small range current measurement

[zeeshan728]

Hi,
I want to measure dc current in the range of pico ampreres(pA) and time scale is from microsec(usec) to some seconds.Can anyone suggest some method. kindly give ur comments Thanks.

 

[andre_luis]

There are very agressive specifications.
However ( not sure if this works well ) you could perform by capacitor charge ramp - wich slope charge is linear at constant current :

Q = I.T = V.C
( 10^-12 A ) x ( 10^-6 ) = V . C
V.C ~ 10^-18

Assuming a capacitor of some picoFarads ( 10^-12 F ) :

( 10^-12 A ) x ( 10^-6 s ) = V . ( 10^-12 F )
V ~ 10^-6 v

Means that the minimum scale voltage is in range of uVolts !!!
Even using an AmpOp the noise could take impossible obtain a good result.

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[FvM]

Besides noise properties of amplifiers and resistors, the ultimate limitation is elementary charge. 1 pA x 1 µs is just 6 electrons, so the statistical uncertainty will be high.

 

[andre_luis]

hi FvM,

Do you mean it is possible think about that solution ?
I was considering relation Signal/Noise ( despite magnitude varies according typical application ).

Maybe is necessary to know about where that solution is intended to be applyed, it´s wright ?

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[FvM]

Do you mean it is possible think about that solution ?

I assume, that 100 or may be 10 pA can be measured with µs resolution using generally available electronic devices. But a full problem specification should be given first. Integrating charges instead of using (high ohmic) resistors for direct current measuremet may be a way to reduce the circuit noise level.