[time-nuts] TIC model

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Feb 20 13:43:07 EST 2014


For a 10MHz synchroniser clock  A C1 value of around 220pF or so should 
be appropriate.
The exact value depends on the ADC reference voltage.
A n ADC reference less than 5V may be useful.
I'll run some simulations to check the sensitivity to R2's tempco.

Bruce

Bob Stewart wrote:
> Now you've lost me.  What 2.5 MHz synchronizer clock?  Everything I have external to the PIC is 10MHz.  The PIC is running HSPLL at 40MHz, though I don't think that makes any difference to this.
>
> Bob
>
>
>
>
>    
>> ________________________________
>> From: Bruce Griffiths<bruce.griffiths at xtra.co.nz>
>> To: Bob Stewart<bob at evoria.net>; Discussion of precise time and frequency measurement<time-nuts at febo.com>
>> Sent: Thursday, February 20, 2014 3:07 AM
>> Subject: Re: [time-nuts] TIC model
>>
>>
>> R2 is dominated by the adc sample switch on resistance and thus has a
>> relatively high  tempco (~4000ppm/C).
>> C2 has a relatively low tempco (~100ppm/C or so)
>>
>> To reduce the effect of the sample switch on resistance tempco on the
>> gain tempco of the TIC R1 C1 need to be proportioned so that R2 has
>> little effect on the gain temcpo.
>> R1 = 470 ohm, C1 = 1nF (NPO) appears to be about right for a 2.5MHz
>> synchroniser clock and the PIC you intend to use.
>>
>> This should reduce the effect of the sample switch on resistance tempco
>> by a factor of 10 or more.
>>
>> The minimum value of R1 is governed by the output resistance of the
>> tristate buffer and its tempco.
>>
>> Bruce
>>
>>
>>      
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