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ADS1198: Frequently 1 ADC code sample glitch in data on custom PCB with ADS1198

Lancer Parter
Prodigy 10 points

Part Number: ADS1198

I'm seeing a small issue in the data on our custom PCB and wanted to get your review.

  1. Raw ADC data occasionally changes by 1 ADC code for a single sample e.g.: 15, 15, 15, 14, 15
  2. This creates a visible spike or glitch in the calculated voltage and plotted signal.
  3. The is present in both internal test square wave signal and external input signal with ECG simulator.

Current configuration

  • SPI clock: 5 Mbps
  • Sampling rate: 500 SPS
  • Continuous read mode enabled
  • Data is read only after DRDY is asserted
Register Internal Test Signal ECG Signal
CONFIG1 0x04 0x04
CONFIG2 0x18 0x20
CONFIG3 0xCC 0xCC
LOFF 0xBF 0xBF
CHnSET 0x65 0x60
RLD_SENSP 0x00 0x06
RLD_SENSN 0x00 0x02
LOFF_SENSP 0xFF 0xFF
LOFF_SENSN 0x06 0x06
LOFF_FLIP 0x00 0x00
CONFIG4 0x02 0x02
WCT1 0x0B 0x0B
WCT2 0xD4 0xD4

Attaching schematics and plots. Also wanted to review decoupling capacitor placements etc.


image.png
internal_1.csv internal_2.csv internal_3.csv sim_1.csv sim_3.csv ads1198.pdf 

  • 0
    TI__Guru 68916 points

    Hi Lancer,

    It seems reasonable to expect ±1 ADC code simply due to the noise of the ADS1198. For a 16-bit ADC with VREF = 2.4v and Gain = 12, your LSB size is only 6.1µV. 

    Decoupling caps should be placed near the device, ideally with power traces running through the caps and then to the pins. It looks like your schematic has the recommended capacitor value pairs.

    Regards,

    Ryan