On Dec 2, 2008, at 12:25 AM, Bruce and PHK wrote: >> <clip> >> There was a recent Design Idea in EDN or Electronic Design (don't >> remember >> which one) where someone claimed 32 bit resolution out of two 16 >> bit dacs. That >> claim is ridiculous of course due to noise and matching etc, but >> you can >> probably get 20 - 24 bits resolution and accuracy out of a >> cascaded system with >> some care. >> >> bye, >> Said >> > If you build this circuit using the values shown in the schematic you > won't actually achieve 20 bits performance. > One of the resistor values is incorrect. > The resistor matching requirements aren't anywhere near as > stringent as > one may think on first glance. > With some subtle alterations to the operating mode the resistor ratio > matching tolerances can be relaxed considerably. > In closed loop digital control systems the circuit noise should be at > least 1LSB or so to maximise performance. > The trick is to increase the DAC resolution until this condition is > achieved and not increase the noise to meet the condition. > 20 bit resolution performance is easy to achieve, 24 bit performance > requires 2 a little more work. > > Actually monotonicity to about 26 bits was claimed with noise a bit > below 1ppm. > However since the noise varies with the DAC output such claims are > perhaps a little too simplistic. > The noise can be reduced when required by using a better reference. > Any drift in DAC gain and offset could be compensated by a Kalman > filter > should they become significant. > > The major advantage of such a DAC is the inherent monotonicity which > cannot be achieved and maintained (around coarse DAC transitions) > without frequent calibration when the outputs of 2 16 bit DACs are > combined. > > Bruce
Bruce, is there a way for non-subscribers to see this article? Dick Moore > ------------------------------ > > Message: 8 > Date: Tue, 02 Dec 2008 08:25:00 +0000 > From: "Poul-Henning Kamp" <[EMAIL PROTECTED]> > Subject: Re: [time-nuts] V standards > >> Coarsedac changes are an issue as you mentioned, but one can get >> around them >> by scaling the two dacs in a way to only use a couple of bits from >> the >> coarsedac, and operate most of the time using just the fine dac. > > If you are controlling this with a microcontroller, there is an > alternative > you should consider: > > Use only a single DAC and then PWM modulate its output, followed > by a low-pass filter. > > -- > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > [EMAIL PROTECTED] | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by > incompetence. > Poul, how do I PWM a DAC that is itself a PWM device? This question is prompted by the DAC I'm using in Bert Zauhar's FLL GPSDO, which is a dithered 10-bit DAC in a PIC chip that results in 14-bit resolution. I'd like to have a simple way to get 2 to 4 extra bits of resolution out of this. Can you recommend some design resources to do this? Dick Moore _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
