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From: firstname.lastname@example.org (Ed Edmondson)
Date: 10 Oct 2002 02:35:12 GMT
Organization: AOL http://www.aol.com
X-Newsreader: Session Scheduler (Queue Name: gng-fy)
Subject: Re: Digital Meter Scaling Question
Tom & Others,
What I need to do is change the meter scaling for a power supply I have built.
I have two Datel DPM's on the power supply. A 4.5 digit to measure voltage
(1.25 to 25 volts) and a 3.5 digit to measure current. The actual connection of
the meters was provided by Datel apps engineers. The volt meter works as it is
supposed to. It is the current meter which has a problem.
I am measuring the current being drawn by the power supply by measuring the
voltage drop across a 0.1 ohm 1% 5 watt kelvin resistor located on the negative
side of the power supply output (per Datel). At the moment 1 amp reads X1.00
volts. I want the meter to read 1.XXX amps. If my meter was a 200 mV FS meter
there would be no problem. However, with a 2 volt FS meter the reading is not
as accurate (correct)?
I have some Analog Devices AD8551R op amps as previously suggested in this
thread. I have connected pin 2 (- In) though a resistor to ground and from pin
2 to pin 6 (output). The voltage from my current sensing resistor is applied to
pin 3 (In +). Pin 5 (V plus) to pin 7. Pin 4 (V minus) to ground. The output
from the op amp goes to the In High pin on the meter and ground goes to In Low
on the meter.
I was going to use a 100K resistor from In minus on the op amp to the output
(feedback) and another resistor 10K from the same In Minus pin to ground. Would
this be correct?
What size bypass resistor should I use on the op amp power pin? Is 0.1 or 0.001
What kind of accuracy can I expect using 0.1 % metal film resistors?
I realize I could just change the value of the sense resistor or the meter
itself. However, I already have these components and they are working so it
would be more trouble than just adding a op amp, 2 resistors, and a capacitor
between the sense resistor and the meter input.
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