Single supply lowpass filter for analog inputs

lokki

I would like to sample a bass guitar signal with one, eventually 4, of the analog inputs (not audio inputs). I am using small humbucking pickups (the ones found in the GK pickups) and need to amplify, lowpass and bias the output in order to read it via analog in. Are there any recommendations for single supply opamps that work well with belas 5v? I looked at the lct1062 as active lowpass.

Any hints are welcome.

giuliomoro

The audio expander capelet has exactly the circuitry you are looking for. Look at the schematics and check out the actual part in use listed in the BOM https://github.com/BelaPlatform/bela-hardware/tree/master/capelets/audio_expander/audio_capelet_A2

You may also consider getting the audio expander capelet itself (for Bela) or the Bela Mini Multichannel Audio Expander (for BelaMini), where the latter gives much better quality as it uses a proper audio codec for the additional channels. Note that the Bela Mini Multi (for short) can in principle be used directly on top of a PocketBeagle without a BelaMini, in case you only care about audio inputs and don't need any analog inputs.

lokki

thanks, it seems i cannot find the add8704 at mouser anymore.

the BOM mentions OPA4348AIPWR but they are on backorder...64 weeks. jeez.

what is the non connected jumper on the input stage (the one that connects r2 to pin2 for example)? optional gain?

if i were to use the audio-expander, i don't suppose it is easy to mod it for more analog gain and lower cutoff on the lowpass. basically i need four inputs with cutoff frequency at

E: 131HZ
A:175HZ
D: 233HZ
G: 311HZ

EDIT: and the same question goes for the bela-mini-multi:

any analog gain stage that i can use? and i guess i would still need to create a lowpass filter upfront.

although, maybe with a higher quality signal i could do that digitally...

giuliomoro

lokki what is the non connected jumper on the input stage (the one that connects r2 to pin2 for example)? optional gain?

yes, see https://learn.bela.io/products/accessories/audio-expander/#using-the-jumper-wires

lokki if i were to use the audio-expander, i don't suppose it is easy to mod it for more analog gain and lower cutoff on the lowpass.

uff, needs to change two resistors and two capacitors per channel ... are you any good at SMD soldering?

lokki EDIT: and the same question goes for the bela-mini-multi:

there is no input filtering there, as that's done by the codec via sigma-delta modulation.

lokki any analog gain stage that i can use?

How much gain do you need? All the inputs on the Mini Multi are essentially the same as on the Bela / BelaMini cape, so you could try it out with some of those you already have, and the input noise performance will be similar for the Mini Multi, too. There is the PGA gain that you can adjust on each channel as you need.

lokki although, maybe with a higher quality signal i could do that digitally

that's what I thought, yes: sample with audio ADC and then apply any filter you like without soldering 🙂

lokki

giuliomoro thanks for your in depth answer. I will try with my belas here and let you know. Fun fact: I will be in London Sunday..but you're not there anymore 🙁

lokki

giuliomoro there is no input filtering there, as that's done by the codec via sigma-delta modulation

So could this be adjusted?

giuliomoro

no (I mean yes, but you shouldn't have to do that), but you can apply any digital processing you want to the full-bandiwdth signal to filter it as needed.

lokki

giuliomoro yes, got it. Just thought I'd ask if there is a "free" (as in no additional processing power) lowpass...

giuliomoro

you can set the filter coefficients for the ADC filter, see 10.3.3.2.1 here https://www.ti.com/lit/ds/symlink/tlv320aic3104.pdf ...

lokki

Interesting, there is even an AGC I could potentially use...

lokki

ok, i've got plenty of gain with the pga on the bela-mini. also the quality is sufficient for my needs. so i guess i'll buy a bela-mini-multi. thanks!