How to protect op-amp high resistance input ?

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kekon

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I've just designed high impedance amplifier for a pH electrode. As one knows such electrode needs to be connected to an amplifier with very high resistance. Of course I chose AD820 op amp with FET inputs. A typical pH electrode has internal resistance of about 200MOhm or higher. This electrode will be connected through BNC connector but I have no idea how to protect AD820 input against ESD or voltage spikes. One cannot do it by placing any diodes at the input as they will introduce large errors (their reverse currents are too high). Maybe someone has experience with the problem described ?
 

You could have resistors and a mechanical switch. The resistors are in the circuit when there are no electrodes connected to the input and out when there are.
 

flatulent said:
You could have resistors and a mechanical switch. The resistors are in the circuit when there are no electrodes connected to the input and out when there are.
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Well, it would work very well but if someone forgets to switch the resistors on ?
Proffesional pH meters do not have any switches to protect their inputs. Maybe there are BNC connectors with integrated switch (such as with some jack connectors used for headphones). But I consider to use your idea.
 

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