Measure AC signal using PXI 4070 DMM card

[TPJ]

Can I measure 0.025 Vrms signal in a detection bandwidth of 0.2Hz-600 KHz ac signal using PXI 4070 DMM card?

-TPJ

[Dennis_Knutson]

Specifications.

It has a bandwidth of 300 kHz and a 100 nv range sensitivity at the +/- 100 mv scale

[TPJ]

But as per the specification document of PXI 4070 it has a sampling rate of 1.8 MS/s. Can I not use this feature to sample the 0.025 Vrms signal in the detection bandwidth of 0.2Hz-600 KHz?

-TPJ

[TPJ]

Can I measure AC noise of 0.025 Vrms (0.2Hz-600KHz) on a DC supply of 5 volts using PXI 4070 in the digitizer mode?

{The maximum sampling rate seems to be 1.8MS/s in the digitizer mode}

-TPJ

[TheWoost]

TPJ,

I recommend that you refer to the following document to get a better understanding of the terminology found in the specifications:

Bandwidth, Sample Rate, and Nyquist Theorem

While this article has many references to products within our line of High-Speed Digitizers, the theory applies for any measurement device with an ADC.  A high-speed digitizer may be more suitable for your application since you are interested in sampling a signal that can have frequency components up to 600kHz (well within the bandwidth specifications of these devices).

Here's a couple of high points from the article that address your question:

"Bandwidth describes the frequency range in which the input signal can pass through the analog front end with minimal amplitude loss - from the tip of the probe or test fixture to the input of the ADC.  Bandwidth is specified as the frequency at which a sinusoidal input signal is attenuated to 70.7% of its original amplitude, also known as the -3 dB point."

"Sample rate is not directly related to the bandwidth specifications of a high-speed digitizer. Sample rate is the speed at which the digitizer’s ADC converts the input signal, after the signal has passed through the analog input path, to digital values that represent the voltage level. This means that the digitizer will sample the signal after any attenuation, gain, and/or filtering has been applied by the analog input path, and convert the resulting waveform to digital representation."

Regards,

Andrew W.

National Instruments

[TPJ]

Thank you Mr. Andrew for providing an understanding of the terminolgies associated with the ADC.

I looked at the High Speed Digitizers PXI modules. They have far too capacity than what I need. Hence not very cost-effective.

Instead I was looking at the PXI 6259 card. It has a small signal bandwidth of 1.7 Mhz and a maximum sampling rate of 1.25MS/s

I was thinking of using this card to capture 25mVrms noise signal (on a 5 volt dc) in a detection bandwidth of 600 KHz.

Please provide your comments. (Kindly note that I am already using this module PXI 6259 for other purpose)

Also kindly brief me about what "aggregate sampling rate" means and why it differs slightly from the "maximum sampling rate"?

-TPJ

[TheWoost]

TPJ,

The PXI-6259 does meet the minimum needs for your application in terms of bandwidth and sample rate.  The bandwidth of the signal that you are wanting to measure is 600kHz, making the Nyquist rate = 2 * 600kHz or 1.2MHz.  The maximum sample rate of the PXI-6259 when sampling on a single channel is 1.25MS/s, slightly higher than the Nyquist rate.  One thing to keep in mind is that this sample rate is sufficient if you are only concerned with observing frequency information from your signal.  Sampling a 600kHz signal at ~1.2MS/s will result in 2 samples per period of the waveform, and will be represented in software as a triangle wave with the same frequency as your real-world signal.  If you are interested in observing both the frequency and shape of the signal, it's recommended that you sample at a much higher frequency.  I typically recommend a sampling frequency of at least 10 times the bandwidth of the signal (in your case this would be at least 6MHz), because this results in 10 samples per period of the waveform and gives a pretty accurate representation of the shape.

All of the M-Series data acquisition cards use a multiplexed architecture to sample their analog input channels.  This means that although you have 32 analog inputs, there is only one ADC that must be shared among them.  If you sample on a single channel, no multiplexing has to take place and you are given full use of the ADC on that single channel.  However, if you sample on more than one channel in a task, the overall sample rate must be divided among all the channels being sampled.  For the PXI-6259, the maximum aggregate sample rate (sample rate that is divided among multiple channels) is 1.0MS/s.  As an example, if you were sampling 4 channels in a task the maximum sample rate on a single channel would be (1.0MS/s)/4 or 250kS/s.

-Andrew

[TPJ]

Thank you once again Mr. Andrew for your clarification to my query.

Appreciate your help!

-TPJ