Why I Hate Analog Circuits

Published on 2017-10-10 in Trinket Shields.

I’m completely hopeless with analog circuits. I think part of it might actually be a self-fulfilling prophecy — I know that I’m bad at it, so I’m not even trying. Usually I just try and copy any analog pieces I need from other projects, cargo-culting them and hoping they will work. A case in point: the audio shield.

Simplicity itself. A small audio amplifier, connected through a resistor and capacitor to the DAC pin on one side, and to an SMD speaker on the other. What could possibly go wrong?

First of all, I re-used a footprint for the PAM8301 amplifier, using a part for some sparkfun’s DAC chip. It’s exactly the same size, I just changed the labels on the pins. What I failed to note is that the pins on that chip are numbered in the opposite direction than on my amplifier. So to start with, I had to bend the pins of the chip the other way and dead-bug it. Fine.

Next thing I am a little less sure of. I mean, I am sure it doesn’t work, but I’m not sure why. If I connect a piezo to the DAC pin directly, I get the sound that I’m playing (although very quietly). If I use the amplifier (with a 47kΩ resistor and 0.1µF capacitor between the pin and the input of the amp), I get nothing. And it gets warm. Sounds like it’s simply connected wrong, but I verified the PCB and the datasheet a million times already.

I must be missing something obvious, here’s the schematic so that you can laugh at my ignorance:\

Oh, and yes, I copied it. From Adafruit’s Circuit Playground Express board, which also uses a SAMD21 chip and a very similiar SMD speaker.