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I have an embedded system powered by a 3.7V LiPo battery that will be connected to an iPod's headphone out cable. The embedded system will perform some DSP on the iPod's audio out signal and use OpAmps to mix it with a second signal.

Through some of my own measurement, I have discovered that the iPod's audio signal is about 2Vpk-pk and is centered around the ground wire provided in the headphones.

I know it is common practice to connect the grounds of the two systems together to make sure all voltages are at the same level between systems. Instead of connecting headphone ground to embedded system ground, could I connect it to the middle of a voltage divider in my system so that all the signals coming from the iPod are "centered" around (3.7V/2 = 1.85V)?

schematic

simulate this circuit – Schematic created using CircuitLab Thanks, John

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  • \$\begingroup\$ Are you really intending to use TL081s? They seem to be intended for operation on +/-15 volts, and according to the datasheet, are only guaranteed to get the output within 3 volts of either supply - doesn't leave much room for operation with a 3.7 volt supply... \$\endgroup\$ – Peter Bennett Feb 19 '15 at 0:28
  • \$\begingroup\$ hahaa nooo. Those were just the op amps provided in the schematic editor. I haven't picked one yet, but it looks like I will use AD4691. \$\endgroup\$ – John M Feb 19 '15 at 1:33
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You do not want to use 1M resistors for the virtual GND divider. Use a much lower resistor value set. For a practical working scenario the resistors should be about 10 to 50 times lower impedance than load impedance that you place on the Headphone signal from the I-Pod.

If this lower impedance divider does not suit well with your battery supplied application then you should look into using a split voltage rail divider chip or create your own. To roll your own you can use an a resistor divider like you have now, place a cap across the lower divider resistor and then buffer the divider using an opamp that is wired up as a standard voltage follower. The output of the op-amp becomes your virtual head phone ground.

enter image description here

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  • \$\begingroup\$ Ahh. I see. I need to consider the transmission line characteristics in this scenario. Just connecting the grounds together is ideal because of the low impedance. Thank you so much for suggesting the "split voltage rail" thing. I had never heard of a "split voltage rail divider chip" OR "virtual ground" before, so this really broadened my horizons. \$\endgroup\$ – John M Feb 19 '15 at 1:38

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