Redpitaya Forum

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

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.

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


Hope it will help you !

Hi,

Thanks for reply! Did you implement a band pass filter before the Red Pitaya's input? I was considering doing something very similar, but now I'm thinking of modifying rather than bypassing so that I can band-pass filter on board the RP.

I am thinking of switching out the op-amp on the bottom of the board (AD8066) with one that has a higher cut-off (probably the AD8056). Additionally, the passive components near to where you soldered appear to be 2 LC filters (I haven't yet desoldered the components to confirm their parameters), so I will switch these out to appropriately bandpass.

Hi,

Yes, some external MiniCircuits bandpass filters were used, mainly to not saturate the ADC.
For your modification, you have to deal with the 200MHz bandwidth of the LTC6403-1 (differential amplifier placed just before the ADC pairs)

BR

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