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Thank you for your replay,actually I am little confused about the voltage at which the micro ohm meter might measure the required resistance.
Need some help on this also.
If you use a microcontroller to do the calculations, then i reccomend:
* Use a stepdown switching regulator with current limit set to 5A.
* use the enable/shutdown to control power with uC
* measure current
* measure voltage
* calculate resistance.
With that there is no need for a precise "5.000 A" current. The calculation will do it right.
Or do you mean 0 to 2000 microohms with resolution of 1 microohmm?
With DC stimulation you may see errors caused by thermocouple effects. If you need high precision you might think of some kind of AC stimulation.
Hi,
You are right on this,I need a resolution of 1mOhm.
And I know about that,effect of dc will cause drifts on the readings.So a calibrated shunt across the current line will determin the current via ohms law. We get current via shunt
Plus the voltage drop across the DUT in mV.
Am I right!
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Ac exitation will be good to our measurment needs,but doesn't a constant current regulation might be hard to deal with?
just to be sure: resolution is 1 microohm or 1 milliohm?
The current shunt is not that big problem, because you have equal temperature at he joints, so ideally the thermocouple effects will cancle itself.
The bigger problem is the voltage measurement at the DUT. Here the voltages might be much smaller than at the shunt and the temperatures are not predictable.
AC excitation is not that problem.
I´d use some H-bride configuration with MOSFETs. the gate currents are negligible and the voltage drop is no problem. Low voltage types (<20V) with Rds-on of a few mohms are easy to find.
Switching speed is no problem, power loss is also no problem.
If you need precision, then do it with AC.
software is important, too. But that after hardware is finished.
Resolution is 1 micro ohm!
But I want to know about how to limit current in the circuit at minimum range of micro ohm.
And a precision opamp will read a voltage range in milivolts!
I think we would require large current to measure the DUT
Lt 3741 is a pretty good current driver ,I will totally look into it.
And being on the signal conditioning circuit on DUT,the concern are towards measuring the micro volts over to adc acceptable range.
If we would use AC signal instead then the need of AC TO DC RMS converter will increase the Circuit Complexity and Error rates. LTC1966 is a substitute to do so.
What do you think.
Hey,
Its a good idea to choose a bipolar Adc .
So we now require the followings:-
Opamp running in chopper mode
Signal converted via bipolar adv
The a MCU with LCD display
And a current driver using H-bridge to generate AC output with current limiting.
For the current generator, I was thinking of a single ended transformer coupled set up. Run it at 22 HZ (to avoid inductive effects and the mains frequency, also the op-amps will have a higher gain). Put a hall effect current sensor on the output line and use it to control the current by modulating the Vcc to the output transistor.
Frank
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