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ESD protection on audio inputs and outputs, yes or no?
ESD protection on audio inputs and outputs, yes or no?
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Old 14th July 2018, 06:26 PM   #1
JayShoe is offline JayShoe  United States
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ESD protection on audio inputs and outputs, yes or no?
Default ESD protection on audio inputs and outputs, yes or no?

Hello,

I'm designing a mixer. It will go through a decent amount of torture in the field with users unfamiliar with audio. Inputs in particular will have a lot of plug and unplugging. There will be digital ADC and DAC parts.

Is ESD protection on the circuitry inputs and outputs a good idea? Is it standard on most mixers? Is it overkill?

Maybe a simple resistor to ground? Maybe a part from littelfuse like SP1001-02JTG?

Thanks for the help.
Jay
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Old 14th July 2018, 08:34 PM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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I don't know if it is standard on most mixers, but it is probably a good idea, especially when the inputs go straight to the ADC and DAC chips rather than to a discrete amplifier or an op-amp made in some antique IC process. Besides ESD, you also have consider electrical overstress from accidentally connecting a 48 V +/- 4 V phantom supply.

On top of that, there is latch-up: when the power supplies are on, short current peaks into or out of an IC pin can trigger parasitic thyristors and cause complete destruction of the IC. Usually the ICs are designed to handle 100 mA, but anything higher can cause trouble.

When it doesn't harm the performance too much, you can use an AC coupling cap rated for more than 52 V, a series resistor and two small Schottky diodes, one to the supply and one to ground.
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Old 14th July 2018, 09:19 PM   #3
MarcelvdG is offline MarcelvdG  Netherlands
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I forgot to mention that the parasitic thyristors normally only can get triggered when the current peak drives the pin more than a diode forward voltage above supply or below ground. Hence the Schottky diodes: they have a lower forward voltage than a normal silicon junction diode.
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Old 14th July 2018, 09:42 PM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by MarcelvdG View Post
I forgot to mention that the parasitic thyristors normally only can get triggered when the current peak drives the pin more than a diode forward voltage above supply or below ground. Hence the Schottky diodes: they have a lower forward voltage than a normal silicon junction diode.
We used to put mains through a resistor into PIC micro inputs for zero crossing detection, and they survived no problem at all.

It blows them if you put too much current into an input or output pin.

ESD isn't usually a problem when components are in circuit as there are usually other paths to ground. I have never in 40 years ESD protected an amp, mixer or pre amp of any type.

Handling IC's out of circuit is a different matter and earth straps are recommended especially with man made fibre clothes that produce static.
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Old 14th July 2018, 10:31 PM   #5
MarcelvdG is offline MarcelvdG  Netherlands
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Maybe I should have phrased my second post differently. What I meant is that you need the combination of voltages outside the range from ground to supply and high enough currents to trigger latch-up.

I know of one commercial product that had a serious latch-up issue: a PCI video digitizing card with a Conexant Fusion chip on it. When you connected a surveillance camera to it, the order in which you powered up the camera and the PC with the card determined whether everything worked normally or the chip would die. The package of the chip would actually burst when you connected stuff in the wrong order.

ICs are indeed much less sensitive to ESD when they are on a board, because the power supply decoupling capacitors are in parallel with the supply-to-ground clamps and improve protection. Nonetheless, there are many products that have extra ESD protection outside the ICs. Antenna inputs of car radios are one example, but as far as I know, the same holds for products like memory cards with exposed contacts.
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Old 14th July 2018, 10:35 PM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by MarcelvdG View Post
I know of one commercial product that had a serious latch-up issue: a PCI video digitizing card with a Conexant Fusion chip on it. When you connected a surveillance camera to it, the order in which you powered up the camera and the PC with the card determined whether everything worked normally or the chip would die. The package of the chip would actually burst when you connected stuff in the wrong order.
.
We designed some equipment for a telephone exchange many years ago and it worked great. Then the customer said they wanted to plug in line cards with the system switched on !
We had to put an extended ground terminal on the connector so it touched first. And also every single data line on the edge connector needed pairs of diodes to VCC and zero volts to stop latch up.
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