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TPS65381-Q1: Analog Mux (AMUX) accuracy - divide Ratio - TMS570LS1114

Helge Pape
Prodigy 160 points
Part Number: TPS65381-Q1
Other Parts Discussed in Thread: TMS570LS1114

Hello,

I am testing the ability of the TPS DIAG_OUT pin. For DMUX everything is fine.

For AMUX I observed that the voltages are not as expected:

Diag_Mux_Sel Signal Name Divide Ratio Divide Acc V [V] UV [V] OV [V] V [digits] VUV [digits] VOV [digits] Meas [digits] Measdiv [V] Meas [V]
0x01 VDD5 2 2,25% 5,00 4,500 5,450 3102 2729 3458 2653 2,137 4,27
0x02 VDD6 3 3,75% 6,00 5,200 8,200 2482 2070 3519 1715 1,382 4,15
0x04 VCP 13,5 6,25% 4,000 15,000 0 345 1465 819 0,660 8,91
0x08 VSOUT1 4 1,20% 0,00 3,300 9,500 0 1011 2983 16 0,010 0,04
0x10 VBAT_SAFING 10 5,50% 12,00 4,200 36,700 1489 493 4805 475 0,380 3,80
0x20 VBATP 10 5,50% 12,00 4,200 36,700 1489 493 4805 475 0,383 3,83
0x40 MAIN_BG 1 0,00% 2,49 2,364 2,617 3090 2934 3247 2983 2,405 2,41
0x80 VMON_BG 1 0,00% 2,49 2,364 2,617 3090 2934 3247 2963 2,388 2,39

Except MAIN_BG, VMON_BG and VCP all voltages are not in valid ranges. VSOUT1 is not used. I did the measurements ("Meas") with internal ADC of TMS570LS1114 (VRef = 3.3V, 12 bit resolution) as well as with Voltmeter.

The left 7 columns are obtained from the data sheet. "V [Digits]" means expected voltage at DIAG_OUT pin in Digits. "Meas [digits]" means measured voltage at DIAG_OUT pin in digits. "Measdiv [V]" means measured voltage at DIAG_OUT pin in volt. "Meas [V]" means measured voltage with respect to divide ratio in Volt.

The voltages on the output Pins of TPS are all correct (e.g. VDD5 = 5V, VDD6 = 6V,...). Is there a Problem with divide ratio? Because with divide ratio 1 everything is fine.

Greetings Helge

  • 0
    TI__Expert 5600 points
    Hi Helge,

    Device expert will take a closely look on your question.

    Best regards,
    Jari Niemelä
  • 0
    TI__Mastermind 20580 points

    Hello Helge,

     

    I cannot comment for your specific test set up but there are several points to keep in mind with respect to the full measurement range that may cause an incorrect measurements in a system. Below is a list of common issues that could be the source of the measurement errors.

     

    • The AMUX accuracies are specified with no load. If your ADC or measurement method is pulling current through the AMUX output there will be a resistor divider across the output resistance of the AMUX, footnote 2 on table 5-4. Loading will cause measurement error.  
    • Settle time (stabilization) of the signal. If a measurement is made too quickly after selecting a new AMUX output the signal may not have stabilized due to the RC on the circuit. There is a recommended switching order in the datasheet under table 5-4 that switches the AMUX signals in a sequence that under typical use cases has the least voltage steps between the outputs (from high to low voltage) to try and minimize the wait time between sampling the AMUX signals.
    • MCU IO pull down or up influence. To speed up DMUX readings out of the DIAG_OUT, normally the DIAG_OUT pin is connected to both a GPIO and an ADC channel on the MCU. Some MCU GPIO pins have permanent pull up or down or a configurable one. If there is a pull up or down it will cause a resistor divider with the AMUX output and cause measurement errors. Double check this is not the case.
    • These divide ratio accuracies are for the divider only. It doesn't include the accuracy of the source being measured.
    • ADC error and ADC reference error are not included.

    Hopefully these tips will help you find the source of the measurement error. Please let us know if you have further questions.

    Best Regards,

    Scott

  • 0 in reply to Scott Monroe
    Prodigy 160 points
    Hello Scott,

    there was a pulldown resistance configured for GPIO as you described. After removing the pulldown everything works fine for me!

    Best Regards,
    Helge