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Original question:

INA240: INA240

INA240: Does a differential probe also require a DC bias current?

Shyama Agrawal
Intellectual 860 points
Part Number: INA240

Hello,

Looking at the previous discussion between Mr. Imtiaz Khan and Mr. Guang Zhou about how INA240 can not be used to measure voltages which don't have a return path for DC Bias Current. I wonder if a differential probe can actually be used to measure floating differential voltages( As in if there is no return path for any current to flow into the probe terminals) ?

I know that it isn't exactly related to INA240 but still I would appreciate any insight from your side.

Regards,

Shyama

  • 0
    Guru 140200 points
    Hi Shyama,

    which discussion do you mean?

    Kai
  • 0 in reply to kai klaas69
    Intellectual 860 points

    Hello Kai,

    Please refer to the original question to which I made this thread using the ask  a related question feature.

    Link to the original question and the discussion which I am referring to:

    Regards,

    Shyama

  • 0 in reply to Shyama Agrawal
    TI__Mastermind 34250 points

    Hi Shyama,

    Differential probes are not necessarily isolated, therefore DC paths must be provided. they are normally designed in as part of the probe circuit.

    Regard, Guang

  • 0 in reply to Guang Zhou
    Intellectual 860 points
    Hello Guang,

    Thank you for your reply. Although I didn't understand how the DC paths are provided as a part of the probe circuit neither did I understand its implication on the performance of Non-isolated differential probe while probing a system which has no common ground with the probed circuit.

    Does that mean, if I am using an earth referenced oscilloscope with non-isolated differential probes, I'll still be able to correctly probe voltages in a circuit which is completely isolated from earth just by the use of the two probe tips? Will we be earthing the probed point on the circuit to provide a path for bias current for the differential amplifier in the probe?

    I am not an analog engineer and thus, don't know much about bias current. I hope you can share some insight on my doubts.
  • 0 in reply to Shyama Agrawal
    TI__Mastermind 34250 points

    Hi Shyama,

    I recommend you consult a real probe design expert for accurate advice, however here is what I THINK - A voltage source can be completely floating, such as a battery cell. However once you connect to a non-isolated differential probe, it is not truly “floating” anymore. Its terminals are connected to the probe ground, albeit through large resistances normally in the several mega ohm range, these resistances are presented by the high input impedance on both terminals of the diff probe. It is the large, yet still finite, input resistances that enable the high voltage capability of the probe. The high input resistances are also responsible for the independence among probes, thereby a ground referenced oscilloscope can measure multiple differential probe channels at the same time, each with its own common mode voltage.

    Regard, Guang