Design of a surface resistance tester

[Nick_F]

I am trying to design a surface resistance tester. Typically such an instrument uses a high voltage between probes (100V or 10V, depending on resistance value) and it is capable of measuring values from 1 kohm to above 100 Gohm (precision is not essential, a 10% would be ok). My intention is to use a linear ohmeter circuit, so I can convert resistance directly to current or voltage and display it digitally. I am looking at measuring the high end of resistance (from 10 Mohm to 100 Gohm).
After checking some circuits, one possible method relies on using an opamp. The gain of an opamp in inverting configuration is Vout = -Rf/Rin*Vin (Rf is the feedback resistor, Rin is the input resistor and Vin is the applied voltage to input). So, if I connect the test probes in the feedback loop of an opamp, I could get an output voltage proportional with the resistance to be measured. But the test voltage would be limited to the supply voltage of the opamp (it would be nice to use a 9V battery to supply the whole circuit). How can I have 100V test voltage between probes. The 100V can be obtained with a DC-DC inverter.
Perhaps there are better methods than this one.

Thanks,
Nick

 

[FvM]

Having a varying test voltage might be unwanted, in some situations you'll measure a breakdown voltage with fixed current instead of a resistance with fixed voltage. But that's your choice.

The feedback circuit driving the variable voltage to the probe must no necessarily be an OP, it can be a discrete or integrated voltage amplifier with low quiescent current.

 

[Nick_F]

Having a varying test voltage might be unwanted, in some situations you'll measure a breakdown voltage with fixed current instead of a resistance with fixed voltage. But that's your choice.

The feedback circuit driving the variable voltage to the probe must no necessarily be an OP, it can be a discrete or integrated voltage amplifier with low quiescent current.

The test voltage is fixed and it has two steps (either 10V or 100V, depending on the measured resistance - start with 10V and if the resistance is too large to be measured, then move to 100V). The tester is used to identify if a surface (floor, work bench, chairs) is conductive, dissipative or insulative, for ESD purposes.

I know of CMOS opamps that have very low bias currents (a few femptoAmps). I don't know how to build something with discrete components that would match their huge input impedance and probably it is easier to use a readily available and cheap opamp.

 

[moeedmughal]

I know of CMOS opamps that have very low bias currents (a few femptoAmps). I don't know how to build something with discrete components that would match their huge input impedance and probably it is easier to use a readily available and cheap opamp.

I am developing a resistivity meter based upon wenner array. Used LMP7721 in transimpedence configuration to measure low current. (https://www.ti.com/lit/ug/snou004/snou004.pdf). See if it is of any help.

 

[FvM]

I know of CMOS opamps that have very low bias currents (a few femptoAmps). I don't know how to build something with discrete components that would match their huge input impedance and probably it is easier to use a readily available and cheap opamp.

I did not suggest to build the OP from discrete components, only the high voltage output stage.

 

[Nick_F]

I did not suggest to build the OP from discrete components, only the high voltage output stage.

Thanks for clarification. Are you suggesting using a high voltage transistor at the output of the opamp and use its emitter as the high voltage point for the feedback resistor?

- - - Updated - - -

I am developing a resistivity meter based upon wenner array. Used LMP7721 in transimpedence configuration to measure low current. (https://www.ti.com/lit/ug/snou004/snou004.pdf). See if it is of any help.

Thank you very much for the link and info (I had a quick look at what wenner arrays are). It's very interesting how careful we need to be about cleaning the pcb when dealing with very low currents or high resistances.

 

[FvM]

Are you suggesting using a high voltage transistor at the output of the opamp and use its emitter as the high voltage point for the feedback resistor?

You need voltage gain after the OP to increase the voltage swing. I didn't consider a specific circuit, but there must be at least one transistor in common emitter configuration. Notice that an OP with an external gain stage usually needs specific frequency compensation.

General purpose OPs combined with high voltage (100 - 200V) output stages can be often found in audio power amplifiers.

 

[Nick_F]

You need voltage gain after the OP to increase the voltage swing. I didn't consider a specific circuit, but there must be at least one transistor in common emitter configuration. Notice that an OP with an external gain stage usually needs specific frequency compensation.

General purpose OPs combined with high voltage (100 - 200V) output stages can be often found in audio power amplifiers.

I found a diagram with an output stage supplied from ±120V here: https://www.ti.com/lit/an/snoa600b/snoa600b.pdf
For my purpose, I would need single supply - I'll keep digging.