Input Stage Imperfections

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igttgit
Posts: 15
Joined: Fri Jul 31, 2015 9:22 pm

Re: Input Stage Imperfections

Post by igttgit » Mon Oct 10, 2016 4:26 pm

It looks like you are using the raw data with your own FPGA
I have also encountered the problem, and I have added the FPGA code from the original Red Pitaya FPGA

viewtopic.php?f=14&t=1473&p=5406#p5406
Here is the forum post with my query, the FPGA filter works very well to counteract this problem

https://github.com/RedPitaya/red-pitaya ... a_dfilt1.v#
The filter module can be found here The module is called "red_pitaya_dfilt1"

luigiraffaele124
Posts: 12
Joined: Sun May 29, 2016 5:26 pm

Re: Input Stage Imperfections

Post by luigiraffaele124 » Mon Jan 02, 2017 9:12 pm

I have connected OUT2 generator (with sinusoid of 0,9V @ 1kHz) to IN2 scope, with a T-connection and a 50 Ohm load.
The scope visualization is
https://www.dropbox.com/s/zm3w6vsf99tpg ... s.jpg?dl=0

We have a difference of 0,9V - 0,84V = 0,06 in peak value (that equals to an error of 20*log(0,9/0,84) = 0,6dB.
At 500Hz, 3kHz, 10kHz frequebcy, I have about the same situation.
Dont'you think that 0,6dB in an error too high at this frequency?

luigiraffaele124
Posts: 12
Joined: Sun May 29, 2016 5:26 pm

Re: Input Stage Imperfections

Post by luigiraffaele124 » Wed Jan 25, 2017 3:55 pm

I have also tried a configuration with the generator sending a DC signal to a cable connected to the scope, with T connector and 50 Ohm termination.
The generator is sending a 900 mV signal but the scope is reading a 828 mV signal, so with 72 mV of difference.
Is this a normal behavior?
Does it depend on imperfection on low pass filter or other input stage component?
Or my board can be defective?
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redpitaya
Site Admin
Posts: 883
Joined: Wed Mar 26, 2014 7:04 pm

Re: Input Stage Imperfections

Post by redpitaya » Thu Apr 20, 2017 10:57 am

Hi,

Probably in this cases (amplitude difference) board is not calibrated.
With calibrated board if you set OUT1 to 900mV
connected to IN1 (using 50OHM termination) you should get on IN1 = 900mV +- 0.5mV

Kind Regards, Zumy

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