[General] How to calculate bandwidth needed high speed ADC with Serial Output

[amartin32]

I want to build a analog to fiber optic link and am currently looking into using parallel ADC's with Serial outputs and then sending the serial information to a serial to fiber optic link.

Being a newbie, I do not quite understand how to calculate the amount of bandwidth I need for my system to work. I would like to transmit 4 analog signals with a sampling rate of 250 kHz and a 12 bit resolution.

Then the bit rate is 12 Mbps? (Sampling Rate needed in Hz*12 bits*# of Channels= bits per second)

From my understanding the bit rate calculated for the ADC must be 12 Msps?

I am also confused as what definition and calculation of bandwidth I should use in the context of this system?

Would this Evaluation board be used for I would like to do?
Link: **broken link removed**

I know that the Serial information is sent in the form of zeros and ones but there is no carrier frequency in serial transportation therefore I do not understand where bandwidth would come to play. Also does trouble shooting a serial transmission involve using an eye diagram?

I greatly appreciate any help.

 

[FvM]

ADS5242 is a high speed ADC, it can be operated at a minimum sample rate of 20 MSPS. So it's probably not the device you want.

From my understanding the bit rate calculated for the ADC must be 12 Msps?

A bit rate will be measured in BPS respectively MBPS. 12 Megabit/s is the net bit rate for the assumed digital data transmission, but you will need some kind of framing and possibly clock embedding (in case of a synchronous serial protocol) to transmit the data over a single channel like an optical fibre.

 

[amartin32]

So then the maximum sampling rate for each channel would be 1.4 Mega Samples per Second?

I believe the ADS5242 has a bit clock and a word clock are also transmitted with the serial transmission. Is the same as the clock embedding or do you mean having a external clock?

Thanks for your very fast reply!

 

[ads-ee]

Then the bit rate is 12 Mbps? (Sampling Rate needed in Hz*12 bits*# of Channels= bits per second)
I would like to transmit 4 analog signals with a sampling rate of 250 kHz and a 12 bit resolution.
From my understanding the bit rate calculated for the ADC must be 12 Msps?

What you want is a four channel 250 KHz ADC with serial outputs connected to perhaps a small FPGA/MCU . The ADC's individual sampling rates are at 250 KHz not 12 Msps. The raw data bandwidth sent to the optical channel is 12 Mbps (Mega-bits per second, not Mega-samples per second)

I know that the Serial information is sent in the form of zeros and ones but there is no carrier frequency in serial transportation therefore I do not understand where bandwidth would come to play. Also does trouble shooting a serial transmission involve using an eye diagram?

Your confusing data bit rate with bandwidth. I imagine the optical fiber transmitter has some input clock for serial data and it's probably significantly higher than the 12 Mbps of data you have to send. So a simple approach would be to send it similar to RS-232 with start and stop bits, and the destination has to over sample the signal to recover the data and find the center of each bit. Or you could take a more complicated approach and use 8b10b encoding to allow embedding the clock in the data along with control characters that can then signal start, stop, idle, and framing. You could then either continuously oversample and adjust the sample point or use clock recovery on the data at the receive end.

Depending on which method you use you will see significantly different overhead in the transmitted bit rates.

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So then the maximum sampling rate for each channel would be 1.4 Mega Samples per Second?

I believe the ADS5242 has a bit clock and a word clock are also transmitted with the serial transmission. Is the same as the clock embedding or do you mean having a external clock?

Thanks for your very fast reply!

Not even close, see #4. The sample rate of the ADC is 250 Ksps (kilo-samples per second). The bit rate (nothing to do with sample rates) to transfer the data from a single ADC has to be greater than 12*250 = 3 Mbps (Mega-bits per second). Any serial interface ADC likely allows upwards of 10-20 Mbps transfers over something like a SPI interface to a MCU. The example ADC I posted in #4 probably allows higher transfer rates since it's a 300/400 Ksps ADC.

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Actually the ADC I posted has only a 6.5 MHz SPI so it's not really a good choice for a data acquisition as you can't get all the samples out. You might have to do some searching to find a suitable part. I did notice Maxim has another part which allows for 25 MHz SPI but that part is only a 225 Msps ADC.

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You should really consider using a 4 channel parallel interface ADC, that is more typical of a data acquisition ADC as you don't have to run the interface nearly as fast to get all the samples out. Though then you'll have to deal with sending the data serially to a optical transmitter unless you use one that also has a parallel interface (haven't work with them so don't know if that's feasible).