No, I was suggesting a using a small 7508 and a 7660 inverter chip to 
generate -8 VDC to give your op amp a bipolar supply (did this recently for 
a 4-20 mA sense circuit for a project that I'm working on). There are chips 
designed to 1) sense current and output a voltage, and 2) 4-20 mA current 
chips as well, but they are all pretty expensive and don't drive the 4-20 mA 
down to 0.5 mA (it's a requirement for this project; 0.5 mA indicates a 
sensor fault to the PLC).

----- Original Message ----- 
From: <ChrisEGroup@rocklizard.org>
To: <piclist@mit.edu>
Sent: Sunday, May 08, 2005 4:56 PM
Subject: RE: [EE] Problems using a differential amplifier to measure 
current.


>
> Olin wrote: This sounds like your current sense resistor is on the
> return leg with one side ground.  If so, why do you need a differential
> amplifier in the first place?
>
> -----
>
> I had originally planned to place the sense resistor between +5v and the
> load. Now I am considering placing it between the load and ground. I
> chose the first method out of habit. This is how I usually use an amp
> meter when taking measurements.
>
> I have never made a current sensor before so I'm looking for some best
> practices to do this. Some of the posts here have lead me to think that
> the ground side current sense might be the best. I have not had time yet
> to build up a standard non-inverting amp so I'm not sure of the issues
> with this method.
>
> How do you guys build current sensors? Douglas Wood mentioned that there
> are chips designed to do this. I might investigate one of these as a
> solution but for now I'd like to learn how to solve this problem by hand
> before going to a pre-built solution.
>
> I'd still like to hear comments on the Hall Effect current sensors if
> anyone here has used them.
>
> Thanks again.
> Chris
>
>
> -----Original Message-----
> From: Olin Lathrop [mailto:olin_piclist@embedinc.com]
> Sent: Sunday, May 08, 2005 1:53 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Problems using a differential amplifier to measure
> current.
>
> ChrisEGroup@rocklizard.org wrote:
>> * I made an error on the schematic. The gain is 100x. I am using
>> 10K/100 Ohm resistors in each OpAmp voltage divider.
>
> That's a lot for a single stage.  It will be difficult to get the
> resistors matched well and get good common mode rejection.
>
>> * The OpAmp inputs range from 0V TO 50 mV
>
> This sounds like your current sense resistor is on the return leg with
> one side ground.  If so, why do you need a differential amplifier in the
> first place?
>
>
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