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| Nanoamp current source troubleshooting |
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| ZeroResistance:
--- Quote from: mvs on May 04, 2019, 07:48:26 am --- --- Quote from: ZeroResistance on May 04, 2019, 06:06:02 am ---But when I change the Input voltage from the reference the current does not linearly increase? --- End quote --- It should be linear. But linear means in general case f(x)=a*x+b and your b is not zero. MCP602 has +/-2mV input offset voltage. It is around +/- 45nA of output current in this schematic. --- End quote --- Woah! +/-45nA is huge! I am practically trying it out with an LMC662 and that has a offset voltage like 6mV!! Also caught an error in the schematic the lead R7 had to be 1Meg i have entered 1M and Ltspice considers that as milli :-/O |
| mvs:
Input offset voltage is more or less constant, so you can try to add correction and calibrate it out. Or you can use chopper stabilized opamp (Zero-Drift, Auto-Zero, etc) like MCP6V* series. |
| Kalvin:
In this particular case where using a battery, it is possible to introduce approx. 200 mV offset voltage between the signal ground and the negative power supply using a simple [schottky] diode. Without the offset voltage created by the diode, the rail-to-rail op amp will not be able to swing its output all the way down to the signal ground. However, introducing the diode, the negative power supply is now about 200 mV below the signal ground and the op amp is now able to swing its output down to signal ground potential without difficulty. Although the signal reference ground will have some ripple (60mV in this particular case) relative to the op amp's negative power supply even with the 10uF bypass capacitor, the typical power supply rejection ratio (PSRR) of an op amp will be able to take care of this "problem". Similarly, any temperature effects introduced by the diode will be attenuated by the op amp's PSRR. Br, Kalvin |
| ZeroResistance:
--- Quote from: Kalvin on May 04, 2019, 01:41:38 pm ---In this particular case where using a battery, it is possible to introduce approx. 200 mV offset voltage between the signal ground and the negative power supply using a simple [schottky] diode. --- End quote --- I had done something similar and tried it out practically, with the LMC662, based on what exe had suggested above. Basically he said to insert a diode in series with the load so that the op-amp works extra and is forced to raise the output above 0.7V, above its output swing limit of 0.1V, however I found something interesting in the datasheet. The output swing seems to be load dependant and seems to go as high as 0.7V when the load is 600ohm or so. Pls have a look at the attached image. I inserted a 1N4148 diode, and then 2 and then went upto 4 in series to mitigate this issue. However I found that at loads above 100K to 1Meg, the circuit wasn't performing well, I sort of thought that well the diode wasn't able to conduct at these low currents, and gave up on the idea. But I may well be wrong. |
| exe:
--- Quote from: ZeroResistance on May 04, 2019, 04:02:39 pm ---The output swing seems to be load dependant and seems to go as high as 0.7V when the load is 600ohm or so. Pls have a look at the attached image. --- End quote --- To me it looks like Vmin and Vmax are swapped in the datasheet. Anyway, that's expected, as there is a current limiting resistor. The more current you draw, the more dropout there. But it shouldn't be a problem when there are nanoaps. --- Quote from: ZeroResistance on May 04, 2019, 04:02:39 pm ---I inserted a 1N4148 diode, and then 2 and then went upto 4 in series to mitigate this issue. However I found that at loads above 100K to 1Meg, the circuit wasn't performing well, I sort of thought that well the diode wasn't able to conduct at these low currents, and gave up on the idea. But I may well be wrong. --- End quote --- Did you check on a real circuit, or in a simulator? |
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