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[SOLVED] Compare two 0 to 10 vdc and output 0 to 10 vdc

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meanie

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Hi Can any body help an aging brain? I want to compare two 0 to 10 vdc signals and output the resulting difference as 0 to 10 vdc. I have tried a couple of circuits using comparators but have come up with the problem of a limited output range. I thought it would be easy top achieve but I seem to be wasting a lot of time going round in circles. Any body have any ideas? Many thanks.
 

Hi meanie,
you may use differential amplifier with unity gain.

If all the resistors are all of the same ohmic value, that is: R1 = R2 = R3 = R4 then the circuit will become a Unity Gain Differential Amplifier and the voltage gain of the amplifier will be exactly one or unity. Then the output expression would simply be Vout = V2 - V1.
Hope this helps
 

Hi Vinoth
I have completed this circuit as shown using a comparitor LM 393 but the output was caped. It appears that the comparitor limits the output to a certain max level and doesn't allow a true calcultaion for the full range of 0 to 10 vdc.
 

Hi Meanie,
I referred the LM393 datasheet. From the typical applications given in data sheet, I am thinking LM393 is not suitable for linear amplification. It has been optimized for comparator action. The comparator does not have feed back and hence output would be either +ve or -ve Saturation. The solution would be better trying with simple opamps like LM741 as an differential amplifier.
 
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    meanie

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Exact.
I think you use lm741 with correct reference then it will work correct.
 
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    meanie

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The problem isn't phrased quite clear. Substracting input A from input B (or vice versa) means, that the output signal covers the voltage range -10 to +10V. Is this what you want? Then a +/- 15V supply would be needed. The said 0 - 10 V output would require an absolute value function, or in other words, a full wave rectifier.

Using a comparator in an OP circuit will result in oscillations. Furthermore, LM393 has an open collector output and would need an additional pull-up resistor to produce positive output voltages. Any standard OP will be good for this circuit.
 
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    meanie

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Yes LM393 is a comparator ,not an opamp, the output doesn't swing, it goes either to 0 or +supply.
Try with a LM358 but if your power is equal to the input you want (10v) then you have to use a rail to rail opamp that can accept and output a voltage equal to the power supply.

I assume that your input is 0-10 and V2 should be higher than V1 if you intent to use single supply.

A differential amplifier like this should work
DiffAmp.jpg

Alex
 
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Hi Vinoth
I had made a stupid mistake, I had tied the R4 to the negative rail effectively. I have just tested a circuit and it's perfect. Thanks for the help. Sometimes just talking to someone helps you see the obvious. Thanks again.
 

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