Bypassing front end filter

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Sean McKee
Posts: 1
Joined: Tue May 19, 2020 4:03 pm

Bypassing front end filter

Post by Sean McKee » Thu Aug 06, 2020 6:10 pm

Hello all,

I'm working on a project that will be sampling in higher nyquist zones. The ADC on the Red Pitaya 125-14, the LTC2145-14, has the specs to do what I need, but the Red Pitaya itself has a 62.5MHz low pass filter on the front end I'll need to bypass. Does anyone have experience performing this type of hardware modification?

Thanks!
-Sean

alex123go
Posts: 14
Joined: Mon May 04, 2020 4:46 pm

Re: Bypassing front end filter

Post by alex123go » Thu Aug 20, 2020 8:37 pm

Hello,

By looking at the schematic, there's two filters between the ADC and the FPGA.

1st one is where you select the LV vs HV voltage input and 2nd one is after that.

You can bypass the 1st one but not the 2nd one.

When I started at the lab, I remember a colleague that also wanted to bypass the filter and he just used the provided jumper to connect the input of this stage to the output (connect pin 2 to pin 5) https://imgur.com/a/QrpiwwZ

I'm not sure about the results he obtained, but he continued to use the Red Pitaya like that.
Maybe it's worth a try.

bma
Posts: 2
Joined: Wed Sep 23, 2020 9:54 am

Re: Bypassing front end filter

Post by bma » Wed Sep 23, 2020 10:43 am

Hello,

I have already done this HW modification to demodulate 80MHz and a 220MHz signals with successful results on 125-14 redpitaya.

The basic idea is to pass trough the passive and active low pass filters, but we must pass through the single to diff amplifier, which will limit the bandpass of the redpitaya.

1. remove jumpers (HV/LV) to cut the connection between the SMA connector and the LP inputs
2. remove the coil in front of the single to diff amplifier
3. solder a wire between the + input of the single to diff amplifier and the sma connector

Here an image to help you:
https://ibb.co/NF2G2ky

Hope it will help you !

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