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# small range current measurement

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#### zeeshan728

##### Newbie level 6
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.

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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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.

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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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.

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