confused about the chosing hardware (really frustrated)

[go2tariq]

Hi to all;

I am working with electronics being a beginner. Probably someone here to help me. My sampling frequency is 5000 s/s. I want to make a circuit (amplifier) which detect the current flowing around 1fA. Please someone help me to find out how to chose component to make current integrator people use 10pF cap for integrator I don't understand why. What is cut off frequency in this case to apply filter.

 

[FvM]

Unfortunately you didn't manage yet to tell your intentions in an understandable way, also in your previous thread, where you had sketched a charge amplifier diagram.

Particularly, I don't understand the meaning of the 5000 S/s rate. A current of 1 fA (10e-15 A) correspondends to 6250 electrons per second. Sampling the measurement with 5000 Hz results in about 1 electron per sample. So you are actually talking about counting single electrons. It's possible e.g. with channel electron multipliers, but not with charge amplifiers, I fear.

Currents in a fA range can be measured with regular amplifiers in much slower time scale, with about multiple seconds measurement time.

 

[go2tariq]

Thanks for your help. I used feed back resistance to discharge capacitor but problem with using resistor I get the current as output. Let say if I send sine wave after passing through the amplifier I receive the sine wave which I am not interested. Let say a charged bead if I drop this bead (negatively charged) into a sensor and I get sine wave as a result and I can't calculate voltage. I need to calculate the voltage basically.

 

[FvM]

I think, you should specify a typical input signal to illustrate the signal processing requirements. They are not completely clear to me.

The shown circuit can be either understood as a low-pass, or a lossy integrator, depending on your main interest. For a current input signal, R1 would be omitted, so you get a classical charge amplifier circuit.

 

[go2tariq]

yes it is integrator. I am using this as integrator I just mention this diagram to show that by using feed back resistance we get sine wave again but I am interested to get voltage at the end. Could you please help me to make switch to discharge the cap instead of using feed back resistance.

 

[FvM]

Sorry, I don't understand where you expect to get a sinewave. I also don't see a need to discharge the integrator in a basic charge amplifier application, as you described it.

 

[go2tariq]

Well if I don't use switch to discharge the capacitor it will saturate. I don't want to use feed back resistance. And if I don't use feed back resistance as a result there is a need of switch to discharge cap isn't it.

 

[FvM]

I don't want to use feed back resistance. And if I don't use feed back resistance as a result there is a need of switch to discharge cap isn't it.

Yes, but why don't you want the feedback resistor?

 

[go2tariq]

Ok let me explain you after using feed back resistor the result was like sine wave. and by cap only I get only positive or negative peak. That's why I don't want to use feed back resistance.

Yes, but why don't you want the feedback resistor?

 

[FvM]

Perhaps the resistance has been selected too low.

 

[go2tariq]

I used 50 M ohm resistance with 10 pf cap.

Perhaps the resistance has been selected too low.

 

[FvM]

They should be rather 50 G ohm. May be you have difficulties to get resistors in this range.

 

[go2tariq]

That's why I need switch to discharge cap. Do you know something about Photon coupled FET switch.

They should be rather 50 G ohm. May be you have difficulties to get resistors in this range.

 

[FvM]

I guess, you are talking about photovoltaic optocouplers. There are a few fast, low capacitance types like AQY221N2V.

But most integrator reset circuits are using a regular JFET or MOSFET across the capacitor.