configure PCI 5112 for two channel acquisition at different sampling rates

[Dhiraj]

I have a PCI 5112 digitizer and wish to sample the same input signal at two different sampling rates. The two sampling rates have to be strictly pairwise relatively prime. How do I achieve this?

[Luke_G]

Hi Dhiraj,
Unfortunately there is no way to sample at different sample rates on this or any National Instruments device. The limitation is that each board relies on a single timing device. Without knowing the context of your application, I can think of two options.
1. Use two separate devices.
2. Use a single channel on one device and sample at the faster of the two rates. Then in software, use the "Resample Waveforms" function to resample the data at an arbitrary sample rate.

[Dhiraj]

Hello Luke,
I am trying to sample a signal at say 125 MHz using the ni5112 digitizer using sub-Nyquist sampling. Basically I have a bandpass filter preceeding the ADC. Now in order to do this, I need to specify my sampling rate accurately. How do I get control over the sampling rate? I can't figure out how to sample a signal at say 23.278 MHz. Some help here would be really appreciated.
Thanks
Dhiraj

[falkpl]

Usually the sampling is in DAQ is dome at some multiple of the onboard clock or an external clock can be used.  This is all setable in DAQ tasks.  The 5122 is a scope and uses a different api so I am not sure about external clocking for acquisition.  But usually if you need specific acquisition or generation rate you can bring in an external clock source.

[jaced]

Hi,

You will need to use a decimated value of the onboard clock, which is 100Mhz, In the decimation method, the ADC samples at the rate of the onboard clock and then sends its digital data to a decimator that essentially discards samples at a specific interval to achieve slower effective sampling rates. In this case you can sample at 20Mhz or 25Mhz based on devisors of 5 and 4 respectively. Your device also does not support an external sample clock. I suggest using a device that does support an external sample clock such as the NI 5124 and using an external sample clock. The input range of the NI-5124’s sample clock begins above your frequency so you would still need to decimate to achieve your specified frequency.  I hope this clears things up.