How to do aquisition at low sampling rate with 5122?

[d_touch]

I want to do it less than 30MHz. According to high speed digitizer's help, external clock is devided and it can be less than 30MHz. But the signal is only distributed to routing matrix and clk out. It can't be used for ADC, I think. Is it correct?

[Shea C]

d_touch,

The external sample clock can be divided down using decimation. Consider the following piece taken from the NI High-Speed Digitizers Help:

"External Sample Clock
Some digitizers and oscilloscopes can accept an external sample clock. This external sample clock is used to replace the onboard clock (the VCXO) for synchronization or to achieve a sampling rate that cannot be specified by using the onboard clock. Some devices can also decimate the external sample clock to achieve a sampling rate that is an integer divisor of the external sample clock. For example, if the external sample clock is 70 MHz, you could decimate the clock by a factor of 2 and achieve a 35 MS/s sampling rate."

I think the valid divide down values are 1<65,535. This should give you the ability to aceive the rate you need. I hope this helps!

Regards,

Shea C
Applications

[d_touch]

Shea C,

Thank you for your reply.
>The external sample clock can be divided down using decimation.
I understand that the external sample clock can be divided down using decimation. But I think that the lower limit of sampling clock for ADC is 30MHz. So it means ADC can't do sampling under 30 MHz. Is it correct?

d_touch

[Nick_Storton]

Hi d_touch,

The NI PXI-5122 can sample at rates of 100/n MS/s, where n is an integer value between 1 and 216 – 1. This give a minimum sample rate of 1525.9Hz

Nick

[Leonard]

Hello d_touch,

You are correct that you cannot clock the ADC on the 5122 lower than 30 MHz. The decimation allows for lower sample rates but the ADC is clocked at the rate of the clock that is decimated.

For example, the VCXO is 100 MHz on the 5122 and this is tied directly to the ADC, so the ADC is always running at 100 MHz when you use the onboard clock. If you specify a sample rate of 50 MS/s in NI-Scope what happens is the data comes out of the ADC at 100 MHz (100 MS/s) but only every other sample is stored in memory. If you asked for a sample rate of 25 MS/s, every 4 sample is saved to memory. Does this make sense? The NI 5122 allows you to do the same thing with external clocks. You can input a 30 MHz clock but you can configure for sample rates lower than 30 MHz, such as 15 MS/s. The 30 MHz clock is connected to the ADC though and for the 15 MS/s rate, every other sample coming out of the ADC is stored into memory.

Hope this clears this up.

-Brian

[d_touch]

Brian, I'm clear. d_touch

[d_touch]

Thank you Brian, I'm clear. d_touch