[volt-nuts] Homebrew LTZ1000 reference?

[Didier Juges]

I think one reason it is expensive is because the simple "digital comparator" requires the composite DAC (the combination of the two 16 bits DACs considered as one device) to be monotonic. This is done by using two pretty good 16 bit DACs and having a significant overlap range to smooth out the differences.

If you can deal with longer settling time (milliseconds instead of microseconds), you can use a microcontroller to perform the digital comparison. Your code can correct for non-linearity in the composite DAC (via calibration, and/or real time closed loop correction) and reduce cost significantly. You may be able to use two 12 bit DACs for instance, at the expense of more software to develop and a slower response time, but much lower cost in production (you can buy a dual 12 bit DAC for a few $). You may need a sample and hold to reduce transients on the output while the circuit does its job, if that could be a problem with your application, or use a large (noise-free) capacitor :)


I built a breadboard for something similar to that a while ago using the 24 bit ADC in a Texas Instrument MSC1210 microcontroller and two available 8 bit DACs, to give me about 12 bits of linearity, as a proof of concept.TI advertises the MSC1210 as a smart ADC, because it integrates a
pretty good 24 bit ADC with an 8051 microcontroller and 32k of flash... The idea was to expand the concept later to 20 bits using two 12 bit DACs to drive the OCXO of a GPS disciplined oscillator. I never finished the work, but conceptually it was simple because a GPSDO does not need speed.

For metrology, where response time is not typically as critical as in industrial applications, or for something like a GPSDO, that may be a valid alternative.

Didier


----- Original Message ----
> From: Randy Scott <scottr9 at yahoo.com>
> To: Discussion of precise voltage measurement <volt-nuts at febo.com>
> Sent: Thu, November 19, 2009 9:59:52 AM
> Subject: Re: [volt-nuts] Homebrew LTZ1000 reference?
> 
> > Linear Technology has an application note
> > on this very subject:
> > 
> > cds.linear.com/docs/Application%20Note/an86f.pdf
> 
> Has anyone here actually built the 20-bit DAC described in the app note?  I've 
> been thinking about it and actually started laying out a PCB.  I am hesitant to 
> pull the trigger, given that the DAC ICs that I was planning on using (LTC1595 
> in SO package) are $48 each and the design requires two of them.
> 
> Randy.
> 
> 
>       
> 
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