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NNTP-Posting-Date: Wed, 09 Oct 2002 22:41:27 CDT
From: firstname.lastname@example.org (John Fields)
Subject: Re: Digital Meter Scaling Question
Date: Thu, 10 Oct 2002 03:46:11 GMT
Organization: Austin Instruments, Inc.
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On 10 Oct 2002 02:35:12 GMT, email@example.com (Ed Edmondson)
>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?
No. For a non-inverting opamp with the configuration you describe
the gain is described as Vout = ((R1+R2)/R1) * Vin, where R1 is the
resistor from the - input to gnd and R2 is the resistor from the
output to the - input. For a gain of 10, then, the resistance of R2
should be 9 times the resistance of R1.
>What size bypass resistor should I use on the op amp power pin? Is 0.1 or 0.001
Bypass resistor? You probably don't need one, but if if you mean
capacitor, a 0.1µF ceramic Won't hurt.
>What kind of accuracy can I expect using 0.1 % metal film resistors?
1%, since you're using a 1% shunt.
>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.
Professional circuit designer
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