ADS4249: Technical Inquiry about ADS4249

Hi

    Due to the differential input the kickback does not affect the ADC sample precision.Is this right?

    Reguards

    Aaron

Hi Rob

    The tipical output voltage value of VCM(ADS4249 PIN23) is 0.95V,but the VCM output voltage is 0.978v on our pcb board.Is this voltage of 0.978v normal?What is the output voltage range of the VCM?Will the diffierence between the actual output voltage(0.978V) of the VCM and its theoretical value(0.95V) affect the ADC measurement results?

Reguards

Aaron

Hi Rob

We use the ADS4249 to acquire signals and encountered some issues during the process of configuring its registers.

1:The ADC was configured in High-performance mode, High-frequency mode, and High-speed mode (with the following register settings: write 03h to register 03h, write 01h to register 4Ah,write 01h to register 58h, write 40h to register 02h, write 18h to register D5h,write 0Ch to regiser D7h, write 20h to register DBh). We measured the voltage of  Vcm is 0.903V (not the expected 0.87V) . When applying a 0.5V VPP(±0.25V) sine wave input, the maximum value ADC output is 2095 and the minimum value ADC output is -1945.

2:We attempted offset correction by writing 2Ch to register CFh and 20h to register 3Dh. After 9 seconds, ACh was written to regiser CFh.
The measured Vcm remained at 0.903V (not the expected 0.87V).With a 0.5V VPP(±0.25V)  sine wave input, the maximum value ADC output is 2095 and the minimum value ADC output is -1945.

3:We attempted to modify the channel gain by writing C0h to register 25h and C0h to register 2Bh.When applying a ±0.25V sine wave input, no observable change was detected in the ADC output values.

4:We attempted to adjust the offset pedestal by writing 80h to register BFh and 80h to register C1h.With a ±0.25V sine wave input applied, no measurable change was observed in the ADC output values.

Please assist us in analyzing the following issues:

1:Under the configured mode, the voltage of the VCM should be 0.87V. Why is it measured 0.903V on the circuit ?

2:For a ±0.25V input, the expected output should be ±2000. Why are we observing larger positive values (+2085) and smaller negative values (-1945)?Shouldn't the offset correction function adjust the range from 2095/-1945 to ±2000? Why is it ineffective?

3:After setting a 6dB gain, the full-scale range should become 1Vpp. However, experimental results shows no impact on the  ±0.25V sine wave input. Does gain adjustment only modify the input voltage limitation range?

4:Why does adjusting the offset pedestal also fail to affect the captured output values? Specifically, when set to midcode-32, the output should decrease by exactly 32 codes—why is there no observable change?

Hi Aaron,

Yes, a higher VCM can affect performance. Can you measure the DC voltage on both of the analog input pins, nearest to the ADC's analog inputs?

What are those two voltages, they should be the same.

Regards,

Rob

Hi,

It might be best for you to send over your schematics so I can review them first and make sure everything looks good from that standpoint first.

Please advise.

Thanks,

Rob

Hi Rob,

    I will provide you the  schematics two days later.Tomorrow and the day after tomorrow are weekend.We powered the ADS4249 through 1.8v power supply.but we use 2.5v I/O to communicate with ADS4249 through SDATA SCLK SDATA and RESET.Can the 2.5V I/O affect the vcm.

Reguards

Aaron

Ok, thanks Aaron, send over the schematics when ready and I will look them over.

Thanks,

Rob

Hi Rob,

    In the E2T forum,I can't find the way to add an attachment,SO I have send over the schematics throuth email(noreply-e2e@mail.ti.com) ,please check the email.

    Thanks

     Aaron