Impedance Balanced Output Resistor

[Gold]

When making an impedance balanced output what is the best way to match resistors? The buildout resistor goes from the opamp output to xlr pin 2 and a matching resistor goes from xlr pin 3 to ground.

The output impedance is usually stated as the buildout resistor value. Isn't this an approximation that assumes the output impedance of the opamp is 0 ohm? If you are trying to match the impedance to ground matching the resistors will have an error.

If the actual opamp output impedance is 0.1 ohm I guess it doesn't matter but if it's 1 ohm and the buildout resistor is 50 ohm that's a 2% error.

Do I need to worry about this?

 

[ruairioflaherty]

Hey Paul,

I know this has been addressed here recently enough (I think by PRR).  I can;t remember anything specific to search for, I guess try the usual search strings and see what you get.

Cheers,
Ruairi

 

[ruairioflaherty]

This thread may help

http://www.groupdiy.com/index.php?topic=33750.0

 

[JohnRoberts]

When making an impedance balanced output what is the best way to match resistors? The buildout resistor goes from the opamp output to xlr pin 2 and a matching resistor goes from xlr pin 3 to ground.

The output impedance is usually stated as the buildout resistor value. Isn't this an approximation that assumes the output impedance of the opamp is 0 ohm? If you are trying to match the impedance to ground matching the resistors will have an error.

If the actual opamp output impedance is 0.1 ohm I guess it doesn't matter but if it's 1 ohm and the buildout resistor is 50 ohm that's a 2% error.

Do I need to worry about this?

Typically the output impedance of an opamp is not a simple resistance, but a resistance that is reduced by negative feedback to some arbitrarily small number at audio frequencies. Opamps with higher source impedance are generally not used to drive output lines for other reasons, but this too.

Impedance balanced outputs are far from perfect, but worth the pennies to add another resistor. For best output balance symmetrical circuits are better. I wouldn't worry about this unless you are dealing with a severe noise environment.

JR

 

[Gold]

I wouldn't worry about this unless you are dealing with a severe noise environment.

Thanks. I love a simple answer.