Is it suitable to use ADA4930 to drive AD9257？

Hi DPFATE_DP,

Thanks for your interest in AD9257.

As far as your circuit is concern, the ADA4940 is suitable as long as the VOCM requirements and frequency of operation is below the bandwidth limit of the ADC driver. The Vpp of 1.56V is I think normal, a minimal loss is inevitable.

With regards to AD9257 inputs, kindly probe the DC voltage of AD1_VINx+/- if it matches to 0.9V as per datasheet.

I am just looking on where your AD1_VCM is connected to because you claim that it is connected to ADA4940 VOCM from AD9257. If this is true, it should be that AD1_VCM is tied to both ADA1_VOCM1 and ADA1_VOCM2. 

I attached some articles for your reference. 

CN-0252 (Rev. A) (analog.com) (Note: Though this is from AD9265 and ADA4930 but it will help you visualize the role of VOCM.)

"Rules of the Road" for High-Speed Differential ADC Drivers | Analog Devices

Regards,

Xavier

Hi Xavier.

I have another question. In actual applications, the VIN- signal comes from the DC voltage output of the AD5628, and the VIN+ signal comes from an external signal source, which changes approximately every 10us with a peak-to-peak value of up to 1.5V and a DC signal of roughly 3V. In this case, is impedance matching necessary?

Thanks!

DPFATE_DP

Hi DPFATE_DP,

From AD9257 perspective, the VIN+/- of the ADC should be within Vpp <= 2.0V and VCM of around 0.9V. I am more worried of DC signal of 3V at VIN+ that will absolutely trigger the ESD circuit of the ADC causing damage to AD9257 since it is beyond the Absolute Maximum Ratings.

For ADC front-end, yes, impedance matching is necessary. Thank you.

Regards,

Xavier

Hi DPFATE_DP,

From AD9257 perspective, the VIN+/- of the ADC should be within Vpp <= 2.0V and VCM of around 0.9V. I am more worried of DC signal of 3V at VIN+ that will absolutely trigger the ESD circuit of the ADC causing damage to AD9257 since it is beyond the Absolute Maximum Ratings.

For ADC front-end, yes, impedance matching is necessary. Thank you.

Regards,

Xavier

Hi Xavier

Thank you. Does AD9257 have any requirements for the power-on sequence and register configuration sequence? Because even if I do a chip reset through register 0x08, sometimes the power-on state is still wrong.

Thanks!

DPFATE_DP

Hi DPFATE_DP,

How did you able to conclude that the power-on state is wrong? Did you notice errors at the output? Can you elaborate more on this?

Regards,

Xavier

Hi Xavier

The waveforms acquired by AD9257 may be distorted or have obvious spikes, and sometimes restarting it will make it normal. Or after powering on, the waveforms acquired at the beginning are chaotic, and later it will be normal.

Thanks,

DPFATE_DP

Hi DPFATE_DP,

Can you share a scope shot with the issue? Did you ensure that the clock inputs and analog inputs are clean?

Regards,

Xavier

Hi Xavier,

Because I didn't reserve test points on the PCB board, it is difficult to capture the issue of the input signal. Therefore, I intend to make sure that my register configuration order for the AD9257 is correct first.