Hello! First post here. I'm a second year electrical engineering student.
My current (hobbyist) project is a 100Msps/20MHz USB oscilloscope using an FPGA.
I have it all planned out (schematics all drawn, too) and was about to start laying it out on PCB.
Fortunately, I decided to simulate it (the analog input frontend part) before continuing on, because I realized the design won't work at all.
The problem is that the input amplifiers have a relatively low input impedance of ~1Mohm, and that doesn't play well with the 1Mohm input impedance of the probes!
So I did some research, and apparently MOST high frequency amplifiers have low input impedance. Some even in the few hundred kHz range.
Except for those with JFET input stages. They go up to about 1Gohm, and sound just right for my needs.
However, my question is, what are their disadvantages compared to "regular" op amps? Google didn't want to tell me anything... and I'm sure there are disadvantages since otherwise all op amps with be JFET already, and I want to be aware of them.
We haven't learned about amplifiers in classes yet (well, only how to use them, not their construction), so I don't actually know what that means.
Many thanks!
My current (hobbyist) project is a 100Msps/20MHz USB oscilloscope using an FPGA.
I have it all planned out (schematics all drawn, too) and was about to start laying it out on PCB.
Fortunately, I decided to simulate it (the analog input frontend part) before continuing on, because I realized the design won't work at all.
The problem is that the input amplifiers have a relatively low input impedance of ~1Mohm, and that doesn't play well with the 1Mohm input impedance of the probes!
So I did some research, and apparently MOST high frequency amplifiers have low input impedance. Some even in the few hundred kHz range.
Except for those with JFET input stages. They go up to about 1Gohm, and sound just right for my needs.
However, my question is, what are their disadvantages compared to "regular" op amps? Google didn't want to tell me anything... and I'm sure there are disadvantages since otherwise all op amps with be JFET already, and I want to be aware of them.
We haven't learned about amplifiers in classes yet (well, only how to use them, not their construction), so I don't actually know what that means.
Many thanks!