# Homework Help: Current to Voltage converter

1. Sep 19, 2011

### hami1992

hello everyone

I am facing a problem in designing a current to voltage converter using op amp I have to convert the current in the range of 40 micro amps to a suitable voltage using a T network having 3 resistors on the feedback. Please guide me what component values I have to use and which op amp I should use.

Thanks to all

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Last edited: Sep 19, 2011
2. Sep 19, 2011

### skeptic2

1. What is a suitable voltage?
2. Why do you have to use a T network having 3 resistors on the feedback?

3. Sep 19, 2011

### hami1992

Well I also did not want to use three resistors on the feedback but my teacher said that you must use such a circuit , well I also dont have the idea that what should be the suitable voltage which I should get but I have to further use that voltage for working on LabView

4. Sep 19, 2011

### hami1992

I have now attached the figure of my circuit which I have been trying to use for current to voltage conversion, please have a look at it and let me know what i can do?

Thanks

5. Sep 19, 2011

### skeptic2

Is this homework?

6. Sep 19, 2011

### skeptic2

What are the factors that determine what a suitable voltage is?

7. Sep 19, 2011

### hami1992

Well you can say that I need a voltage in the range of 1 to 5 volts

8. Sep 19, 2011

### skeptic2

Can you explain the parameters you have specified for your current source? 40m sin(1,40,20k,0,0)

9. Sep 19, 2011

### yungman

What you want is called "Transimpedance" amp. Your teacher is wrong. You do not need three resistors to do this.

To find the feed back resistor to transform 40uA to 5V full scale is:

$$R=\frac V I = \frac 5 {40\times 10^-6} =125K$$

This is a very small resistor. You should be able to do it with even some basic op-amp. If you want to feel good about it, you can use a J-FET input op amp.

If you teacher disagree, tell him to join in this thread and I'll convince him. I design transimpedance to transform sub pico amps using feedback resistor greater than 10 giga ohm. That is not even uncommon. There is big disadvantage using 3 resistor network like your teacher want. You amplify the noise much much more and you increase the offset error. This is all well documented. There are very very good reason people don't do 3 resistors.

10. Sep 19, 2011

### rude man

Except for offset current effects. Input current offsets are exacerbated by a single large resistor in lieu of a lower-impedance T network. Have to look at the op amp i(offset) and
e(offset) specs to find out. JFETS tend to have crummy offset voltages.

BTW I'm not the teacher ....

Last edited: Sep 19, 2011
11. Sep 19, 2011

### hami1992

Thank you yungman I will see today what my teacher has to say about it :)

12. Nov 23, 2011

### snehlata

Hi
We need an I/V Converter which may handle the input currents of the order of 10 nanoamps to 50 microamperes (or at least 0.1microamperes to 25 microamperes) & will give a measurable/ sufficient voltage signal.
The scheme is like this, the Channel electron multiplier's anode will be directly fed to the I/V converter. The signal at I/V converter will come at max. 13kHz frequency.
Power supply or power consumption is not an issue.
Please reply as soon as possible, if anyone knows how to make an I/V Converter for this current range.
Regards,
Snehlata