Electronics > Projects, Designs, and Technical Stuff
low Signal amplification ?
namster:
and What about 4 kelvin measurement used by HP 3458A 34420a 34401a , can you achieve a 10µOhm resolution ? , Also AC excitation has disadvantage of Skin Effect which can affect the measurement
Marco:
--- Quote from: namster on October 26, 2019, 04:35:43 pm ---also a standard instrumentation Amplfier don't give correctly at low voltage input
--- End quote ---
Minimum common mode for MAX4209 is Vss - 0.1 (ie. below the rail). Even if it wasn't though, simply adding an extra negative power rail is hardly a big deal.
Kleinstein:
Most of the modern high resolution DMMs use multiplexer and do a kind of pseudo differential measurement in 4 wire ohms mode. So they measure one end first and than the other. So the actual amplifier sees kind of modulated DC and thus low frequency AC. In addition they usually offer an more where the excitation current is turn on an off to compensate for possible thermal EMF at the DUT. In addition possible offsets from the multiplexer and protection circuit are also corrected. AFAIK they usually don't reverse the current flow, but still use a kind of modulated excitation.
With the modulation slow (e.g. some 1-20 Hz) enough skin effect should not be such a big problem. In the extreme one could test at 2 frequencies and warn the user if the DUT shown a significant difference.
To really get very high resolution an AC current (could be a kind of square wave) is the way to go, because of possible thermal EMF at the DUT. DC wise some 1 µV resolution is feasible, better gets tricky. AC can go down to the 1-10 nV range with some patience.
namster:
@Marco
the MAX4209 is not verry stable with a typical Gain error Temperature of 50ppm and 200ppm for non linearity that is not verry good specs for my application .
@Kleinstein
for reducing Thermal EMF it exists a methode called current reversal , or delta method , for my milliohmeter project i want a instrument that give a good performance for a not expensive cost , in the first version i used a H bridge wich reverse current but there was a high leakage current , in second version I would like to try to keep the same concept but by improving all the circuit , for know i try to amplifiy the signal wich has a full scale of 20mV
i tried a circuit wich have two Opamp with two gain of 10 and 35 (same as 34220a ) but the gain is not linear what can be the problem ?
the offset can be componsate by a DAC
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