How to interface FLASH ADC with FPGA?

Safiqul wrote:
> Hi,
>
> I am trying to measure the RC time constant (exponential decay time) by
> FPGA. I have already done it by micro-controller
How fast is your uC? Is it that uC solution to slow?

> with two comparators.
How fast are your comparators?

> For that, I have choosed Flash ADC (MAX105)
Do you really need an 800MHz ADC? Sounds like "technical overkill"...

> But, I have no idea how to interface between Flash ADC and FPGA.
To connect them you will need some copper wires...

> I need also the counter value that can count the time for each sample.
What language?

> The RC time constant only 100 us.
Thast fairly slow compared with your ADC...   :-o
Because with T=100us you will get about 4.000.000 samples for a 5T 
charge.

Lets do it this way:
tell, WHAT you want to do, not HOW you want to do it.

> 1. Capture the time by counter for each sampling. To get the precision
> points, I need higher ADC clock as well.
What about noise? Any uncertainity of 1 bit will lead you false ways...
And this ADC has much of it:

1

DC ACCURACY 

2

Resolution RES 6 Bits          min    typ  max

3

Integral Nonlinearity     INL   -1   ±0.2    1 LSB

4

Differential Nonlinearity DNL   -1  ±0.25    1 LSB

5

Offset Voltage VOS (Note 2)     -1  ±0.25    1 LSB

> 2. So, for 8 bit Flash ADC, I will get 255 counter values.
But you have a 6 bit ADC...

> Then add all these counter values to get the RC time constant.
Are you sure?

Usually the time constant is to take the charge function of a capacitor 
for a calculation of the tim constant. So, the time to reach a level of 
approx. 63% is the time constant.

> For each degree change, the RC time constant changes only 5ns.
Just the practical way: why not choosing a much more simpler temperature 
sensor?