The Mute Audio pupDAC
The primary tweak that one can perform on the pupDAC is the output opamp selection. The default BOM and Beezar kits include the Burr-Brown OPA2835. This is a very low distortion, ultra low-power, rail-to-rail opamp. The prototype pupDACs used the National Semiconductor LMH6643 opamp. TI recently purchased National Semi. They had purchased Burr-Brown many years ago. So both opamps are actually owned and produced by Texas Instruments currently. TI is also producing a new opmp, the OPA2836. It offers similar specs as the OPA2835, but with a much higher bandwidth (205MHz vs. 56 MHz). We haven't specifically tested it, so be sure to have a back up LMH6643 or OPA2835 handy if it doesn't work well. Of course, there may be other opamps that will work, but again - we haven't tested them. If you do and they perform acceptably, let us know and we'll add them to this page.
Update: The OPA2836 has been tested in a production version of the pupDAC. Offset is negligible and performance is outstanding! The OPA2836 is highly recommended!
Besides making certain to select a dual opamp, packaged as 8-pin SOIC for the pupDAC, one must consider the voltage requirements and the ability of the opamp to swing as close to the power rails as possible. Being solely USB-powered, the pupDAC is limited to 5VDC maximum. The actual voltage supply for the opamp is completely regulated with both negative and positive TPS low dropout (LDO) regulators at -2.5 and +2.5VDC. This is possible because the USB power provides up to +5V (regulated down to +2.5VDC by U10) and -5V is provided by the charge pump, U7 (regulated down to -2.5VDC by U8). So with +or- 2.5VDC available, one should ideally select an opamp that can use that voltage swing with as small a voltage floor as possible (ability to swing "rail-to-rail").
Here's a quick look at the voltage and amperage specs of the OPA2835, OPA2836, and LMH6643:
OPA2835 - 56MHZ, 2.5V to 5.5V supply voltage, 160V/us slew rate, 0.00003% THD, 0.85mA continuous input current, 40mA output current
OPA2836 - 205MHZ, 2.5V to 5.5V supply voltage, 560V/us slew rate, 0.00003% THD, 0.85mA continuous input current, 50mA output current
LMH6643 - 130MHz, 2.7V to 10V supply voltage, 130V/us slew rate, 0.079% THD, 2.7mA continuous input current, 75mA current drive
An important caveat to selecting opamps - there are no output capacitors on the pupDAC, so offset is a concern, as is oscillation. We are confident that with proper construction, offset with the OPA2835 should be 1 or 2 mV at most, and with the LMH6643. Oscillation is also not present with eaither opamp. However, this result may change with different opamps. Offset can be easily checked with a DMM. As for oscillation, two things can help you pinpoint this if you don't have an oscilloscope: 1) the opamp may run hot (to the touch), and 2) you may notice a false sense of detail in the highs and a noticeable absence of bass. Please be aware of these potential faults. If you choose to use a different opamp than those listed here, carefully check the offset voltage of your pupDAC output and critically listen to the sound, .