[time-nuts] 5 MHZ PIC PPS Divider?
brooke at pacific.net
Wed Apr 16 20:53:44 EDT 2008
The A/D used is only 12 bits @ 300 kHz and Linear now has very similar 12 bit
A/D at 600 kHz and 24 bit much slower. Since a s/h is used to capture the ramp
value the key spec seems to be that the hold time exceeds the conversion time.
Are there s/h circuits that hold for more than 130 ms?
Hard to say what "hold" means when talking about 24 bit conversions. I'm not
sure of how droop would effect the result.
I'm asking because the LTC2400 is about $8 compared to the LTC1273A for $25.
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Bruce Griffiths wrote:
> SAIDJACK at aol.com wrote:
>>did you or anyone every build this LTC application?
>>I wonder about the tempco matching of the reference 1N457 diodes
>>compensating the 2N5771 etc...
> Not this particular implementation but have built similar circuits
> decades ago.
> Tempco isnt an issue.
> There's plenty of time to interleave calibration cycles of say 100ns,
> 200ns, 300ns, 400ns duration between PPS measurements.
> It should even be possible to correct residual low order non linearity.
> With some elaboration is easy to construct a BJT current source with a
> residual tempco of around 100 ppm or so due to the BJT's hfe tempco.
> Replacing the polystyrene cap with a C0G/NP0 cap is probably worthwhile
> especially if one wishes to avoid hand soldering the polystyrene cap
> using heat sinks on the cap leads.
> This technique and variants thereof have been in use for decades in
> nuclear instrumentation.
> Another widely used technique is the Wilkinson converter wher the
> capacitor is charges by a large current during the time interval to be
> measured and the discharge time with a current perhps 1/1000 of this is
> measured using a comparator and a counter clocked at 10 or even a 100 MHz.
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