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Strange behavior of the inputs of ADS7830. No input connection but still there are 5V.

Svetoslav Valkov
Prodigy 30 points
Other Parts Discussed in Thread: ADS7830

Hello, 

I have got an end product I should repair. There is an analog joystick connected to ADS7830. It is using pins 1 to 3. Each of the inputs is tied down to GND by a 10k resistor.

The problem is that sometimes it has 5V on the input pin3 /same as the supply voltage/, although the joystick is not connected. It is not a problem of wiring or PCB. I've measured the inputs with an oscilloscope and I found that actually there are pulses with timing similar to SDA pin - SDA input is 3.3V because of the MCU. Osciloscope paramters: 1V/div; 2ms/div. Please, see pictures below. 

However, sometimes, when the device is restarted it worked fine - there is no voltage on the inputs. So, the question is if it is a chip failure or bad initialization? 

Fig.1 Schematic diagram

Fig.2 Pin3 reading with no input 

Fig.3 SDA pin

  • 0
    TI__Guru 54576 points

    Hello Svetoslav,

    The CH0-CH3 are analog inputs and there is no internal circuitry of the ADS7830 that would drive the inputs to +5V.  Since you have pull-down 10kOhm resistors to GND on CH0-2, the expected voltage should be  ~0V when there is no source driving the inputs.

    Are there any other external sources in the input path that can be un-intentionally driving the inputs?  Since you see SDA pulses on the analog inputs, any chance that the SDA/SCL signals are crossing the input signal path or a parasitic connection is present between them?  How many devices show this behavior?

    If possible, please post PCB gerber layout plots of the design showing the supply, GND, bypass capacitors, Digital I/Os and analog input connections. 

    Thank you and Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 30 points
    Hello Luis,

    Unfortunately, there was no parasitic connection between SDA/SCL lines and analog inputs. This chip is ESD sensitive as it is written in the datasheet. Obviously, these 10k resistors are not enough to suppress the ESD coming from the joystick /ADS7830 is reading a joystick position/. That's way I've added some 10n capacitors in parallel with the resistors - I've seen it somewhere in the forum. Of course, I've replaced the chip and now it is working fine. I've even tested it with one of my highly electrostaticaly charged colleagues :) - it still works!

    So, I've blew and replace 5 chips, but finally came to a solution.

    Thank you for support! Best Regards!

    Svetoslav