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DRV135: Confusion about the load resistance

Part Number: DRV135
Other Parts Discussed in Thread: INA134, INA137

Hi,

I am designing a balanced audio output using the DRV135, that will drive another equipment (An amplifier, exact reference unknown at this moment) via XLR connector and several meters cable. 

I may have missed this in the datasheet, but i don't know what do do with R1, R2 and RL in the schematic below:

I understand that one would either use 600 Ohm RL, or two 600 Ohm R1 and R2. So, my questions are:

  1. From figure 31, it seems having 600 Ohm RL is the best option in terms of THD+N, then would we even consider the other option (using two 600 Ohm resistors R1 and R2) ? Is there a situation where it makes more sense to have R1 + R2 instead of RL? 
  2. I assume the value of 600 Ohm is related to the impedance of the cable that will be driven, right? Does this value need to match the cable characteristic impedance?

Thank you very much,

  • The three resistors shown in the schematic are just for the test circuit.

    R1 and R2 are driver-side pull-downs (high-ish resistance, probably 100k or so). RL is for the test circuit -- it sets the load resistance to 600 ohms because that is how the device is rated in the data sheet. So the idea is that a signal is fed into the DRV135, the amplifier drives a long cable, and an INA134/137 converts back to single-ended.

    600 ohms is a vestige of the old telecom days where mile-long lines were terminated with 600 ohms. Audio cables do not have a characteristic impedance, or at least one which is advertised and guaranteed. The frequencies involved are low enough where the cable is just a lumped impedance, not a transmission line.

    A real audio circuit won't present a 600-ohm load. Modern audio designs use bridging impedances. This means a low output impedance feeding a moderate input impedance. We never try to match impedances in line-level audio circuitry.

    The DRV135 has built-in matched 50-ohm resistors which sets the source impedance. They must be matched for best CMRR. Also these resistors ensure that the chip won't blow up if one of the outputs is shorted to ground, which is common for audio circuits. Also, the resistors isolate the amplifier from the cable for stability reasons.

    The INA137 in the test circuit presents a 12k-ohm input impedance to the driver. That is a common value. So there it is: 50-ohm source impedance driving a 12k load impedance, for minimal voltage-divider effects and best stability. It means you can hang several inputs off of one driver, which is very common in professional audio.

    (BTW: when connected properly, the DRV135 has an internal 10k pulldown on the outputs, so the R1 and R2 in the test schematic isn't necessary.)

    So the point is that you don't need R1, R2 or RL in your actual design.