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.)?
ADI offers clamp amps that could suit your needs like AD8036/AD8037low distortion voltage feedback clamp amplifier. I'm not sure what exactly is your operating BW but this clamp amps has small signal BW of 270 MHz. Here are some good stuff to read that might help you. ADC Input Protection and Clamping and this app note might also be helpful: AN-402.
By theory, you can extend the existing dif amp with the same configuration with active output clipping from the app note by having two pairs of this circuit but we cannot guarantee that best performance of the ADC having this frontend circuit without having try it on the lab. There are lot of things needed to consider when driving ADC inputs.
Hope it helps,