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ADS1235: ADS Loses connection when connected to load

João Almeida
Prodigy 30 points
Part Number: ADS1235

Hello everyone. I am designing a project using the ADS1235 as the ADC for converting load from a strain gauge connected in full wheatstone bridge (amplified and filtered by AD8557 opamp), controlled by a STM32F405 MCU. The device is powered via USB using a FDTI (3.3V), and communicates using the MCU serial ports.

I am having trouble with the device, since it is perfectly functional and detectable from the serial monitor, and when the load is connected to the pads I lose communication with the ADS, and the serial monitor only shows "-1" instead of the converted data.

Any thoughts on how to solve this issue, or where the problem might be? I appreciate the concern in answering this question.

Thanks!

  • 0
    TI__Mastermind 39811 points

    Hi Joao,

    Can you send a schematic of your system? This will help us determine what is going on here

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 30 points

    Hello. Yes, of course:This is my layout, using the adc and opamp. The SPI pins go to the STM32, as well as the Reset and DRDY pins. THe input SIGX of the opamp is the wheatstone bridge. The strange thing is that when the bridge is not connected, the ADC works fine. I already tried connecting the system to an external power supply than the USB, the result is the same.



  • 0 in reply to João Almeida
    TI__Mastermind 39811 points

    Hi Joao,

    Is the VDD voltage = 5V? The power circuitry is not shown in the images you provided.

    Since you have the START pin pulled high, you should see pulses on the DRDY pin of the ADC assuming that AVDD and DVDD are powered correctly. This indicates that the ADC is converting. Do you see such pulses on the DRDY pin? They should come out of the ADC at a frequency similar to the default data rate (~20 SPS).

    What is the nominal resistance of the wheatstone bridge you are connecting to the ADC / amp?

    Have you tried connecting a different load to the ADC to see if it can convert correctly? For example, use a precision source with low output impedance just to see if you can get a valid response from the ADC.

    Finally, can the USB transceiver supply sufficient current for the ADC, the amplifier, and the load (as well as any other circuitry you may have on the board)?

    -Bryan