Picoamp Precision Amplifier

Dear Thomas Kuehl,

Thank you for your quick response.

I was using LMP91000, since I need a potensiostat.

Since it is possible to use an external resistor gain with LMP91000, I wonder if a megaohm order resistor will work to measure picoamp currents.

Thanks again,

Alejandro

Hi Alejandro,

The LMP91000 is handled by another group within TI. I notified one of their application engineers about your questions.

Regards, Thomas

PA - Linear Applications Engineering

Hello Thomas, 

Thank you for your help. May be if I could have the email or phone number of the application engineer  I could contact him directly.

Thanks again for your help.

Regards,

Alejandro

Hi Alejandro,

I am not exactly sure who handles the LMP91000 since it resides with another TI group, but I am attempting to find out who it is. I believe it is fellow in Europe. Once known for certain the plan is to have you answered by him, on the forum.

Regards, Thomas

PA - Linear Applications Engineering 

Hi Alejandro,

the external gain resistance option in the LMP91000 has been thought to well tune the gain of the TIA in order to maximize the output range of the LMP91000. In the DS we suggest 350Kohm (which is also the max selectable value in our selection of resistances) as maximim resistance. That value guarantees a very stable behaviour with most of the electrochemical cells presents in the market. Usually the output stage of an electrochemical cell can be modeled wiht a capacitance (its value in most of the case is not showed in the Datasheet of the electrochemical cells), that capacitance togehter with the transimpedance gain resistance generates a pole which can reduce the stability of the TIA.

The current that you are trying to measure is very narrow and you will need a transimpedance resistance in the order of tens of Mohm, which will not work with the LMP91000.

In order to measure that current I suggest to use the LMP7721, already suggested by Thomas.

regards,

Domenico

Hi Domenico, 

So, what's the lowest current LMP91000 can measure ?

Regards,

Alejandro

Hello Thomas, 

I am studying the potensiostat circuit with LMP7721, I wonder what is the minimum and maximum currents I can measure with this device. May be I could use LMP7721 for very small currents and LMP91000 for larger currents...

What do you think ?

Thank you.

Regards,

Alejandro

Hi Alejandro,

the LMP91000 is able to read currents in the order of hundreds of pA.

For instances if is able to work with electrochemical cells which have a sensitivity in the order of .5nA/ppm.

regards,

Domenico

Hi Alejandro,

the LMP7721 used in the TIA stage of the potentiostat allows you to connect high value of feedback resistance, the device has huge BW respect to the LMP91000.So for sure you can read very small currents in the order of fraction of pico, but than you have to take in account the noise of the feedback resistance and the input  current noise of the LMP7721.

So I would say that in the range between hundreds of fA to hundred of pA you can use the LMP7721. For higher currents you can use the LMP91000.

regards,

Domenico

Hi Domenico,

So bottomline, LMP91000 can read currents from 100 pA up to 750 uA (as reads in datasheet) ?

SInce the lowest sensitivity I need is 0.5 pA/ppb, is it possible to preamplify the Working Electrode output and then feed the LMP91000 for this low current level ?

Thank you.

Regards,

Alejandro