[time-nuts] ADC sample voting algorithm?

[Bob Camp]

Hi

If you are looking at a high impedance source with a “normal” ADC, you need a buffer amp. 

If you have a signal that decays, it is generally not a good idea to toss all the samples into a 
single bucket. You probably need to do some sort of slope estimation. 

The still better solution is to get an ADC that is not super noisy. Most built in ADC’s on 
MCU’s are quite noisy when evaluated against their claimed resolution level. Even with a few
thousand samples, this may not improve as much as you would wish for. 

Better still, use a converter with a fast enough slope that the ADC noise is not an issue. 

Bob

> On Oct 5, 2016, at 8:22 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 
> 
>> On Oct 5, 2016, at 5:00 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> 
>> Hi
>> 
>> What does the signal you are sampling look like? 
> 
> The last time I actually looked (it was a while ago), it looked reasonable as closely as I could look, but the ADC resolution is something like 1mV per LSB, and I’m not sure I looked that closely.
> 
>> 
>> Does it (maybe) have a bit of noise on it?
>> 
>> If it is the output of a “normal” TDC, then the answer is to sample once.
> 
> I dunno about a “normal” one… This one is the one I got from Jim Harman. The phase difference between the divided-by-10 output and PPS goes through a diode, JFET and then an RC (with a much higher discharge resistor).
> 
> As long as you sample at least 1 µs after the PPS (and it takes at least that long to get into the ISR anyway), it *ought* to be stable for dozens of µs.
> 
> That’s kind of why I’m going down the road of multiple samples - to see if there’s anything to it.
> 
> 
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