alvays
Joined: 07 Jul 2004
Posts: 64
Helped: 3
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 23 Dec 2004 8:37 How to test and design a nA order circuit |
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I need to design a circuit,Current-to-Voltage, with 0.6um CMOS MixedSignal Technology. which has a current as low as 1nA.
What is the essential point to design that? And how can I test a 1nA order current, by which test instrument?
Sorry, I have not any clue yet now.
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VVV
Joined: 26 Nov 2004
Posts: 1466
Helped: 269
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| 29 Dec 2004 10:04 Re: How to test and design a nA order circuit |
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Basically, measuring low currents is easily done with an opamp having a resistor in the feedback path. Since your range is in the nA, you will need a low offset, low bias current opamp. I would suggest the LMP2011 from National, but other parts are also suitable. I had no other design parameters, so I selected this part.
This particular part is offset-compensated, which means you can only measure slow-varying currents; basically the offset is adjusted periodically inside the part; this can be done with discrete parts, too, but it gets complicated, this part only costs $0.98.
Another method would be using a chopper/ syncronous rectifier, but is just about as complicated.
So, I selected a pretty basic circuit which is presented in the figure. The bias current for the part is 5pA, which is 0.5% of your range. The offset is 60uV max. over temperature (lower if the range is restricted). The voltage drop across the 10MΩ resistor will be 10mV at 1nA, so that is another 0.6% error. Another contributor to the error will be the 10MΩ resistor. Try to get a resistor with a low tempco. Suppose you get one with 25ppm/°C. If your temp range is 0-40°C only, then you have 0.1% from it.
The gain is set to 100 by R2,R3. That allows a lower value resistor for R1, which is easier to get and more stable. To adjust the gain in order to compensate the tolerances, I would simply add another resistor in parallel with either R2 or R3, no pots. Just solder different resistors and check calibration until you get it right.
As for producing nA currents, you will have to get some very large value resistors (a few 100MΩ ones) and connect them in series with a very low voltage, 0.1V or so.
With the values shown, the maximum output will be Vo=-R1*(R3/(R2+R3))=1V for 1nA input. The input resistance is R1/(R3/(R2+R3)*Ao-1)=3.3kΩ for Ao=300000. (You can take this into account when calculating the current you produce for testing).
A clean layout is required, PCB should be free from flux residue, dirt, etc. Input shielding is recommended.
For more on this, please see also this appnote: http://www.national.com/an/AN/AN-242.pdf
Circuits 11 and 12 are suitable. Circuit 12 can be simplified a little.
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VVV
Joined: 26 Nov 2004
Posts: 1466
Helped: 269
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| 29 Dec 2004 20:02 Re: How to test and design a nA order circuit |
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Just wanted to add a few more things:
Resistors R2, R3 also need to have a low tempco.
An offset adjustment may be needed. Use a pot to add a very small voltage to the + input of the opamp. The adjustment would be done with the input shorted. As I said previously, no pot should be used for gain.
Shielding of the whole circuit may be needed.
And finally, it may be best to pot the whole assembly to protect it against moisture.
Back to producing nA currents, you can get a so-called DC voltage standard to produce higly accurate low voltages and use high precison high-ohm resistors in series with that voltage to get extremely low currents. I am not aware of any commercially available nanoammeter.
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