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Nanoamp current source troubleshooting
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ZeroResistance:
I built a simple nanoamp current source.
Op amp is the MCP602. The circuit runs on 3VDC.
I have attached the circuit diagram, R7 is the load through which I measure the current.
Instead of V2 I have used LM4041 For the reference, its a 1.225V fixed reference. The series resistor for the reference is 22K (not shown in schematic).

The input at the voltage divider node R8/R9 shows around 12mV in both LTspice and the practical circuit. However when I check the output voltage across the load the real world circuit shows around 130uV while LTspice shows 13uV?

What would be the cause for this anomaly?
ZeroResistance:
So here are the different voltages LTSpice vs Practical

MCP602
Vout    6.95mV     693uV
Vin-      136mV     659uV
Vin+     1.12mV    556uV

Vref      1.224V
Vref div 11.96mV    12.117mV
VL          133uV        13.3uV

The first column is the practical circuit and the 2nd column is LTSpice. For some reason the Vin- and Vin+ is showing a huge variation. Probably due to the multimeter leads.
exe:
I'd try a bipolar power supply for your opamp. The output may not be able to swing that wide. Or, you can increase R4 or add a diode to increase required voltage drop.
ZeroResistance:

--- Quote from: exe on April 11, 2019, 01:25:59 pm ---I'd try a bipolar power supply for your opamp. The output may not be able to swing that wide. Or, you can increase R4 or add a diode to increase required voltage drop.

--- End quote ---

Yes I thought so too, but I guess the MCP602 is a single supply opamp. Also when I check the datasheet of the Op-amp the Common mode input range is
Min: Vss- 0.3
Max: Vdd-1.2
So I think the operating range should fall within the spec of the Op-amp.
I  am powering the op-amp using a 3V supply.

I  didn't get the part of increasing R4 or adding a diode?
exe:

--- Quote from: ZeroResistance on April 11, 2019, 01:46:31 pm ---
--- Quote from: exe on April 11, 2019, 01:25:59 pm ---I'd try a bipolar power supply for your opamp. The output may not be able to swing that wide. Or, you can increase R4 or add a diode to increase required voltage drop.

--- End quote ---

Yes I thought so too, but I guess the MCP602 is a single supply opamp. Also when I check the datasheet of the Op-amp the Common mode input range is
Min: Vss- 0.3
Max: Vdd-1.2
So I think the operating range should fall within the spec of the Op-amp.
I  am powering the op-amp using a 3V supply.

I  didn't get the part of increasing R4 or adding a diode?

--- End quote ---

The input may be fine, but output is not. You are asking your opamp to swing down to less than 1mv to ground. I never seen an opamp that could do that. So, my theory is, you want impossible from opamp. Solutions: 1) bipolar supply 2) move operating point so that output is within the range of opamp capability. So, if you increase R4 then for the same current output should be higher, right? If not, then forget this suggestion. Same with diode, if you place diode on output, then opamp will have to raise the output further away from ground to "overcome" additional dropout. Dave has a video about these technique, in this video:
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