger.

[Hasham622x]

Thanks for your reply.

Could you please guide me on the unity gain buffer? Is it related to the DAQmxBuffer property node, and should I set the input buffer size (bufSize) to 1? Or would adding a small resistor to APFI0 help reduce the transient behavior?

I would appreciate your guidance on this.

Thank you!

[GerdW]

Hi Hasham,

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@Hasham622x wrote:

Could you please guide me on the unity gain buffer?

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That "unity gain buffer" is an additional external piece of hardware. (There's nothing to change in DAQmx tasks/properties.)

[Hasham622x]

Thanks for your reply. Can I use 100pF Capacitance between APFI0 and AIGND or op-amp voltage follower as unity gain buffer.

Kindly please guide ..

[GerdW]

Hi Hasham,

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@Hasham622x wrote:

Can I use 100pF Capacitance between APFI0 and AIGND or op-amp voltage follower as unity gain buffer.

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How should a capacitor work as "unity gain buffer"?

That buffer is created by using an amplifier, in this case with "unity gain" (aka G=1)…