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OPA875: Single power supply and mid-supply reference

Angel Garcia
Prodigy 50 points
Part Number: OPA875

Tool/software:

Hi,

I have used the OPA875 for switching 2 videos sources, but as there is a single power supply (3v3-GND) it doesn't work.

From what I got from other threads in the forum, it's possible to make this circuit works with a single power supply, but it needs a mid-supply reference.

What is not clear for me is, can I solve the issue by placing a 75-ohm resistor "over" R14 and R15, connected to 3V3 for having the mid-supply reference? Do I have to do it by placing a clamping diode (supply/2) between pin 2 and ground for setting the mid-supply reference?

Thanks in advance.

  • 0
    TI__Genius 14440 points

    Hi Angel,

    To maintain a 75ohm input impedance, you could add resistors over R14 and R15 but adjust the values to be 150ohms. This would ensure the inputs are set to mid supply. For the GND pin, I believe a split resistor configuration would work, similar to the inputs. You could use 500ohm resistors, I believe this would work to bias the GND to mid-supply. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 50 points

    Hi Ignacio,

    Thanks for your answer.

    What I already did  is to disconnect R14 and R15 from the ground and connect them to 5V, whereas the supply for the OPA875 is now 12V, so the resistor are more or less at mid-supply level.

    On the other hand, GND pin is still connected to GND, which from your comment is wrong, so as far as I understand in my setup needs to be connected to 5V (mid-supply) (I'm carrying out with this modification)

    What is surprising for me is that even having GND pin connected to GND, when the OPA875 is enabled, there is nothing in the output, not even a dimmed signal or something that is not correct. Is this issue related with the reference voltage?

    Thanks in advance.

    Ángel

  • 0 in reply to Angel Garcia
    Prodigy 50 points

    If the reference is set at 5V (mid-supply), do we need an enable signal levelled at 5V instead at logic level? Would the output be raised at 5V as well?

  • 0 in reply to Angel Garcia
    TI__Genius 14440 points

    Hi Angel,

    Having this setup should work if your input signal is also centered around 5V. If the input is not centered around 5V your options would be to AC couple the input or provide a negative supply. As far as the GND pin being biased to ground, if your input is centered around ground, the output would try to go to ground and would be hitting the lower output range of the device. As for the enable and select pin, these are referenced to the GND pin potential. To toggle these two features, you would have to apply a voltage with respect to 5V in this case. With this configuration, assuming the signal is also centered around 5V, you would also see a signal centered around 5V at the output.

    Best Regards,

    Ignacio