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ADS8328 SDO pin is occilate oscillate after CS is LOW

Other Parts Discussed in Thread: ADS8328, ADS8323


I have some problem with ADS8328 and I try to solve by myself but it still not work yet.

The problem is when I try to read data from ADC. After the CS is low, the SDO pin will oscillate until the falling edge of SCK.

The waveform is showed in the picture.

I decided to use auto-trigger and auto-channel mode. The connection of ADS8323 pins are

VBD = +3.3 v

VA = +5.0 v

EOC = No Use

CONST = Pull up to VBD with 5k Ohm

Ref-, COM, AGND, BDGND = Ground

Please give me the advice. What is the reason of this problem and how to solve it.

Thank you

  • Hi Pisut,

    Sorry you are having problems with the ADS8328. I haven't seen anything quite like this before with this device. I can't see any time-divisions on your capture or what the division spacing is. Can you share a cleaner shot that includes these or just share what your SCLK is configured as? I worry that you're operating the device either extremely fast or extremely slowly because SDI and SDO seem to be out of phase.

    If you have a schematic you are comfortable with posting here or sharing with me in a private conversation that would be helpful as well. I'd like to see what happens if you read back the configuration register rather than reading a conversion result. In this way we can validate your serial bus before moving on to looking at the conversion result.

  • Hi Kevin,

    Thank you for your suggestion. I have tried with many timing configuration with the last with

    CS active time (CS low period) = 1 ms

    SCLK frequency = 25 kHz with High period = 5 uS, Low period 35 uS

    Delay after CS low to first rising edge of SCLK = 35 uS

    You told maybe the problem comes from slowly operate. Is the current timing according to your assumption?

    Thank you.

  • I am sorry that I forgot to tell you one important thing.

    In the datasheet page 23 that tells that Vref should  be 0.3 to 4.2 V. But in absolute maximum ratings tells Vref can be maximum at VA + 0.3 V.

    Since I use 5 to 5.5 V with VA, so I use LM4040-5 to make precise 5.0V with Vref.

    I thought maybe the problem come from too high of Vref. But I when change Vref to some voltage in range, the problem is still exist.

  • Pisut,

    I'll have to review the timing you shared, but in the mean time I've got some feedback for you. 

    First, there should be nothing wrong with using a 5V reference with a 5-5.5V supply at VA. The absolute maximum table indicates that permanent damage will potentially be done unto the device if the ratings are exceeded. Take note, however, that the specification table used a 4.096V reference. This means the performance you experience with the device may deviate from the figures in the table, though I don't expect it will be by much.

    If you can, I'd like to see what happens when you read back the configuration register. Do you still see the oscillation when reading the configuration register? I worry that disregarding the EOC/INT pin could cause you to read data from the device while another conversion is being completed and new results are being written to the data register.

  • Hi Kevin,

    There is also oscillate on SDO while Reading the CFR.

    The oscillation will occur when the MSB is '1', if the MSB is '0', the SDO is not oscillate.

    Anyway, I will capture the result of reading CFR and converted data and share to you.

    Thank you.

  • OK!! I found the reason of the problem. There are two input ports in ADS8323 and one of them (+IN1) is connected directly to COM pin. I use that input to confirm the SPI is work that must read with 0x0000. The other (+IN0) I connect to some voltage and read the converted data.

    When I remove the +IN1 from COM, it work correctly. But I also confuse why this situation happened. If I want to input either +IN0 or +IN1 with zero, how I can do?

  • Pisut,

    That sounds extremely odd. Do you have a schematic we can look at?

  • Hi Kevin,

    I will share the sehematic for you.

    Thank you.

  • Pisut,

    If you attached the schematic to this forum post, it did not come through. You can also email it to if you prefer to keep your schematic private.