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TIDA-00548: 4-20mA analog input over-current protection

MKrasuski
Intellectual 280 points
Part Number: TIDA-00548
Other Parts Discussed in Thread: ADS1263

In 4-20mA analog input, the burden resistor is subjected to excessive power/voltage in case of wiring mismatch or the field transmitter failure.

TIDA-00548 employs an input MOSFETs as a current-limiting devices in each channel, which shall be perceived as potentially more robust, accurate and faster mean of protection than any PTC.

The gates of MOSFETs are routed respectively to AIN8 and AIN9 of ADS1263 DAC. These I believe operate in GPIO mode.

What should be the principle of MOSFET gate control in this setup:

What would be over-current detection condition resulting in pulling the GPIO low? Is it an abnormally high conversion result? A one of single or multiple/repetitive occurrences across the stream of samples? What should be a recommended margin between FSR code and over-current trigger code?

What would be your recommendation in a multi channel configuration with sampling rate up to 50SPS/channel:

Shall the respective channel's MOSFET be activated only through the measurement period in the certain channel or permanently? What would be the recommended MOSFET's setup/settling time prior to conversion begins?

Were this issues investigated in this design?

  • 0
    TI__Intellectual 1280 points

    Hi Marek,

    The gates of the MOSFETs are controlled by AIN8/9 (GPIO mode) only to enable/disable the burden resistors. They are not controlled in a loop, e.g. if the analog value saturates then the signal is switched off. The current limit is done in hardware over the change of Vgs of the MOSFET. See also section 3.2 in the user's guide for detailed explanation.

    Sensor configuration 1: Two independent 4-20mA loops.
    Leave the channel(s) permanently on to measure the current. Disabling a channel would break the 4-20mA loop and cause the field transmitter to shut down.

    Sensor configuration 2: One 4-20mA loop.
    You probably want to use a ping-pong scheme to measure and compare the values. Drawback is that measurements are taken at different times. So the comparison might is somewhat limited and dependent on the bandwidth of your signal.

    I did not perform any measurements on setup/settling, but I think (since you control AIN8/9 and start of conversion over SPI) the delay between two independent SPI commands is sufficient.

    Hope this helps!

    Thanks,
    Lars