conversion question 1

[mehr27]

Hi,

Is it possible to convert from picoAmps to Hertz? If not, what is the relationship?

Thanks,
Glenn

 

[davidbeach]

They're totally unrelated, there is no conversion.

 

[ScottyUK]

davidbeach's post not withstanding, it would be possible to build a circuit to convert one to the other. The current you are measuring is very low - you will need to find a designer who specialises in ultra-low-level instrumentation design or similar.

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[BrianG]

ScottyUK is correct: it could be possible to design a circuit which would produce a frequency output which is proportional to current, but it would be very difficult (=expensive!)at such low currents.

Are you just trying to measure picoAmp currents?

Depending on measurement period allowable, the following approach may be of interest. At a company I worked for some years ago we needed to measure this sort of current very accurately. We used the technique of passing the current into a precision air-spaced dielectric capacitor for a known time (30-40 seconds), then measured the voltage with a very high input impedance (electrometer input)DVM.

The simple calculation I = C.dv/dt gives the current.

 

[CarlPugh]

To convert from current to frequency first convert from current to voltage. High resistance resistor(s) and a high input impedance operational amplifier should do this. The national LMC series of operational amplifiers should work, high resistance resistors are availiable from Caddock and Digi-key.
There are many articles about voltage to frequency converters and there are IC's just for this purpose.

Yes there are other manufactures of operational amplifiers and resistors that will work. Also there are other circuits that will work.

Good Luck

 

[melone]

At the end of the day, the only relationship between current and frequency is the dictated by the cuircuit that is designed. Inherently, there is no relation.

 

[mehr27]

Thanks for the replies. My structural background is certainly showing. It's always nice to learn something new.

 

[logbook]

mehr27,

I am curious as to what you were actually trying to do or think about. Where was the current coming from and what was the reason for wanting to scale this to frequency?

 

[davidbeach]

I wasn't thinking Hertz in proportion to picoamps, I was thinking mathematical formula for units conversion. With that clarification, I stand behind my original post, but doesn't mean that someone couldn't build a circuit with a frequency output proportional to the input current in picoamps.

 

[GOTWW]

Amps = coulombs/sec
Hertz = cycles/sec

therefor the convertion is (1)pA= (1)Hz* (1)picocoulomb/cycle

or (1)pA=(1)Hz*(1)coulomb/(1)Gigacycle.

Obvious I thought.

 

[melone]

So why are pA related to Hz? Just because they are both units per second, doesn't mean that they are related. Your car can go 60 miles per hour, and your washing machine can wash 2 loads per hour, that DOESN'T mean that you can change how many load of laundry you can wash if you simply drive faster.

 

[mehr27]

logbook,

I'm not actually attempting to build or design anything. I was looking at two graphs. One had a y-axis in pA and another had a y-axis in Hz. I was attempting to compare the two on level ground.

 

[ScottyUK]

mehr,

Unless the two are related by some other factor - capacitive leakage current increasing with frequency springing to mind as one example - there is no implicit relationship between the two.

davidbeach - you were right all along!

GOTWW - you have me confused, or you are confused. I am not sure which!




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[BigTom2005]

Mehr,

Im sure that the conversion could be done. Think about this:

if they can use voltages to convert to frequencies (Voltage controlled oscillators), then it makes sense that you can do the same with current.

its been a long time that i've dealt with op-amps, but I think that there might be a configuration out there that takes a current input and gives an amplified voltage output.
(Current to voltage amplifier). But, theres a problem with such a small current signal--noise. You must clean up any noise. If this could be done, then use the current to voltage amplifier, then use the voltage output to drive a VCO. The output of the VCO is a sinusoid (Sine wave), whose frequency changes according to the input voltage.

Good Luck.

 

[GOTWW]

My first thought was a Van de Graaff generator, but I will let it rest.

 

[circuit1952]

BigTom2005

"if they can use voltages to convert to frequencies (Voltage controlled oscillators), then it makes sense that you can do the same with current."

That does not mean there is any inherent relationship between the units being discussed. If you use a circuit to convert from current to frequency, the conversion factors are dtermined by the circuit and the values of the components. If you change the value of a particular capacitor, for instance, you change the conversion factor.

 

[Skogsgurra]

Thanks, circuit 1952!

This thread left reality almost from the beginning. You brought it back to terra firma. I certainly hope that this ends it.

 

[25362]

A side note: GOTWW got it right when expressing the quantities using common symbols to differentiate them. The SI system, however, doesn't recognize the C (the unit is s.A) or the Hz (the unit is s^-1).

When using SI base units one should be careful since, for example, in radioactivity the Bq = 1 decay/s, is expressed as s^-1, the same as for frequency.

 

[Bung]

But 'frequency' as used colloquially here by 25362 is actually the 'units of peaks of a sine wave travelling past a point per second" ie things per second. The Bq is also things happening per second, which is also a frequency. I could express the speed of my car in 'per second' simply by stating it in the form of the rate of rotation of the wheels in radians per second.

I guess it is back to the old issue of making sure that you don't confuse the map with the terrain.


Bung
Life is non-linear...