PIPPYG 3.0
The goal with pippyg 3.0 is to get more useful recording time from a set of batteries. This is not achieved by burning less power, but by allowing the device to keep running when the batteries are almost flat. Version 2.7 stops recording when the batteries droop to around 3.0V, at the point that the LDO powering the SD interface drops out. Version 3.0 should run until the battery voltage is much less than 2V, powered by the Pico buck-boost. Fingers crossed, that might be half a day of run-time, maybe less, maybe more, depending on the batteries in use.
There's no such thing as a free lunch though, and the price of this is more pressure on the Pico buck-boost - on 2.7 the buck-boost only powers the Pico and the tiny dribble of current needed for the analog circuit, now it needs to also supply the surging current demands of the SD card as write cycles occur, and these surges will generate noise within the Pico itself and across the PCB. To try to combat this noise the filter on the analog supply is butcher this time round, it should suppress noise on the supplly rail by 63dB at 20kHz, which should map straight onto improved noise in recordings. Also the supply return from the SD card is straight into the battery, so it shouldn't disturb the GND plane, but the Pico still has to move from not much current to a couple of hundred mA as the SD card kicks in, and any attempt to analyse this is deep into analog black arts.
UPDATE : have built 5 of these, they appear to be both lower noise AND better battery life than 2.7, I will do a side-by-side test over Christmas and see how many hours I can get out of each.
Here's the bottom layer of the PCB showing the approach to managing current surges and supply noise.
A is the ground pour for Pico.
B is analog ground - and the topology of the op amp circuit means the loop from stage 1 output to stage 2 input simply HAS to hop from the top plane to the bottom, hence the annoying thin track half way up
C is SD ground, with its own private supply return to the battery -ve pin. The analog circuit has its own private 3v3 which is more like 3v2, it's the Pico output passed through the LPF with 3dB point at 16Hz.
There copper pour all over the top surface too, but it's covered with tracks so is far from unbroken.
Parts from DigiKey (to build 5) : https://www.digikey.co.uk/en/mylists/list/2SSR4C9UXO
PIPPYG 3.0
*PCBWay community is a shared platform and we are not responsible for any design issues.
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- Pete Lawrence Jan 22,2024
- ioanf Dec 29,2023
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