NI 5752 ouput dependency on input frequency

[MansoorEE]

Hello, I am using ni 5752 with 7966 to meausure ultasound analog singal ended input with the frequency range from 100KHz to 5MHz. I have seen that if I choose the AC coupling for the ADC, the output amplitude varies with the varing frequencies. But, if the DC coupling is selected, the output stays flat for the above freuquency range.

It is also observed that the AC coupling gives higher ouput for the same input signal than the DC coupling.

I followed another post where it is suggested to use 5751 instead of 5752 to receive the flat response from ADC that is independent of the frequency.

I have following questions:

Q1. There is a suggestion in the AFE 5801 data sheet for 5752 to use a 100nF at the single ended input. Will this help reduce the frequency dependence of the output?

Q2. Is the DC coupled ouput of the 5752 be similar to the 5751 output?

Q3. Are the gaind controls of the 5752 and 5751 comparable?

syed

[Henrik_Volkers]

Generally said:

• Your signal source has a source (or output)  impedance.
• The connecting cable has a (cable) impedance.
• Your 5752 (or whatever ADC/Scope/DAQ) has an input impedance.

Sounds complex?  Well usually all these parameters are complex 😄

All these impedances need to be known and regarded to make serious measurements. 

At a first glance you can find these parameters in the datasheets.

The 5752 has a 100 Ohm input impedance,  a twisted pair cable (CatXX) usually have a 100 Ohm impedance, and your source?  (Most probably 50Ohm output?)

5752 Spec also tells something about input filter... 

[MansoorEE]

Thanks for your response Henrik; we have been very careful about impedence matching. Somehow we need to figure out the best configuration to perform our tests.

I have more questions about the 5751 and 5752 DIOs.

Q) On the data sheet it specifies that the minimum pulse width is 10ns but the Max toggling frequency is only 1MHz. How is it possible to generate a pulse of 10ns with 1MHz frequency?

[David-A]

I have more questions about the 5751 and 5752 DIOs.

Q) On the data sheet it specifies that the minimum pulse width is 10ns but the Max toggling frequency is only 1MHz. How is it possible to generate a pulse of 10ns with 1MHz frequency?

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Put the IO node that corresponds to the DIO in a timed loop that runs at 100MHz then pull the line high, then low, then wait 98 ticks and do it again.

[KeygoTech]

I have a question like this,could u help confrim it?

now I need NI 5752  DO to Generate a series of pulse signals, the frequency distribution is as the pic follow. Three pulses once a time, and the frequency between pulses is 3.3Mhz (or 2.25Mhz), the period of the transmitted pulse is 4Khz.

I am not sure if it is possible? thank u very much!

[David-A]

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@KeygoTech wrote:

Three pulses once a time, and the frequency between pulses is 3.3Mhz (or 2.25Mhz), the period of the transmitted pulse is 4Khz.

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Judging from the picture, it looks like you want to generate 3 pulses, at 3.3MHz, with a duty cycle of 50%, repeating every 250microseconds.

If you try to use the DO from the 5752 to toggle at 3.3MHz you're going to get an ugly square wave. So ugly that I wouldn't recommend that you use the 5752 DO to generate that series of pulses. But if you want to try anyway there's nothing stopping you from dropping the digital output node in 33MHz clock domain (or something that easily divides down to 3.3MHz) and trying to toggle at the desired rate. Just make sure that you hook up a scope to the digital output line and see for yourself if the ugliness of the pulses is too much for your application. If it is then you would want to use a different board to generate the pulses.