Electronics > Projects, Designs, and Technical Stuff
Very low bias current Op-amp to buffer a Kelvin Varley divider
Kleinstein:
The resistance dependent offset is just a different description of bias current. One they compensate for the bias, the offset is not effected by the input impedance.
With AZ OPs the offset can also depend on the AC impedance at the internal sampling rate. So one may need to have some capacitance at the input.
The 30 pA or so for a modern meter is not only the amplifier but also includes protection circuits, range switching and the layout. With such low bias ratings it's important to look at the temperature and maybe humidity for which this is specified.
High end meters often use a very slow auto zero cycle and a discrete JFET amplifier. Like with integrated AZ OPs its finding the right balance between low bias and low noise. The overall performance seems to be not significantly different.
Even with a noisy amplifier there may not be much need for filtering, as most meters are not that sensitive to noise. The usually long sampling time is already a very efficient low pass filter.
Gyro:
--- Quote ---You have to admire the designer's skills given these date to the 70's (60s?)
--- End quote ---
Absolutely. Early 70s certainly. The days when men were men and sheep were nervous ;D It's all too easy today. Lots of lessons still to be learned from the challenges overcome in vintage stuff.
As for the input current, yes I guess leakages start to dominate - lots of PTFE standoffs in the Datron, don't know about the Agilent, but both would have active guarding.
As for the modern AZ amps, Ib and Vos compensation probably have much less importance but supply bootstrapping would probably still give a significant improvement in CMRR and probably other parameters, as well as allowing a greater i/p swing (outside the supply limits of the AZ amp).
TimFox:
To avoid current from common-mode voltage, the guarding referred to above is essential. Even a PTFE standoff can have leakage resistance from humidity (although PTFE is naturally "hydrophobic"), so having a guard ring with appropriate drive at its base is a good idea.
Kleinstein:
Having a bootstrapped supply, bootstrapped protection circuit and driven guards can be often combined to to together in one circuit.
With guards and a few cuts, one may get around extra standoffs.
Whether you need bias compensation depends on the AZ amplifier. The very low noise versions have a bias that is to high to be tolerated - in the 100 pA range. With supply bootstrapping the bias compensation may be as little as a very high value resistor and a trimmer.
Echo88:
Maybe someone can help me out with the understanding of the KVD-buffer mentioned here and in the AN86 Page 9: http://cds.linear.com/docs/en/application-note/an86f.pdf
I only ever thought about CMRR in difference amplifiers which measure tiny signals. In this case its mentioned together with a voltage follower; how does the CMRR manifest as an error in this circuit?
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