Hello:
I want to design an adder with an amplifier to support more than five inputs.
Just refer to the example as below.
Besides, the frequency of the inputs is almost 200Mhz.
Would you please suggest me an amplifier and a referrence schematic?
Thanks a lot.
You will likely need to use a current feedback summer to get the BW. A part like the OPA695 or LMH6702 might work. You will have to use lower R values, around 402 ohm each if you want unity gain on each channel, yes, this looks pretty good using 402 for all R's
What simply works at low frequencies need not necessarily to work at high frequencies. Already mounting five or even more feedback resistors to the -input of HF OPAmp alone, might introduce so much wiring inductance and stray capacitance that the OPAmp might begin to oscillate.
Because of this, you need to decrease the number of channels which shall be summed by one single OPAmp. And to reach the number of channels you want to sum, cascade these summing amplifiers.
Keep also in mind, that the input summing resistors must be driven by a low source impedance, sufficiently low up to the highest frequencies. Otherwise the summing amplifier can become unstable. Therefore, it would be a good idea to drive the input summing resistors by individual OPAmp buffers.
Then, as it is difficult (or impossible) to match the impedance of microstrip line to the impedance of input summing resistor, keep the wiring length to the input summing resistors to the absolute minimum. A 51R isolation resistor directly at the output of driving OPAmp can be helpful.
The following schematic shows how I would do it:
Of course, this is only the first stage of cascading. Further cascading stages are necessary, if you want to sum more than five input signals.
And, please remember, what I said above: If the unavoidable routing inductance and stray capacitance of five input channels already results in instability, you might need to decrease the number of input channels.
Kai
Hello,
Thank you for the information. I forgot to mention that I would also need the amount of power to determine the current using power, wavelength, and photosensitivity (found in datasheet).
For now, I have it as max current at 100uA, and I have the gain set to 10kOhms which will give you a closed loop bandwidth of around 603MHz for a Butterworth response using the OPA855, which is our 8GHz decompensated amplifier. This would follow the setup Kai proposed above. For a very high speed part like the OPA855, wiring inductance and stray capacitance will be critical or the amplifier will oscillate.
We can help create a simulation using the OPA855. This will work for pulses with min rise times of around 580ps. Will this work for your type of application?
Thank you,
Sima
Hello Michael and user,
That is correct, the feedback capacitance calculated would be very difficult to realize in practice. Thanks for the additional knowledge on PRP resistors! I was not aware of these.
Also for noting the correction on the bandwidth. I was going by the APD given, but I now see that the application would need down to 200MHz. This makes this easier, here is the updated calculation while still assuming the same input current and output voltage requirements as stated earlier:
Would only now need a sub 1GHz GBW part. These products should work for this type of application. Can be filtered down on noise/dc/power/supply requirements.
Thank you,
Sima
