Resistor Ohm Meter

[khetanikrunal]

see My test voltage is 500 V DC...

Even i must use amplification of lower current(from Piko amp to Nenoamp)..

system is like this

HV(500v dc)----->Resistance of Oil(1mohm to 2000Tera Ohm)------>LV.

And i have to Take current from LV point and convert it to voltage send to controller via ADC.

[Gusse]

So your current sense capability is limited to ~0.5nA with 1Tera ohm measurement equipment. Definetely it would be much easier to try lower sensing capability than increase test voltage.
Most sensible would be increase test voltage by 2x (or 4x) and try to improve current sensing (what ever it might need).

Pico amper range current sensing is very demanding and any noise from environment will affect your results (even measuring at DC). Most probably you have to do your measurements in shielded rooms, etc..

Even if you could build up measurement system that could measure up to 2 Peta ohm then limiting factor for accuracy would be ADC (even 16bit). One step would be ~30.5G ohm. Probably you should consider some better ADC than in PICs. (This is valid only if you have just one range for input, full range).

BR,
-Gusse-

[khetanikrunal]

hello

I can measure up to 3 nenoamp and 6 uV.
event thats not problem.

I am not able to get linearity.

i.e., if ADC o/p 500mV for 1 teraohm.

then i am not getting 250 mV for 2teraohm.

[Gusse]

hello

I can measure up to 3 nenoamp and 6 uV.
event thats not problem.

I am not able to get linearity.

i.e., if ADC o/p 500mV for 1 teraohm.

then i am not getting 250 mV for 2teraohm.

Even i must use amplification of lower current(from Piko amp to Nenoamp)..

Now I'm a little bit puzzled.

Do you mean that you can measure:

1) 3 nano A (3 * 10^-9)

500V -> Rmax = 170G ohm

or
2) 3 pico A (3 * 10^-12)

500V -> Rmax = 170T ohm

Rmax is maximum resistance that you can measure.

When you try to measure 1T and 2T, how you know that insulator is exactly ~1T and ~2T? This linearity issue is also bit unclear.

BR,
-Gusse-

[Melanie]

There comes a point where you have to look at the problem from a different angle.

Why not connect the 'Resistance' to a precision Capacitor and measure the rate of charge? You will still need a high-impedance amplifier, but at least you're not going into territory with silly values that are likely to change drastically if someone walks into a room without breathing apparatus.

[Gusse]

Capacitor charging method sounds far better solution than current sensing of some pico ampers.

..., but at least you're not going into territory with silly values that are likely to change drastically if someone walks into a room without breathing apparatus.

Well said Melanie

BR,
-Gusse-

[mtripoli]

Head over to Cyrustek (http://www.cyrustek.com.tw/product-1.htm) and have a look at their line of IC's. I was looking at them not to long ago for a project I have going. Most of them have pretty simple serial interfaces. "Smart-Tweezers" use one of these devices.

A question: What's a "precision Capacitor"? High-impedance amplifier that doesn't need silly values?


Mike Tripoli