AD9629 input clock design

Hi CrazyFrog,

Thanks for using the AD9629.

• You do not mention what is driving the clock in your current system. What is the drive capability and/or power output of your clock source?
• You said that each ADC has the same clock circuit as shown in your diagram. Is the same clock source driving all four of these, or does each one have its own clock source?
• For clarification, in your new system, are you proposing to use one transformer to drive all four ADCs?

Thank you.

Doug

Thanks you for answering:

My answers just below

- Yes, the ADCCLKIN is an output of a microcontroller, it can output few ma, i guess.

- On my current design, yes each ADC has its own clock source ( one PWB-1-BLC per ADC)

- Yes i would like to use just one transformer for driving all four ADCs.

Regards

Hi CrazyFrog,

Thank you for your reply. I'm still not clear on one detail. In your present system, is there one clock source (one microcontroller output) driving the four transformers, or is each transformer driven by a different microcontroller output?

Thank you.

Doug

Yes there is only one gpio output pin ( of the microcontroller) driving the four clock system (see the picture attached)

Thanks you

Hi CrazyFrog,

From a drive strength perspective, it seems to me that your single microcontroller output will have to supply about the same amount of drive power to one transformer that fans out to 4 loads, as it does driving four transformers each driving one clock load.

So, from a drive strength perspective I think you are OK.

Try to keep your clock signal path as short as possible to reduce the transmission line effects.

If there is an output impedance associated with your clock driver pin, you might want to add pads for a possible matching termination resistor, right before the transformer.

Thanks,

Doug

Hi CrazyFrog,

From a drive strength perspective, it seems to me that your single microcontroller output will have to supply about the same amount of drive power to one transformer that fans out to 4 loads, as it does driving four transformers each driving one clock load.

So, from a drive strength perspective I think you are OK.

Try to keep your clock signal path as short as possible to reduce the transmission line effects.

If there is an output impedance associated with your clock driver pin, you might want to add pads for a possible matching termination resistor, right before the transformer.

Thanks,

Doug

Hi dougl,

I have to respawn this post, as am having some issues from EMC testing. 

It seems that the transformer is ringing too much (at about 500Mgh) even when there is no clock driving it.!!

So I had to add 2 pulldown resistors at the end of the transformer of 10k, on both CLK+ and CLK-. 

This has remove completely the EMC disturbance. 

However, I don't know exactly how... 

Could you please give me your opinion on this? 

I know by adding two pulldowb resistors of 10k it does remove the issues but am not sure if it is safe for the AD9629. 

What value would you recommend? 

Thanks in advance 

Hi crazyfrog,

Where are you observing the clock ringing? Is it before or after the back-to-back diodes?

It's safe however those resistors might attenuate the clock amplitude and band-limit the frequency to the ADC clock inputs. 

Have you tried supplying a clock to the ADC with the 10k pulldown resistors installed? Does it able to source properly the clock and does it able to get a data capture?

Regards,

Mark