i am looking for a circuit to limit or clip the output voltage of a differential op amp (ADA4938) which drives the input of an ADC (AD9259). With the ADA4938 i am converting a single ended 50Ohm terminated signal to a differential signal for the ADC. My current schematic is the same as in the datasheet of the ADA4938.
But now i want to limit the outputs (out+ and out-) to the maximum voltage which is allowed at the inputs of the AD9259 by -0.3 to 2.0V as the frontend is used for data acquisition. That is why i can not prevent the input signal (vin_single) from overdriving or even destroying the ADC inputs.
First i thought of using comparators which power down the ADA4938 in case of a voltage which is too high. But the power down function of the ADA4938 is too slow and in that case i would not see the clipped signal in digital at the output of the ADC.
A solution with diodes is hard to realize because i did not find any zener diodes with voltages less than 1.8V.
Recently i found a circuit in an AD application note which is used for active output clipping (see below). This works well in this configuration but i don't want to use an additional op amp as this increases noise, power dissipation, etc..
My question is now:
- How do i implement a clipping circuit for not overdriving the ADC inputs?
- How can i extend the existing diff. op amp circuit (see graphic above) so that i can provide an active output clipping function like cicruit below. Would this be suitable (influence of parasitic capacitances of diode etc.)?