| Electronics > Metrology |
| Precision current source single Mosfet or twin transistor setup? |
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| Overspeed:
--- Quote from: Kleinstein on January 04, 2025, 07:32:13 pm ---For a resistance measurement one likely would not need the high settling speed of the 6220 and not much current programming other than maybe on / off / reverse polarity. For a 100 mA range only, the main critical part is the current setting resistor. One would likely want a 4 wire resistor or at least separate traces directly from the resistor. Depending on the meter used for the voltage reading, one could also consider the variant with the DUT and ref. resistor directly in series and not on different sides of the FET. This version would allow easier switching between the voltage at the DUT and ref. resistor and also an easy polarity switching. However it kind of needs a separate supply, and a ratiometric measurement gets a bit more tricky. Depening on the resistance range a true ratiometric measurement may not be needed - very low resistors (e.g. < 10 mohm) may well have other limitations. --- End quote --- Hello Thanks I agree on the simple need compared to the possibilities o 6220 . for the 4 wires resistor I agree that in use on the eevblog 577 amp and also on Zlymex ( eevblog post ) I link the picture of the circuit and the current values. architecture looks similar , a precision voltage source Zlymez use a AD780 and a VR4 resistor and Eev use a ltc6655 and a VPR221Z with an OPA376 both voltage source are 2.5V A concern is the resistor heating as the sense resistor handle the power so a direct link wih the voltage and the current . Regards OS |
| Overspeed:
--- Quote from: iMo on January 04, 2025, 09:50:55 pm --- --- Quote from: Overspeed on January 04, 2025, 06:49:55 pm ---Question is for who ? OS --- End quote --- The Q is for you. You want a precision current source, but except the "100mA" there is none other indication what you actually want (provided this is the Metrology section people wait on some indication). Based on some specific parameters (meaning numbers) people may judge on the actual circuit and parts to be used. --- End quote --- Hello I agree , base on data provided by Zlymex on his 1ma source ( see pic linked ) and the table linked or the 6220 concern is more the current level increase , more difficult to reach tight specs . Times Electronic 1021 DC Current Source is advertised for ± 0.02 % of setting ± 0.02 % of range, ± 0.2 μA and 25 ppm/hr stability which sound acceptable Times Electronic 1024 is accuracy ± 0.02 % of setting + ± 0.005 % of range + ± 0.2 nA As I measure low resistivity alloy I need 10 mA and 100 mA , problem with 100 mA is the Keithley 6220 is not wonderful on he 100mA range with 0.1 % accuracy and no real value regarding the noise or the stability under load I need something better than the performance of the 6220 without any other option as that a 100 mA only DC Times Electronic 1021 looks better for my purposes. I will try the Trobbins circuit with selected resistor and a ref55xx , perhaps set two Mosfet in // can improved the thermal concern of the Mosfet ? in the aim to built a better 1024 time Electronic model Regards OS |
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