[time-nuts] Simple Time to Amplitude Converter

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Dec 28 23:24:45 UTC 2008


Bill Hawkins wrote:
> Bruce Griffiths wrote:
>   
>> Poul-Henning Kamp wrote:
>>     
>>> In message <4956D31B.8040809 at kasperkp.dk>, Kasper Pedersen writes:
>>>     
>>>       
>>>> Bruce Griffiths wrote:
>>>> The charge redistribution adc would have been good, true, but there 
>>>> wasn't one in my junkbox.
>>>>         
>
> If it wouldn't be too much trouble, could you change the Subject line?
> This is not about happy days at all.
>
> Bill Hawkins
>
>
>   
To account for charge injection effects from the ADC multiplexer and the
residual voltage on the ADC sampling capacitor as well as the inevitable
gain mismatch between the 2 channels a more comprehensive model of the
TAC's + ADC will be required. To a first order approximation such
effects will be small and linear a model something like

t2-t1 = G*(V2-V1) + e*V1 may suffice.

Where
V2 is the voltage across the second TAC ramp cap as measured by the ADC
V1 is the voltage across the second TAC ramp cap as measured by the ADC


This equation will need to be elaborated slightly when the initial
voltages across the TAC capacitors (during reset) are also measured.
The exact form of the correction will depend on the order in which the
voltage measurements are made.

Ideally the 2 synchroniser circuits would be include the capability of
producing calibration signals of known duration to facilitate
calibration of the gains of the 2 TACs whenever required.


Bruce



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