Help with voltage adder/summer circuit

[tiny747]

I am trying to make a voltage adder/summer circuit using a LM358 op-amp. I got the schematic off the internet and it matches the one that I have in one of my electronics book at home. I bread boarded the whole thing using a 5 volt voltage regulator. I made 2 voltage division networks using a 20K ohm variable resistor and placed it from (+) to (-) with the wiper output to my fixed resistor that went to pin 2 of the LM358. The fixed resistors are 1k ohm and the reference resistor (fixed resistor) is also 1K ohm. I measured the voltage drop across the fixed resistors. It came to about 1 volt each. So as a summing circuit it should put out about 2 volts. For some unknown reason though it puts out only .6 volts. When I change the voltage drop across the fixed resistors to 2 volts each I still only get a .7 voltage output between Vo and Gnd. Does anybody know why. I used the circuit diagram found at **broken link removed** for my circuit. Only I am using a voltage division network to get my different voltages. I have tried using a spare op-amp with the same results. Please help.

 

[Nigel Goodwin]

I would suggest you need a split supply (+/-9V or so), and a single 5V is too little anyway.

 

[MikeMl]

Post a circuit diagram. I cant tell from your description, but it sounds to me that you are asking the whimpy output from the LM358 to drive resistors that are way too low in value. The feedback resistor should be 10K or higher.

Also, if you are using 20K pots to create the voltages you want to sum, you will have to use extra 358s as voltage followers to buffer the wiper voltage, especially if the summing/feed back resistors are only 1K. To make this work with the voltage followers, the Pots would have to be 1K or lower, and the summing/feedback resistors on the order of 20K to 100K.

 

[tiny747]

I need a maximum of 2 volts output out of Vo of the op-amp. I can use a different voltage regulator (10volt Max). I can use different sized fixed resistors. I didn't know what size to use. I can also use different size pots also. I would like the accuracy to be +- 20mv. I think I attached a poor schematic of what I am trying to do. Hope this pic helps? I'm trying to do this with the least amount of parts. All of your help is really appreciated.

 

[MikeMl]

Ok, I see the problem. You cannot ground the non-inverting input! It has to be biased somewhere near Vcc/2, or you can leave it grounded if you operate the opamp on split-supplies.

The gain from each pot wiper to the output is -Rfb/Rin. So that the Rin doesn't load the pot, make it ~10 times the Pot resistance, i.e. I would use 2K pots, and make Rin about 20K. If you want a gain of -1, then make Rfb 20K too.

Make a voltage divider consisting of two 10K resistors tied to Vcc and Gnd. Tie the center tap to the non-inverting input, and then the output and the two inputs will be relative to Vcc/2. If you set the two pots exactly halfway (2.5V?), the output will be at 2.5V. Offsetting either pot from the mid point will move the output in the opposite direction from 2.5V. Moving them both a like amount will move the output twice as much...

Better yet, make the gain only -0.5, then each input individually can swing from 0 to 5V without the output of the opamp being overdriven. Look at this.

 

[tiny747]

Wow! Thanks so much for the advice and all the wonderful pic. that really makes a difference.

I'm going to try to get the right resistor inbetween ground and non-inverting input. If I tried (which I probably wont) doing it with split supplies would I tie the grounds of both voltage regulators together? Just curious.

Once I get this working I want to build a non-inverting summer. But I want to learn all the steps as I go along that's why I am doing it this way. I have to learn to walk before I try to run.

Thanks again for all of your help!

 

[tiny747]

Sorry about that last post. I read what you said to quickly and realized I should have spent a little more time thinking about what you said before I posted. Anyway I followed your schematic to the "T" and it worked well thanks for all the help. Would you happen to have a schematic laying around for a non-inverting summer. with again of 1. The max Voltage output only needs to be about 2 volts? Thanks again for all the help.

 

[MikeMl]

Two ways: Invert the two inputs (if pots, then just turn the shaft the other way ), or add a second opamp with a gain of -1 behind the first. Unfortunately, to act as a summer, the two summing resistors must go to the inverting input, hence the summer inverts the sum.