[sdiy] DAC selection in MIDI-CV Part #1

[Paul Schreiber]

I'm designing one at the moment (16 channels + 4 gates).  It will be based around a PIC and
MAX525BCPP 12bit D/As.  These Quad-D/As are relatively cheap, but accurate enough (+/- 1 LSB) for
pitch control over a full 10V scale.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

No, they are NOT! (speaking as Maxim's DAC FAE).

Although I've discussed this in the past several times (check archives), I'm doing some DAC apps
now (on the older MAX502 parallel 12-bit DAC), it's time for a ADC/DAC refresher course :)

Here is a DAC FAQ.....

a) What errors are in DACs, and what errors can I control?

There are MANY sources of error in DACs and their 2 main "outside" components: the voltage
reference and the buffer amp. You *must* do a full "system level study" because all the errors
add up, and you have to address ALL of them.

The first thing to address is simple but often overlooked: what voltage accuracy do I need. This
has NOTHING AT ALL to do with the number of bits in the DAC! This has to do with the VCOs. What
ideal voltage does the VCO need to receive in order to be on pitch (assuming the VCO is
'perfect')?

At 1V/octave, a full step is 1/12 or 83.333mv. If we desire 1% of that (pretty much the limit of
pitch perception), then the voltage fed to the VCO out of the DAC system can not have an error of
more than 833uv. That's 833 MILLIONTHS of a volt. It doesn't matter if we are in the 0-1V octave
or a 9-10V octave, the DAC system MUST be able to 'hit' a voltage within 833uv.

Setting the DAC chip aside, let's look just at the op amp buffer (this may be inside the DAC chip
in some parts). The critical spec here is the op amp's Output Offset Voltage Drift spec. We
really don't care what the initial offset voltage is (it's a constant), but, it had better not
drift around over temp or the VCOs drift. The best op amps have drifts of 5uv/C. or better. Be
sure yours does (like the MAX427 or MXL1013, used extensively in MOTM, see www.maxim-ic.com for
datasheets and FREE samples).

The next thing to worry about is the temp drift of the reference. The reference may be internal
or external to the DAC. As a rule, external references are *much* more accurate and stable. There
are again 2 specs of concern: accuracy and drift. Accuracy is part of the overall "voltage
offset", as the output of the DAC is some fraction of the voltage reference (DACs work by
precision dividing down the reference). Drift is in PPM (Parts Per Million) per degree C. A 5V
reference with 5PPM drifts 1uv/C. Choose a reference that has 50PPM or better (the same as a
really good 0.1% resistor). Get 10PPM and be done with it :)

Now it's time to look at DAC errors (Part #2).

Paul S.