digital mega ohm meter designing

[yogece]

hi,
   can anyone help me with the schematics of mega ohm meters or some suggestions

[Rerouter]

create an accurate current source probably ~10-100nA sourcing from your supply rail to 1 terminal, and the other terminal tied to a signal ground (not just the miserable ground plane)

measure the voltage on your sinking terminal with an ADC if you really want digital,probably including an op amp that lights up if its close to the supply rail as its then considered open circuit when your source cannot accurately produce it.

or if you just want a digital display, buy a cheap lcd / 7 segment display voltmeter module, and adjust your dot point as necessary, remembering to tweak the offset before you use it,

edit: in both cases the sinking terminals voltage should be buffered by an op amp with a very high input impedance and low current draw on the inputs.

[kripton2035]

here is one open source : http://www.edaboard.com/thread287707.html#post1230438

[kripton2035]

silicon chip did some too : article 103504 and article 112015

[TiN]

First step in designing should be define required specification. Is it only megaohm meter? For resistor measurement or in-curcuit probing capable device? Or maybe insulation meter? Does it need any controls, if yes - then which ones? Display or remote readout? If remote - which interfaces? Then power supply Answering this questions will bring idea of what needed to be done, as resulting design approach will change a lot for every different case. If it's material/insulation meter may be enough to generate higher voltage and get away with uA/mA range measurements. If in-curcuit/low voltage - entering nV/uV ranges for signals will bring lots of headaches.

Also if need just working reliable tool , then often it's more cheaper to buy existing meter. But if design purpose is education or some special requirement meter - that's another story. 

[brian_stine]

Do you just care about resistance or resistance at 1000V?  Most mega-ohmmeters are insulation testers that test the current at 1000V.  Such meters are not useful if you just want to measure the resistance of high impedance DUTs.  I once tried to test the resistance of a 1GOhm resistor with an insulation resistance tester and blew it to bits because 1000V was too high for it.