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

[meanie]

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.

 

[vinoth8051]

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

 

[meanie]

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.

 

[vinoth8051]

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.

 

[dv_sa559]

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

 

[FvM]

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.

 

[alexan_e]

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


Alex

 

[meanie]

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.