Impedance measure circuit ?

Discussion in 'General Electronics Discussion' started by forever074, Jan 23, 2015.

1. forever074

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Mar 14, 2014
I want to design a circuit to measure impedance of device in rang 10OHM to 1MOHM. Does anyone have any idea to share me?
Thanks.

2. KrisBlueNZSadly passed away in 2015

8,393
1,267
Nov 28, 2011
Do you mean impedance, or just resistance?

If you mean impedance, you have to specify at least the test frequency.

If you mean resistance, the usual way is to supply a known current and measure the voltage drop that it produces.

Is this the kind of answer you're looking for? If not, explain yourself clearly.

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3. forever074

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Mar 14, 2014
@KrisBlueNZ . I mean resistance.
1. How can I design serveral uA current source to measure serveral MOHM resistance with 5% accuracy? ( With a current source of serveral mA, I think I can do, but with uA it is so difficult to design)
2. Why dont we supply a known voltage to a known resistor connecting in series with an unknown resistor and measure the drop voltage of the unknown resistor.
Thank you.

4. KrisBlueNZSadly passed away in 2015

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1,267
Nov 28, 2011
Google op-amp current source schematic to find examples of current sources that can operate down into the µA range. You just need to use a high resistance for the current shunt.

The reason you wouldn't use a voltage divider is that the voltage you measure would not be linearly proportional to the resistance so a calculation would be needed. Here is the graph of measured voltage vs. resistance for a circuit that applies 10V through a 10 kΩ resistor to the test resistance.

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5. forever074

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Mar 14, 2014
@KrisBlueNZ . When I use a 1uA current source to measure the resistance of a capacitor (100uF) connecting in parallel with a resistor (1MOHM), it takes too long to reach the balance state ( i=C*du/dt-->dt=C*du/i with i=1uA). So do you know how to make this process faster.

6. KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
I guess the obvious answers are to either supply a much higher current initially, to bring the capacitor voltage up to the measurement maximum voltage quickly, then switch to the final current to do the resistance measurement, or avoid using a resistance measurement circuit to measure a capacitor!

The first option isn't ideal either because when you switch to the final current to measure the resistance, the capacitor may have to discharge, which will also take time.

Are you trying to measure the leakage current of large capacitors? If so, you should have said so at the start. You'll always get a useful answer more quickly if you don't play guessing games.

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7. forever074

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Mar 14, 2014
@KrisBlueNZ .
I am designing a circuit to measure the resistance of electronic circuit following what u have instructed at last email. But I got a problem when measuring the resistance from 3.3V (voltage after LDO TLV1117-3.3) point to GND caused by a 100uF capacitor placed between 3.3V & GND. So could u please show me how to measure the resistance from 3.3V to GND at this case?

8. KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
So you have a TLV1117-3.3 regulator with a 100 µF electrolytic capacitor across its output, and you want to measure the leakage resistance of that capacitor?

So you measure the "resistance" from the +3.3V rail to 0V without the capacitor, then add the capacitor and measure again?

Have you made any measurements of the resistance from the +3.3V rail to 0V with the capacitor removed? What were the current, voltage, and resistance values?

What other circuitry is connected to this +3.3V rail?

Please describe the whole project in a lot more detail.

9. forever074

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Mar 14, 2014
@KrisBlueNZ .
I have just designed the schemmatic, not set up the board yet. But this is one situation that I think may occur when measuring, and I want to make sure that it can work well so I ask you in advance.
Our company plans to manufacture a large amount of electronic board and we need to measure automatically the resistance ( impedance) at some points of the board to assure the quality of the board after soldering.
My project is: To design a circuit to measure resistance of those boards above.
I see all measuring points are OK except for the one below ( from 3.3V to GND) which takes a lot of time to finish measuring. So could u please show me how to measure the resistance from 3.3V to GND at this case?