@@ -60,7 +60,7 @@ Or back to board drawing, as it were. Anyway, I wanted to make a few changes bef
- Switch to a single-ended op-amp configuration
- Add mounting holes and pads for an external audio jack and volume knob
The first is just good practice for microcontrollers. I added 0.1uF caps near all the XMEGA's power pins, and a few 10uF caps near potentially large current sinks (i.e. LEDs and op-amps). I had added the 22ohm resistors because all the [examples](../../../../../863.17/CBA/people/tomasero/index.html) I [looked at](../../../../../863.14/CBA/people/andrew_mao/week11) had them. But after going through the XMEGA's datasheet in more detail, I found that it has an "integrated on-chip USB transceiver, no external components needed". I wanted to go to a single-ended op-amp configuration to make it simpler to add a potentiometer to control gain. With my current differential setup, I would have to use a double-ganged pot, and if the two resistances didn't stay exactly the same at all times there'd be additional error in my output. So I'll just connect one side of the input to ground, and send the other through an AC-coupled non-inverting amplifier with a DC bias. Finally, the gain control knob and audio jack will be better off mounted to the chassis of the device rather than the board, so that the board doesn't bear any unnecessary mechanical load. It would be nice to attach the USB jack to the chassis as well, but the USB jacks we have in stock are surface mount, and since it's good to keep the data leads as short as possible, I'll leave it on the board.
The first is just good practice for microcontrollers. I added 0.1uF caps near all the XMEGA's power pins, and a few 10uF caps near potentially large current sinks (i.e. LEDs and op-amps). I had added the 22ohm resistors because all the [examples](../../../../../863.17/CBA/people/tomasero/index.html) I [looked at](../../../../../863.14/people/andrew_mao/week11) had them. But after going through the XMEGA's datasheet in more detail, I found that it has an "integrated on-chip USB transceiver, no external components needed". I wanted to go to a single-ended op-amp configuration to make it simpler to add a potentiometer to control gain. With my current differential setup, I would have to use a double-ganged pot, and if the two resistances didn't stay exactly the same at all times there'd be additional error in my output. So I'll just connect one side of the input to ground, and send the other through an AC-coupled non-inverting amplifier with a DC bias. Finally, the gain control knob and audio jack will be better off mounted to the chassis of the device rather than the board, so that the board doesn't bear any unnecessary mechanical load. It would be nice to attach the USB jack to the chassis as well, but the USB jacks we have in stock are surface mount, and since it's good to keep the data leads as short as possible, I'll leave it on the board.
