Need help in designing V2I converter

Status
Not open for further replies.
F

findsriharsha

Member level 3
Joined
Sep 27, 2009
Messages
65
Helped
7
Reputation
14
Reaction score
8
Trophy points
1,288
Location
California US
Activity points
1,644
Hi All,

I need to design a V2I converter. Spec is +/-50mV => +/- 1.25uA.

Please suggest a good architecture for this.. I have +5V and -3V supply.

Thanks in advance.
 

What about a 40kΩ resistor? ;-)

Ok, perhaps some more devices:
 

Yes, I initially thought of this circuit. But input range is +/- 50mV with respect to ground!
 

Consider if input is -50mV, then the FB voltage tries to be -50mV which is across the R. BJT will turn off then, isnt it? corret me if I am wrong.. Thanks
 

Yes, you're right, sorry. To get around you could use the following circuit, even saving the transistor:

Its disadvantage, however, is the small complying voltage ("headroom") which is equal to the input voltage. This means the current measurement circuit must have a very low input impedance, i.e. « 40kΩ (< 400Ω for 1% error) -- which also could be achieved by using only the resistor.

This isn't really helpful, I know. We have to look for more complex circuits.
 

Yes, Thanks Erikl!

In the mean time, please post if anyone else has some suggestions...

thanks!
 

erikl said:
We have to look for more complex circuits.
Click to expand...

I think the easiest way is to amplify the original input voltage and then generate the required current via an appropriate resistor to GND. With your neg. supply voltage of -3V you should be able to amplify the +/-50mV to +/-2.5V , then create the current via a 2MΩ resistor. I'd suggest to buffer the +/-2.5V output by another opAmp:

 

Final note: this issue has been solved. I implemented a diff pair with source degeneration resistor to achieve the required transconductance.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…