ADC Protection

[CallumSmithSound]

The ADC that I was planning on using (PCM1865) has a non-standard input:

• The analog side of the device is powered of a single 3V3 supply
• The full scale input voltage is 5.94VP-P (2.1VRMS) (-1.35V MIN / +4.65V MAX)
• The absolute maximum ratings are -1.7V to 5V This will cause issues if the volatage from the mic preamp becomes too high. The standard way to protect this is to use diodes from the signal to GND and 3V3. However the full scale voltage is ±1.7V more than that range. The only types of diodes with a forward voltage of ~1.7V are very large and expensive.

[CallumSmithSound]

One solution is to use zener diodes with a zener voltage of ~12V between the ±9V rails. However, most zener diodes have ~5% tolerance which could mean that the max ratings are exceeded or, clipping ocours before the full scale voltage is reached

[CallumSmithSound]

It seems external diodes might not be necessary if the current is limited to below ~5mA

[CallumSmithSound]

To limit the current through the ESD diodes, a resistor can be added in series: The max voltage from the opamps is ~8V which is ~9.65V when biased. ESD diodes engauge at around ~5V so the voltage drop is 4.65V. To limit the current to about 4.5mA a resistor of ~1KΩ will be needed