Will redpitaya be suitable?

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MateoConLechuga
Posts: 2
Joined: Tue Nov 07, 2017 6:10 pm

Will redpitaya be suitable?

Post by MateoConLechuga » Tue Nov 07, 2017 6:22 pm

Hello everyone,
I am currently looking to use the 14 bit 50MHz models for my application. Is the sample rate still 125Msps? I am curious if the input impedance can be changed from 1Meg to 50 Ohms? Ideally I am looking for a device that can sample between the 1millivolt to 300millivolt range; and this is looking more and more like a good solution; however I'm not entirely confident yet. Any interesting specifications would be nice to know as well. Thanks for your time!

EDIT: I'm ideally looking to capture between 5-10 seconds of continuous samples, which will later (potentially) be sent to an external device. Is it feasible to store this amount of samples into internal memory?

JohnnyMalaria
Posts: 28
Joined: Fri Apr 28, 2017 12:43 am

Re: Will redpitaya be suitable?

Post by JohnnyMalaria » Thu Nov 09, 2017 10:03 pm

1. 50ohm - use an SMA T-connector and a 50ohm terminator. I think you need the probe package to get these - I can't remember.
2. Spec - 14 bit resolution with 1V input (or 10V via jumper). Hence, 61uV resolution (not allowing for noise).
3. This, to me, is a big limitation of the RP. You are very limited with data sampling rates (you can research the decimation levels available). From memory, it is something like this: choose one of the following values to divide the 125Msps sampling rate by: 1, 8, 64, 1024, 8192, or 16384 (or something like this). These correspond to available sampling rates of 125Msps, 15.6Msps, 1.95Mbps, 122ksps, 15.3ksps, 7.6ksps. That's all you can do (though I'm sure I managed to get 65536 to work even though the documentation says otherwise.) And you are restricted to capturing 16384 samples if you want continuous, uninterrupted data. So, for your target of 5 to 10 seconds, 16384 samples means sampling every 0.3 to 0.6ms (3.3ksps/1.7ksps). You have to choose the decimation factor nearest to what you want. Depending on the resolution you require, you may have to live with grabbing less than 1 second at a time. I had a similar need to capture at 0.1ms continuously for a minute. Search for a post by me that shows my (almost) successful way to do this. Unfortunately, I couldn't get the RP to deliver what I needed in the end. So, the bottom line comes down to: what frequency do you need to sample at and do you need continuous, uninterrupted acquisition beyond the 16384 sample limit?

Note, at the other extreme, at 125Msps you can capture only 13microseconds at a time.

John.

JB
Posts: 13
Joined: Tue Jul 12, 2016 10:34 am

Re: Will redpitaya be suitable?

Post by JB » Fri Nov 17, 2017 10:12 am

Hi Mateo,

your input range problem can be solved by using an amplifier. For the long acquisition time you might want to check out our project at https://github.com/tknopp/RedPitayaDAQServer
It's still work in progess but acquiring data works well (at least from MATLAB). We are planning to add a Python client in the near future.

Greetings
Jonas

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