Deglitching a DAC

On a sunny day (Thu, 19 Jul 2012 14:16:30 -0400) it happened Spehro Pefhany wrote in :

Yes, maybe use the dual gate, as Dr Hombs mentioned, but HC4053 (not 4053) I have used extensively for video, even at higher impedances than 75 Ohm, and had no spikes problem due to charge injection. I think if you did get a few ns spikes, then THOSE can be easily lowpassed, as your signal is max 100kHZ.

Really, a slew rate limiter, or a lowpass filter, would help a lot. The glitch energy is in the 1-2 MHz range, and your DAC update is

I suspect most of Spehro's "glitch" is due to the DAC buffer... I've never seen _anything_ approaching that bad coming straight out of the DAC's I design. But I have an advantage in that my stuff stays on chip... going thru buffers that have to drive capacitance is where you can get hosed. ...Jim Thompson

Don't think so. If it was the buffer it wouldn't be code-pattern dependent. A uniform glitch would not be nearly as much of an issue.

A fast spike glitch from the DAC could disturb some opamps and make things worse, and that would be code sensitive. Most opamps do weird things if you shoot charge into their inputs or outputs. A passive RC section would be a good first pole of a higher-order lowpass filter.

A 3-pole Sallen-Key Bessel filter would be good. The odd pole is passive, so put it first. An S-K is good because the DC gain is 1.000 and doesn't depend on resistor values or TCs. PPM settling will of course be an issue, not just in the filter but everywhere in your signal chain. I usually have transient-response tweaks in my NMR gradient drivers to get to PPM settling in a hundred microseconds or so.

Microdyne had glitch problems in one design, so every board had to be tested though the full range of steps while the tech watched the sawtooth waveform.

Bad engineering can produce employment opportunities, if only in performing rework and installing kludges.

The odd pole is usually first anyway. But it won't help. Speff needs

I think the problem cropped up after the original part went from ceramic to plastic dip. Once the test was part of the process, it was never removed. I never saw a bad DAC on the control boards I tested in a four year period. It was the oldest product still in production, and some parts had to be bought form brokers, so I figured they were afraid some old parts would surface. Maybe even the ones they had sent back to the OEM. The test only took a couple minutes, and we probably built 75 of that obsolete item a year so it was no budget breaker.

Maybe he's seeing an opamp disturbance thing, from charge injection into the opamp front end. Any time a CMOS switching thing connects to the input of a bipolar opamp, expect troubles. IF that is part of the problem, a model that assumes a voltage source glitch followed by a filter isn't the real story.

Nope, it's from the internal switches. I got that straight from the designer. It's a combination of the architecture and the voltage range. Big switches to stand the high voltage, and (relatively) high impedances.

It doesn't have to be bipolar. Overdriving the input of most op amps can cause them to go nonlinear and produce serious recovery problems. It's the same as not feeding RF to the input where it can be rectified, not putting a capacitor from the negative input to ground, not driving a capacitor at the output, and so on. Be we already know these things. The point is your suggestion of a 3-pole Sallen-Key filter won't work.

Regards,

Mike

I got the box in the mail. Thank you for the DACs.