LMP7721MAEVALMF/NOPB: false output signal

Hi Ishara,

can you please share your whole testing configuration in a schematic? Your configuration might be causing too much noise or it is floating.

To understand your issue better please share:

- What is the supply voltage, the common mode voltage, the reference voltage?

- Why did you choose a high feedback resistor of 1T?

Thanks

Miriam

Hi Miriam,

Thanks. Please find TINA file for the schematic. Answers to most of your questions are in the file. 1T resistor because of the low current signal is in 10-100fA range. So, according to the LMP7721 EVAL. data sheet I choose large value resistor. Please let me know if you need any further information.

Thanks Again

Ishara 

LMP7721_Eval_simulation_TI.TSC

Hi Ishara,

thanks for sharing. I guess you're currently using TINA 10. Unfortunately, we're not able to open the schematic but can only see the preview. Can you please share again your schematic using another TINA version this time?

The concern we have with such a high feedback resistor is the increased noise interfering with the signal and causing it to slam against the rail. It seems like you're using a 1pF capacitor in the feedback but the preview is too small to show it properly. Please correct me if I'm wrong.

Thanks,

Miriam

Hi Miriam,

Please find attached the TINA-TI version of the schematic. Sorry about the issue. 

My signal never goes beyond 100fA, therefore, implemented the 1T resistor. However, I tried 10G with both the evaluation board, still output stuck near -1.5V.

Yes, I have used 1pF capacitor in the feedback.

Thanks Again

~Ishara

LMP7721_Eval_simulation_TI_2.TSC

Hi Ishara,

we checked your simulation and when we run it the output does not slam against the rail like in your measurements.

One reason for this might be leakage current caused by a dirty PCB. You have to make sure that the board is cleaned properly in a ultrasonic bath after soldering since femtoampere circuits are very sensitive towards contaminated boards.

Here are some helpful links regarding this topic:
 

www.edn.com/.../Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

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

Furthermore you have to choose the value of the feedback resistor based on the maximum value of the current you want to measure and not the dark current. Therefore, 1Tohm resistor you've chosen may be too high and adds to the problem in your measurement.