out put of LMC6001AI is saturated, what is the problem

Hello Peng,

At picoamp levels, the layout and design of the circuit is critical. It is very easy to create leakages in the pA's to nA's. The circuit needs to be clean and tidy, and not cobbled together on a solderless protoboard. Shielding and guarding will be required if you want any kind of accuracy.

Please see the following articles, as well as the Keithley Low Level Measurements Handbook (particularly chapter 2.1)

http://www.edn.com/design/analog/4368681/Design-femtoampere-circuits-with-low-leakage-part-one

http://www.edn.com/design/analog/4375459/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

http://www.edn.com/design/analog/4395651/Design-femtoampere-circuits-with-low-leakage---Part-3--Low-current-design-techniques

http://www.edn.com/electronics-blogs/the-signal/4400915/Input-bias-current-of-CMOS-and-JFET-amplifiers

http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf

If you disconnect the 1pA current, is the output still railed? It will take some time to settle. Are you SURE it is 1pA?

Send a photo of the circuit...as it is more critical than the schematic.

Regards,

Hi, Paul:

thank  you very much for your advice. it is very helpful.

i checked the circuit and added two tantalum capacitors to the power supplyies, it seems the chip works well. i added -0.1V on a 100G ohm resistor, then the current was inputted into the inverting input and a 400G ohm  feedback resistor is used. the output has a p-p noise of about 10fA, not bad.

meanwhile, could you please take a look at my new post " A question about LMC6001 amplifier, do feedback resistors affect each other?"

again, thank you and have a nice weekend

Peng