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Low Current Resistance Measurement using TS5A23159. questions about power sequencing, default logic states, and minimum current requirements.

nickelson
Prodigy 50 points
Other Parts Discussed in Thread: TS5A23159

Hi,

I want to use an analog switch (TS5A23159) to read out a Thermistor in two different ways with a DMM:

  1. One way is without linearization. The resistance measured should be identical to the Thermistor resistance.
  2. The second way I want to have a resistor network connected to the Thermistor. One resistance in parallel, one serial. This leads to a linearization of the Thermistor resistance.

The Thermistor can range from 2MOhm to 10kOhm. When the Thermistor is high-ohmic the readout current of the DMM is very small. Down to 100nA. Is that a problem using an analog switch? Is there a minimal current necessary?

Thanks,

Niklas

  • 0
    TI__Mastermind 45340 points

    Niklas,

    The TS5A23159 is a bi-directional passive FET switch.  The switch can pass any current up to 200mA.  There is not current minumum needed for this switch so you can use a digital multi-meter (DMM) to measure the resistance across the switch. Keep in mind for your measurement that the TS5A23159 has an on-state resistance between the drain and source of the FET switch (Rdson) ~0.7 ohms and you can measure this for your calculations. 

    Adam 

     

  • 0 in reply to Adam Torma
    Prodigy 50 points
    Hi Adam,

    it shouldn't be a problem to have the supply at +5V and the digital input at only +3.3V for the high state, correct? As long I'm able to have more than +2.4V, everything should be fine?
    I have a digital potentiometer left for setting the digital input. Do you see any possible issues?

    Thanks,
    Niklas
  • 0 in reply to nickelson
    TI__Mastermind 45340 points

    Niklas,

    Correct operating the TS5A23159 with 5 V supply and the digital input at 3.3V is within the recommend operating conditions in the datasheet.

    You are correct.  With a 5 V on the supply pin the minimum voltage on the digital input IN pin that the device will recognize as a logic "high" is 2.4 V.  Using 3.3 V as your logic high is valid. 

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 50 points

    Adam,

    Thank you very much! A few more questions occurred:

    - Do we have to look on timing during startup? Is it important, which voltage is first? +5V VDD and then +3.3V or the other way around?

    - Is it necessary to toggle the switch once to have defined and stable operating point? We will use a 100k digital potentiometer as a voltage divider to set the logic voltage. Could we come into an undefined state during startup?

    - Logic input is sensitive to thresholds and not rising/falling edges, correct?


    Thanks,

    Niklas

  • 0 in reply to nickelson
    TI__Mastermind 45340 points

    Niklas,

    - Do we have to look on timing during startup? Is it important, which voltage is first? +5V VDD and then +3.3V or the other way around?

    If you look at the recommended operating conditions above the IN pin cannot exceed the voltage on Vcc.  If you supply the IN pin with 3.3 V while Vcc = 0V you will violate this recommended operating condition. 

     

     

    - Is it necessary to toggle the switch once to have defined and stable operating point? We will use a 100k digital potentiometer as a voltage divider to set the logic voltage. Could we come into an undefined state during startup?

    No you do not need to toggle the switch upon start up.  The switch will start up in what ever state you have set by the voltage on the IN pin. 

    - Logic input is sensitive to thresholds and not rising/falling edges, correct?

    Correct, this device cannot detect rising or falling edges on the IN pin.

    Thank you,

    Adam