Design help

Hi Randy,

I am not familiar with AD9833, however, according to the datasheet, LTC6930 output is compatible with the MCLK input of AD9833.  Since the interface between AD9833 and LTC6930 is CMOS logic. I recommend placing a 30 Ohm resistor closer to the LTC6930 between the output of the LTC6930 and the input of the AD9833

 If synchronization is important between motor drivers, I recommend giving attention to the layout planning. If not, these distances are fairly small for 32KHz CMOS signals, There should not be a problem with the propagation of the signals. Depending on the input impedance of the motor drivers, a 1:2 buffer might be needed to drive multiple drivers from AD9833. 

One recommendation from my side would be about clock accuracy. What are your clock performance requirements? Did you compare the EPSON CMOS oscillator performance and LTC6930 performance? 

LTC6930 is a silicon oscillator with a fairly good frequency accuracy and temperature coefficient, However, it might not catch the performance of a good crystal oscillator.  I added the clock accuracy vs temperature graph.

Thanks,

Emrecan

Hi Randy,

I am not familiar with AD9833, however, according to the datasheet, LTC6930 output is compatible with the MCLK input of AD9833.  Since the interface between AD9833 and LTC6930 is CMOS logic. I recommend placing a 30 Ohm resistor closer to the LTC6930 between the output of the LTC6930 and the input of the AD9833

 If synchronization is important between motor drivers, I recommend giving attention to the layout planning. If not, these distances are fairly small for 32KHz CMOS signals, There should not be a problem with the propagation of the signals. Depending on the input impedance of the motor drivers, a 1:2 buffer might be needed to drive multiple drivers from AD9833. 

One recommendation from my side would be about clock accuracy. What are your clock performance requirements? Did you compare the EPSON CMOS oscillator performance and LTC6930 performance? 

LTC6930 is a silicon oscillator with a fairly good frequency accuracy and temperature coefficient, However, it might not catch the performance of a good crystal oscillator.  I added the clock accuracy vs temperature graph.

Thanks,

Emrecan

Hi Emrecan-

emrecangidik said:

30 Ohm resistor closer to the LTC6930 between the output of the LTC6930 and the input of the LTC6930

May I assume you meant the input of the AD9833?

emrecangidik said:

There should be a problem with the propagation of the signals

Is that "should NOT be a problem"?

The steppers operate independently, so sync is not an issue.

The device is in a laboratory environment at room temperature, so we are in the "sweet spot" according to the temperature graph.

Below is a schematic based on your suggestion. Please verify that I understand your meaning.

Thanks for responding,

Randy

Hi Randy,

My apologies for the typo. I corrected them. You are right. 

Your diagram looks good. It works as expected. However, I would remove the RA and RB and fan out the output signal after a series resistor. I added a diagram to explain.

Thanks,

Emrecan

Emrecan, Unless you are matching the trace length of MCLKA and MCLKB, it is better to use two separate resistors as originally shown. (And even if you do match lengths, it is still better, although the difference is minor.)