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Title: Please suggest for this op amp design Post by: MicroMaster on January 30, 2009, 12:57:45 12:57 I am newbie in hardware design!
(http://images.elektroda.net/53_1233311274.jpg) 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 Title: Re: Please suggest for this op amp design Post by: Walkura on January 30, 2009, 03:17:32 15:17 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 ;D (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 :) Title: Re: Please suggest for this op amp design Post by: MicroMaster on January 31, 2009, 04:24:12 04:24 (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 Title: Re: Please suggest for this op amp design Post by: Walkura on January 31, 2009, 09:14:56 09:14 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 :) |