Limiting opamp output to 3.3V

[binu G]

Hi,
i have a design with opamp powered at 10 volt but i need to fix the output clamp at 3.3 V. additional circuit should not affect the linearity of the circuit.

do any one have a topology to make this working ?

Binu G

 

[FvM]

To set 3.3V limit exactly, you have to use an active diode circuit. If you simply add it at the OP output, a series resistor
would be needed, which is possibly unwanted. For ideal clamping behaviour (no additional series resistance), the clamp circuit
would control the OP input and has to be designed according to the intended OP function, that wasn't mentioned yet.

 

[cameo_2007]

wont the 3.3V Zener work here?

 

[binu G]

using a Zener on the signal path degrade the analog voltage supplied to the ADC pin of the controller.

 

[FvM]

analog voltage supplied to the ADC pin of the controller

Unfortunately, you didn't mention the practical problem background. In a short, you don't need to limit the voltage to 3.3V exactly.
You need to limit it in a way, that the controllers maximum ratings are kept, and that AD operation isn't affected. In many cases,
a series resistor and the controllers input protection diode are a suitable means. This is possible at least, if a maximum input
current to the analog input (effectively a protection diode maximum rating) is given and you can be sure, that conduction
state of the protection diode doesn't cause errors at other analog inputs.

If you won't rely on internal protection diodes, use external schottky diodes to limit the input voltage to the supplies, below
protection diode threshold voltage. In practice it's the only usable means at high frequencies, where other limiting circuits
don't work any more.

 

[erikl]

using a Zener on the signal path degrade the analog voltage supplied to the ADC pin of the controller.

With 2 OpAmps you can achieve a relatively sharp voltage limitation without jeopardizing the adjacent voltage range too much, s. the PDF below.

Or use one of the active Zener Op Amp Limiters from the .