[time-nuts] Simple Time to Amplitude Converter

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Dec 28 23:30:26 UTC 2008


Bruce Griffiths wrote:
> 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
>
>   
Equation should have been

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

Where
G is the  TAC gain
E is a small coefficient that accounts for TAC gain mismatch and the
residual voltage on the ADC sampling cap.
offset is a time offset principally due to propagation delay mismatch
between the 2 synchronisers.

Bruce



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