Scoppy front end hardware for RP2350-plus system - Page 1
Forum section: electronics / general

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 23, 2025, 08:58:25 am
content: I want to use Scoppy on my Android-Pi Pico 2 interface and I need a better analog front end for it. The developers of Scoppy have kindly put up plans for analog front end boards, but I'm not sure what the best options are. It is my understanding the analog front end is meant to keep the read voltages within safe ranges for the Pi Pico's ADC. Would it be better to use something like the ADS1115 to bypass the Pico's ADC entirely? My scope will be measuring output from my AD831 mixer boards and possibly also from my transmitter. So all measurements will be from running equipment and thus in safe ranges if everything isn't on fire. I am not picky on performance or specs, but I do need something that can read signals from 1Hz to at least 1MHz(ideally 5MHz or more) and interface with Scoppy.Here is the build topic for the AFE:https://github.com/fhdm-dev/scoppy/discussions/63

author: tunk
author_rank: Super Contributor
timestamp: June 23, 2025, 10:35:13 am
content: Don't think the ADS1115 is suitable as it's"up to 860 samples per second". And I guessthat you will have to do quite a bit of coding ifyou want use something else than the PicoADC.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 23, 2025, 11:21:35 am
content: Could you kindly tell me what a good rate would be? I'm not entirely sure how to compare ADC units.

author: tunk
author_rank: Super Contributor
timestamp: June 23, 2025, 12:01:44 pm
content: No expert, but I think the Nyquist theorem saysyou have to sample at least twice as fast as thehighest frequency component. In practice you mayneed 5-10 times higher sampling rate. So for 5MHzyou will need 25-50 megasamples per second.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 23, 2025, 08:14:50 pm
content: Thank you. It seems like the AD9248 might be more suitable. If anything it might even be overkill. I can't seem to find a good middle ground for this type of thing. The AD9248 has 20/40/65 MSPS across the three modes; so it would seem fast enough even in the minimum mode. There seems to be a handful of Chinese boards actually available for that part too, but it would be great if I could find something cheaper. $40-60USD is a as expensive as all the other parts in my system combined.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 25, 2025, 10:56:01 am
content: I have come to realize that what I am really looking for is an RF sampler paired up with some sort of analysis software. If I get the AD9248 and can get it working with my system that would give me a solid SDR system capable of operating from >10KHz-1MHz(maybe up to 3) with no up or down conversion. If I can add some conversion then it could be an SDR up into much higher ranges.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 25, 2025, 01:53:32 pm
content: Thanks to its PIO and DMA, you could indeed transfer samples at 65 Msps directly to internal RAM. That part doesn't look problematic.Now even with 2 cores, these are Cortex-M33 @150 MHz (that can be easily overclocked, but still) so don't expect to do much at all in terms of processing at 65 Msps. That's the key issue.And if you want to just use it as an interface between the ADC and say a PC, you'd need to transfer data at 65 MHz x 2 bytes (you probably don't want to pack data on the fly at this speed), which is 130 Mbytes (not MiB) per second. You won't be able to with a RP2350. You'd need USB 3 SS and no way you can implement that with a RP2350, even with an external chip such as the FT600 series.So you could buffer samples up to available RAM, but any processing or transfer would have to be done on just that limited buffer and not in real time. I don't think you can implement SDR this way. Even at just 10 Msps, you'll struggle to do anything useful per sample in real time on a RP2350.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 26, 2025, 12:28:48 am
content: Thanks! Do you think I can use the HDMI trick?https://www.reddit.com/r/raspberrypipico/comments/1gtr20q/stream_up_to_75_mbytes_from_your_raspberry_pi/

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 26, 2025, 01:56:32 am
content: I have also been doing some research on this and I have discovered that the AD9248 does not need an MCU to function. So I am wondering if I can just use a USB-C interface card of some kind to turn the SPI card interface into a type-C; bypassing the MCU entirely. I think the Adafruit FT232H Breakout might work for this, but I am not sure. Does anyone know if this would be sufficient, or if there's another part that would do the job more adequately?