Hi, I am trying to measure the RC time constant (exponential decay time) by FPGA. I have already done it by micro-controller with two comparators. But now, I want to do it by FPGA and want to use VHDL. For that, I have choosed Flash ADC (MAX105) and FPGA - ACTEL ProASIC3E (A3PE1500). But, I have no idea how to interface between Flash ADC and FPGA. I need also the counter value that can count the time for each sample. The RC time constant only 100 us. Please help me to find out this! Thank you!
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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.
Hi, Thanks for your step by step comments :D! In my system, RC time constant represents the temperature constant. For each degree change, the RC time constant changes only 5ns. That is why, I have chossed that ADC. I want to do it by
1 | VHDL code |
: 1. Capture the time by counter for each sampling. To get the precision points, I need higher ADC clock as well. 2. So, for 8 bit Flash ADC, I will get 255 counter values. Then add all these counter values to get the RC time constant. That what I want to do. Please help me!
> 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?
As far as I understand, you simply want to measure the temperature? Why not taking a integrated temperature sensor, which does everything for you? e.g. LM73 with I2C interface or LM74 with SPI interface. They are easy to access with a microcontroller or an FPGA. And the resolution is mostly sufficient.
You need to know how to develop digital Hardware (Printed Circuit Boards).(FlipFlops,and,or,not,nand,nor,xor,counter,muxes,latches,adder,s tatemachines) And you have to know how you combine them. You have to know what are Setup times,hold times, Signal runtime and Routing, Clock domains ... This is NOT C++ even if looks similar like Pascal VHDL is not a programming language.
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