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ADS8881: ADS8881 Not responding.

Part Number: ADS8881

Hello, Few days back I got an ADC from TI as sample. The ADS8881. Since my application is very simple, I stick on to 3-wire CS mode, without busy indicator. 

The connection is like this:

1. SDI connected to VCC through a 10K resistor.

2. SDO of ADC is connected to SDI of my controller.

3. SCK connected to SCK of my controller.

4. CONV pin connected to GPIO of controller.

and all other VCC and GND connections are made.

The real problem is, the SDO pin of ADC is always low. After giving a low to high pulse with a width of 2uS to the CONV pin, I tried to read the data from SDO by giving 24 clocks, but the pin remains low for ever.

I have probed the CONV, SCK and SDO of ADC, and sure that SCK and CONV signals are there as how they are given, but SDO remains inactive and stays low (GND). The SPI mode is selected as per datasheet. Idle state of clock is low and active state is high, and input data is sampled at middle of data output time.

Please help me with this situation. Its an urgent need.

  • Hello Aravind,

    Are you using a TI evaluation board, or is this a custom board design?  From your description, it sounds like you have the correct signals for the SPI.  What is your SCLK frequency?  Can you provide a waveform capture for the CONVST, SCLK, and DOUT signals over an entire SPI frame, similar to Figure 1 in the datasheet?

    If this is a custom board, have you followed the recommended layout, with power supply bypass capacitors, shown in Figure 77?  If the recommended 1uF bypass capacitors are not placed near the ADS8881, then the device could be constantly resetting due to noise on the supply lines.

    Please measure and confirm the voltages for Vref, AVDD, and DVDD are within recommend voltage ranges.

    Also, with respect to the ADS8881 GND pin, measure the voltages on the input pins, AINP and AINN.

    It is also possible the device is damaged; have you tested more than one ADS8881 device?  If you have more than 1 board, check the second board for proper operation.

    Keith Nicholas
    Precision ADC Applications