Svart
Well-known member
As I chug along on one of my many projects, I came across a fork in the road. Go with the normal impedence converter consisting of discrete BJTs as the frontend to an opamp, or use the inputs of an opamp straight from the source?
As I figure, the opamp already has BJTs or FETs on it's inputs so why series these parts? why not find an opamp with low noise inputs?
I came across some $$L schemos and behold.. the 9k uses opamps straight on the buss with no BJTs before it. I guess I was on the right track here.
Searching through datasheets and info I see that the BJT front should be low noise, but how low is good enough?
the mighty MAT04 that I was planning on using previously has a voltage noise of 2.5nV/rthz. the AD829 that $$L uses are 1.7nV/rtHz!. the AD812 is 3.5 nV/rthz. The 5532 is 5nV/rthz.
besides other qualifications like slew and drive capability, how low should the noise be? what is the threshold of tolerable?
:thumb:
As I figure, the opamp already has BJTs or FETs on it's inputs so why series these parts? why not find an opamp with low noise inputs?
I came across some $$L schemos and behold.. the 9k uses opamps straight on the buss with no BJTs before it. I guess I was on the right track here.
Searching through datasheets and info I see that the BJT front should be low noise, but how low is good enough?
the mighty MAT04 that I was planning on using previously has a voltage noise of 2.5nV/rthz. the AD829 that $$L uses are 1.7nV/rtHz!. the AD812 is 3.5 nV/rthz. The 5532 is 5nV/rthz.
besides other qualifications like slew and drive capability, how low should the noise be? what is the threshold of tolerable?
:thumb: