Electrical – ESD protection diodes on audio inputs / outputs

audioaudio-codecesdinputoutput

I'm building a digital audio device with 1/4" stereo in and outputs that connect to a CS4272 codec. I'm wondering if and where I should place esd protection diodes.

Pictured below is the analog part of the circuit. The left side of the schematic connects directly to the codec.

analog in & outputs

The user could have a discharge trough the other end of a already inserted cable. Are the filter networks enough to take care of any ESD? If not, where would be the best place to place the diodes?

The datasheets of the codec and the AD8656 opamp don't mention any external protection anywhere, and after peering over the schematic of the CS4272 evaluation board I couldn't seem to spot anything there.

Best Answer

If not, where would be the best place to place the diodes?

If on inputs internal protection diodes could be placed right after the connector, or a series resistor on the input and protection diodes. Depending on the signals, series resistors can be placed between the input and the amplifier. Most amplifiers have internal protection diodes, however, these diodes cannot dissipate a lot of power. It then becomes necessary to use external diodes if the input signal could go beyond the ratings of the internal protection diodes. The max rating for the AD8655 is 3kV for the human body model of ESD.

AD8655/AD8656 Data Sheet pg16 INFORMATIONINPUT OVERVOLTAGE PROTECTION

The internal protective circuitry of the AD8655/AD8656 allows voltages exceeding the supply to be applied at the input. It is recommended, however, not to apply voltages that exceed the supplies by more than 0.3 V at either input of the amplifier.

If a higher input voltage is applied, series resistors should be used to limit the current flowing into the inputs. The input current should be limited to less than 5 mA.The extremely low input bias current allows the use of larger resistors, which allows the user to apply higher voltages at the inputs.

The use of these resistors adds thermal noise, which contributes to the overall output voltage noise of the amplifier. For example, a 10 kΩ resistor has less than 12.6 nV/√Hz of thermal noise and less than 10 nV of error voltage at room temperature.

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