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ADS1298: ADS1298 Channel 5-8 Register Issue

Xinyuan Zhang
Prodigy 110 points
Part Number: ADS1298

My team and I are using two ADS1298s with TQFP packages for an EMG application. All negative differential pins are tied to ground and the ADCs were both soldered by hand. 

Our C program (using a pic24fj128ga204 uC) is designed to send a write command to set register values (all channels), then read back the register values to verify that the registers were set successfully. 

When we checked our register read back values (attempted to write (0x91), we noticed that channels 5-8 on our first ADC would only read back (0x00 NUL). When we attempted to write specifically to those registers (button press trigger with different values), the returned value is still 0x00. All other registers matched what was sent. The second ADC functions as intended. 

Replacing the ADS1298 did not fix the issue. the same channel registers would still return 0x00. All PCB connections are identical to the functioning ADC #2 (bypass capacitors, voltage pins). I have attached the Altium schematics for the ADC that is not functioning (Labeled U1) and the working ADC (Labeled U2). 

If anyone would be able to give insight why these specific channels are not functioning, it would be greatly appreciated. Thank you in advance.

  • 0
    TI__Guru 64226 points
    Hello Xinyuan,

    Thanks for your post.

    The first thing I would do is confirm that the ID register is correct. This is a read-only register that corresponds to the specific device you are using. Verify that the value is 0x92 for ADS1298.

    If the Device ID is wrong, it is possible that the power-up sequence was not followed properly and the device misread it's internal registers. These registers are fixed the final test program during manufacturing. If they are misread, the device may think it's a 4-channel device by default, which explains why channels 5 - 8 are not working. Please refer to the "Initial Flow at Power-Up" on page 85 (Figure 93) and also the "Power-Up Sequencing" section on page 96.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 110 points
    We ran out startup program on both PCB ADCs as well as the ADS1298 evaluation board. The 2nd ADC and eval board returned the proper 0x92 ID read, but our first ADC read back 0x80, which corresponds with your advice that the ADC is being read as a 4 channel device.

    Since we have observed this behavior on two ADCs (the original and the replaced one), we are wondering if there is any way to fix issue. There are no discernible difference between both PCB ADCs as for as we are able to check.
  • 0 in reply to Xinyuan Zhang
    TI__Guru 64226 points
    Thanks, Xinyuan.

    For the ADC with the incorrect ID, is it possible to toggle the /RESET pin or send the RESET command again while keeping all supplies on? If so, try this and read the ID register again.

    If this doesn't work, can you measure VCAP1 with respect to AVSS and tell us the result?

    Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 110 points

    Thank you for the reply.

    We tested the voltage on VCAP1 with respect to AVSS, it is 0V instead of above 1.1V, which is what it should be.

    We found out that the cap connecting VCAP1 and AVSS is broken. After we replaced it, all register values are correct now!