3 in1 Precision milliohm meter 10ma ccs and high precision 1.25v Vref

Everything is based around 3 Ic's starting top left. Is the LT 3092 precision CCS it has a 10uA constant current source, opamp and output circuit that can source several hundred ma. in a single ic. This is coupled with the LT1634 0.05% precision Vref to provide a very accurate 10ma constant current and separate 10ma precision 1.25v vref. The 150R and 750R with the LT1634 set the output current to 10ma. This current is passed across the resistor to be measured, this produces a voltage proportionally and is amplified X100 by the AD8551 a zero drift precision microvolt input offset opamp with a gain of well over 125db. If you connect a digital voltmeter set to the Volt's or millivolt range at the output of the 470R resistor bottom right, this voltage will show the resistance. For example a 1.2R resistor will show 1.200v a 0.15R resistor will show 0.150v so the voltage shown on the dvm is proportional to the resistance and is to be read as ohms or milli ohms if your dvm has a milli volt range. The 11.1mili ohm resistor bottom left is the resistor to be measured. The switch next to it disables the the resistance reading and select's the vref output. The switch prevents open circuit or the offset across the measuring resistor adding to the vref . The precision current generated at the measuring terminals can be used for a current source of 10ma from 0 ohms up to 3.3R loading can be acomandated. The whole circuit should use less than 13ma and run for many hours on , 3 or 4 AAA batteries. Supply voltage is 3.7v this provides adequate over head and is made from 3.3v regulator with a schottkey diode in the ground pin. Alot of low power 3.3v regulators are prone to not recover a supply reversal, the Schottky protects this and lift's the regulator output a little to improve head room. Best not to use a 5v regulator as this will encourage thermal drift in 10ua ccs though it wouldn't matter much but would reduce the choice of batteries. The power switch is on the left. The zobel across output stage is required to be close to the ic pins as it help's to null the pole introduced by 47nf on 10ua ccs. Current noise here could be a cause for instability without it. A hand full of precision 0.1% 20 ppm or better resistors are needed but they are really quite cheap nowadays and not many are need. And are 150R 750R 11k 95.3k and two 100R. It will give a milli ohm accuracy of better then 0.5% and can be accurate into the micro ohm's with a 4&half digit dvm. The pot is for output trimming. If you notice on the circuit there are 4 connections to the resistor to be measured and it is important to bring them right to the contact point 2 either side of the target resistor to compensate for the lead resistance. This is difficult to show on the diagram also all opamp grounds and supply bypass caps must go directly to supply ground and go nowhere near the test resistor ground. I hope this point is clear as it affects accuracy greatly in the sub ohm range. The 2.2R and 220nf cap just after the power switch prevent ringing caused in the inductance of long power leads, since this circuit behave like a low ESR cap briefly at switch on. Once started this ringing could build. The 2.2R & 220nf should be adequate to damp this quickly. Similarly the 470R & 100pf prevent capacitative loading and damp out inductance in the output leads as well as giving short circuit protection to the output opamp. The 2k2 is the 1.25v vref output top right. The 2k2 isn't strictly required as this output is current limited to 10ma. I think that's the important matters covered briefly. You could switch in parallel combination's of resistors across the 150R/750R to give other current range's if you wanted. Should cost less than £10 in components and will measure a 1 milliohm resistor accurately if constructed carefully. And measure sub milliohm with appropriate dvm and parts.

published 9 years ago