This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS65381-Q1 Analog MUX maximum output resistance

Other Parts Discussed in Thread: TPS65381-Q1, RM48L952

Please explain the maximum output resistance parameter listed in Table 4-3 on page 28 (SLVSBC4A, December 2013), and also, note (1) below the table?

Is the maximum output resistance specifying the output resistance internal to the TPS65381-Q1 for the DIAG_OUT pin when AMUX is enabled? If not... what is the parmater specifying?

It's not clear to me how to use this information.

Ultimately, I'm trying to connect the DIAG_OUT pin to both and analog input and a digital input of an RM48L952 for diagnostic purposes in a safety application. Any application recommendations would be helpful (example schematic for example).

  • Table 4-3 in the data sheet is defining the available current that can be sourced from the DIAG _OUT when the analog mux is chosen and active. In other words if you choose VDD5 (0x04h) then the output would be a 2.5V (+/- 2%) output that can supply (2.5V / 55k) = ~ 45.5uA. The chart defines the output impedances in order to verify if the output can drive various devices and inputs. You need to set the DIAG_MUX_SEL bits to the values shown in the table to get the desired output level.

    The analog signals are normally used for diagnostic purposes and are usually input into an AD converter to verify that the levels are correct.  If the signal from the DIAG_OUT pin is going to be input into two separate inputs (rather than a single ADC converter input) you may have to buffer the signal depending on the input requirements of the analog and digital inputs that you chose to use on the RM48L952. This is why the note on the bottom of page 27 states that if you want to use the signals to drive an analog and a digital input that you need to make sure that this does not affect the μC ADC converter results (Note A). But given that the outputs from the TPS62381-Q1 are high impedance, you probably should consider using an op amp buffer before the ADC input. If you look at the diagram (figure 4-4) on page 27, I would put the op amp buffer where the rectangular symbol (resistor) is shown before the cap going into the ADC. This would offer greater accuracy in converting the analog signal and still allow the digital signal to be seen. You may not need this, but I believe that you probably will. You will have to test if this affects the conversion results to verify this.

    Please let me know if you have any further questions.

    Richard Elmquist

  • Moving to the right forum

  • Dear Richard,

    is there a typical value of the output resistance for the analog mux output? (Table 5-4; datasheet TPS65381 ver may 2015)

    We want to reduce the offset error of the buffer opamp by matching the feedback impedance to the source impedance, so it would be helpful to know the typical values of the output resistance of the Analog MUX.

    In our case especially for the VMON_BG.

    Thanks in advance!

    Regards,

    Volker