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TMS570LC4357: ADC 1 Pin 31 Incorrect Reading

James Kiessling
Intellectual 380 points
Part Number: TMS570LC4357

Hello,

We are experiencing a problem with AD1IN[31]. We are seeing odd voltages measured on this pin. We have a sensor whose output ranges from 0.66V - 3.0V, and we have confirmed the precision of the sensor. When connecting to AD1IN[31], we see a range of 0.25V to ~2.1V, with an extremely noisy signal at the higher end of voltage. When grounding AD1IN[31], it continues to read about 0.25V.

We believe this has to do with the internal temperature sensor. According to the TRM, AD1IN[31] is multiplexed with an on-chip temperature sensor. By following the instructions in the TRM section 6.5.13 we have set the PINMMR in such a way as to disable the temperature sensor and to enable AD1IN[31], however the problem still persists. 

Could this problem be due to other factors, such as the Launchpad routing? Or is there an alternative way to disable the temperature sensor?

Thank you,

James

  • 0
    TI__Guru**** 197126 points

    Hello,

    Temperature sensor 1 is multiplexed with AD1IN[31]. But the three temperature sensors are disabled by default.

    Can you probe the ADIN[31] before connecting your input signal to this pin? I checked the voltage level of pin ADIN[31] on launchpad, it is pretty clean, around  2mv amplitude. 

  • 0 in reply to QJ Wang
    Intellectual 380 points

    Hello QJ,

    I apologize, I don't believe that I clearly described the issue in my post. Our hardware consists of 3 sensors, two of which output 0.66V - 3.0V and the third outputs 0.5V - 2.25V. We have two sensors connected to AD1IN[29] and AD1[30], and the third sensor to our mysterious AD1IN[31]. Through experimentation, we have seen the following:

    • When all three sensors are connected, AD1IN[31] is read with a significant DC offset and becomes extremely noisy at the high end of the sensor range.
    • When disconnecting the sensor output from AD1IN[31], we see the signal continue to follow that of the movement of the other two sensors
    • When grounding AD1IN[31] with the sensor output disconnected, we still see the signal continue to follow the movement of the other sensors

    To answer your question, we have probed AD1IN[31] and see approx. 0V when the signal is grounded, while we continue to see the A/D readings follow the sensors (not read 0V). With nothing at all connected to the pin, we see a stable 32mV.

    Thank you,

    James

  • 0 in reply to James Kiessling
    TI__Guru**** 197126 points

    Thanks James for your clarification. I will check AD1IN[31] on Launchpad.

  • 0 in reply to QJ Wang
    Intellectual 380 points

    Hello QJ,

    Have you made any discoveries?

    Thank you,

    James

  • 0 in reply to James Kiessling
    TI__Mastermind 49120 points

    James,

    What is the configured sampling time for the group used for these conversions? Can you increase the sampling time to see if that affects the accuracy of the conversions? See this application notes to identify the actual sampling time required given the nature of the external impedance: http://www.ti.com/lit/an/spna118b/spna118b.pdf

  • 0 in reply to Sunil Oak
    TI__Guru**** 197126 points

    Hello James.

    I am sorry for coming back to you late.

    PINMMR174[24] is to enable and disable the Temperature Sensor:
    pinMuxReg->PINMUX[174] |= (uint32)(~(0XFEFFFFFFU)); --> to enable the temperature sensor

    However ADC1N[31] cannot read a value unless the Temperature Sensor is Powered up:
    pinMuxReg->PINMUX[174] &= (uint32)(0XFEFFFFFFU);

  • 0 in reply to QJ Wang
    Intellectual 380 points

    Hi QJ,

    Thank you for the response. I unfortunately won't have the bandwidth to test this for a while, so I'm going to mark that this has solved my issue. 

    Thank you for the assistance,

    James