Help me with using FPGA and ADC(with different output volt.)

[doubledribble]

Help of FPGA and ADC

Does anybody has the experience on using the ADC+FPGA?
My problems is: ADC is 5V output, FPGA(Virtex II pro) is not 5v-tolerant, should I use voltage translator? Or can I use seriece resistors in between, how big of resistor?


Thanks

 

[jay_ec_engg]

Re: Help of FPGA and ADC

I have used ADC and ALTERA FPGA ( ACEX1K) on my board.. but while selecting, u have to take care of these things...

anyways... u can use boost up converter... ( 5V to 3.3V) many companies are providing this ICs...like TI, intersil... search there... dont use resistors... its not a good practice..... and before selecting converter IC.. check current specs of FPGA I/O..

 

[Aser]

Re: Help of FPGA and ADC

Xilinx's inputs are protected by resistor and diodes which are switshed on sequentially to Vcc and GND.
Such circuits proide the input current ca. up to 5 mA.
Therefore if you put to the input the 5V source, and Vcc=3.5V then
you have to switsh on the sequential resistor R>=(5-3.5)/5 =0.3kOhm.
Such attachment is proved by Xilinx but only for inputs.
It slightly increases the delay.
Do not do it for outputs.
In this situation I put the intermediate 5V KMOS buffers.

 

[gold_kiss]

Re: Help of FPGA and ADC

I have seen in few circuits boards that the use of Optocouplers solves such problem.
Thanks,
Gold_kiss

 

[doubledribble]

Re: Help of FPGA and ADC

Thank you guys. Since I chose Xilinx Virtex II pro, it is ok to use sequential resistor. The only thing I concern is that how big of resistors.
I read the data sheet carefully, and it doesn't provide input current ca., as Aser said 5 mA, need to figure out what is exact input current.
Also, wht is 5V KMOS buffers, 5V CMOS? I plan to use 74F574SC, is that ok?