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THS4032: Non Inverting Op-Amp Output Connected to Additional Supply

Fairuz
Prodigy 200 points
Part Number: THS4032


I have a non-inverting amplifier connected to Vout with input Vin (circuit below). However the same output is also connected to a relay to other supply V3 for other operation. the questions is what happen if V3 is used while Vin is floating? Simulation showed the Vout will follow V3 and I need to confirm if this is OK.

  • 0
    TI__Intellectual 1910 points
    Hi Mohd,

    Could you try again to attach the circuit? I do not see any attachment.
  • 0 in reply to Micah B Brouwer
    Prodigy 200 points

    Here it is. i run some spice simulation running V4 when floating V2. looks like at some other time the current can be +ve or -ve at resistor R8. does this will cause issue later on?

  • 0 in reply to Fairuz
    TI__Intellectual 1910 points

    Hi Mohd,

    Apologies for the delay. With the input floating, then the THS4032 will probably rail to either supply. This will cause a problem when the output voltage is different from V3, as the two voltages will compete and potentially cause sufficient current draw to damage the device. Is V3 always 5V or was that just used as a specific example?

    In the datasheet, the short-circuit current of the THS4032 is 150mA. In order to prevent more current from being drawn (or rushing in that could damage the device), a limiting resistor should be placed at the output. The value of the limiting resistor will depend on the expected voltage from V3.

    If we assume worst case scenario where the THS4032 rails low and that V3 = 5V, then the voltage on the output will be approximately 5V - (-1.5 V) = 6.5 V. In that case, about 50 ohms is needed to limit the current.  Please note that this may affect the max output swing by forming a voltage divider depending on the rest of your circuit.

  • 0 in reply to Micah B Brouwer
    Prodigy 200 points

    Hi Micah, thanks for answering. currently the solution i use is to add relay in between THS4032 and the 2nd Supply V4 (U3 is the relay and V3 is the turn on voltage, voltage will be source by V4).

    It will be alternate usage between V2 and V4 supply. for instance, when V2 is used (0-9V), V4 will floating. when V2 is used, U3 cutoff V4 supply.

    i will take note on your suggestion to add resistor for next design. thanks.