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Author Topic: Please suggest for this op amp design  (Read 894 times)
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MicroMaster
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« on: January 30, 2009, 01:57:45 13:57 »

I am newbie in hardware design!



I need to measure current through R1 by measuring the voltage across it. Range is 0 to 500mA current.

I propose to use a .1 or .2 ohm resistor there.
So voltage developed will be from 0 to 50millivolt or 100 millivolt.

I need a resolution of .1mA .So plans to drive MCP3304 in pseudo differential mode.

The voltage range I need to obtain at the input of ADC is 1 volt to 4 volt.

Please give some guidelines to derive my design.
I am using Proteus to simulate my design.(Learning)

Thanks a lot
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MicroMaster

"Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats."
                           Howard Aiken.
Walkura
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« Reply #1 on: January 30, 2009, 04:17:32 16:17 »

For the mosfet i would use a mosfet driver ,doesnt matter really if you choose microchip or the IR21xx series.
With just 5 volt you get a mosfet open but its heels over the canal open with the resistor pushing the source up with rising current i prefer a gatedriver and put 12 to 18 volt on the gate this way your always guaranteed of a fully opened mosfet .
Choice of mosfet doesnt matter much with low currents like this ,although i must confess i never use logic level fets .
With automotive fets as cheap as they are ,i think why bother 4.7 mOhm sounds good 300 Ampere max. cant hurt  Grin (irf1405)
(i do like that you have the mosfet on the positive side ,i use to do that myself for more accurate measuring)

About the opamp you can do with a standard non inverting amplifier since your measuring is referenced to ground there's no need to do difficult .
(if its usefull to you electronics workbench has a opamp wizard then you can generate the amplification and schema needed)

Good luck Smiley
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MicroMaster
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« Reply #2 on: January 31, 2009, 05:24:12 05:24 »

(i do like that you have the mosfet on the positive side ,i use to do that myself for more accurate measuring)

Thanks for the suggestion. Yes I am planning to use a MOSFET driver or a vaccent opamp section to drive the MOSFET.

What is the advantage of using MOSFET on the positive side?

Any stand alone opamp designers other than workbench?

Thanks a lot
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MicroMaster

"Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats."
                           Howard Aiken.
Walkura
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« Reply #3 on: January 31, 2009, 10:14:56 10:14 »

When your mosfet is on the positive side of the measuring resistor you only measure over your resistor .
With small resistors like 0.1 Ohm the Rdson of your mosfet (lets say irf540) is 0,055 Ohm .
With the source on the minus you would have to compensate for the mosfets resistance .
With the mosfet on the plus side you dont have that problem ,also the Rdson goes up with temperature .
It just saves you trouble with compensating those 2 problems ,basicly its just more accurate and easyer to put it on the *high-side*.

If you like to do it in Proteus you can just draw the schema of the opamp and adjust the feedback resistor till you have the right amplification .
I looked with google ,maybe this will help you .
http://focus.ti.com/docs/toolsw/folders/print/resistor-calc.html
(i didnt try it so good luck Smiley
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