Adding voltage to a circuit

[Tobias]

I want to add voltage to the output of a potentiometer using a PIC and a digital POT. For example the output voltage of the POT is 2.3V and I want to add .1 to 2Vs to that signal. Any ideas? I have tried using the Pots output tied into one side of the digital pot and having 5vs on the other side of the digital pot. Doesn't work to good.
Thanks in advance
Toby

[mister_e]

Summing amplifier made with an opamp?

http://www.daxia.com/bibis/uploadasp/flies/%B3%A3%D3%C3%D4%CB%B7%C5%B5%E7%C2%B7%CD%BC.pdf

OR, pot goes to PIC a/D input, it calculate the final voltage, and generate the according voltage with full resolution PWM or DAC. Still possible to use digital pot.

[Tobias]

Thank you for the input. I am going to go the Op-Amp route.

[Tobias]

Here is what I have. It seems to work good except that the digital pot does effect the value from the turn pot. Is there a diode arrangement I can use so the two turnpots don't work against each other. For example with the turnpot at 100% it reads 4.41v when connected to the entire circuit. When standing alone it reads 4.47v at 100%

Thanks for your input

[mister_e]

The non-inverting summing amplifier may cause that as the input current depends on the level of all the other inputs AND the effective input impedance is not constant.

Better to use a inverting configuration.

[Tobias]

HI
Attached is my attempt with the inverting op-amp. I must be out in the woods on the resistor values needed. Any ideas what I am doing wrong?
Thanks

[mister_e]

That's Sim results, or real results?

the Opamp Supply rails are not connected on your schematic though. You must use a dual rail voltage (+V, -V)

What available voltage rails you have available on your board now? (regulated and not regulated)

One thing is sure, as the OPamp pin 2 is considered as Ground, i would use higher value for R4, R5 to don't interfer the Pots impedances.

[Tobias]

Its from NI MultiSim. I looked around and found a basic three pin Op-Amp. The results were more like what I was expecting. Thanks for humoring my learning curve

[skimask]

That's Sim results, or real results?
That's gotta be a sim! And probably some smoke too!

EDIT: well...ahhh...too late...

[mister_e]

:D great sims :D

Did you already tried LTSpice?

One thing is sure, you really need a negative rail.

[Tobias]

No I haven't, what do you think of it? I visited a friend last weekend who has MultiSim and downloaded the 30-day trial. Its pretty fun.

[mister_e]

:D Sims :eek:

I general they're all bull**** and useless... but someone has to make money with something huh?

LTSpice is free from Linear website (http://www.linear.com/designtools/software/index.jsp ). I used it few times already just for fun and evaluation. This is the only one i would trust if i don't have ANY parts on hand. MultiSim has to be good though.

If one day i find a real good Sim i would probably use it, 'till now... none has impressed me. Maybe good enough for education purpose or for proof of concept... but even simple stuff doesn't seems to work.

I always have the same test since day one, which i keep secret... ALL sim failed on. If one day one sim don't fail on, maybe i could says it's somehow useful. Why spend few thousands buck on something that doesn't work while you could have real results with real parts at fraction of the price?

The only useful one that cover almost everything is Proteus... but it's still useless... fun to play with instead of of a Wii or Nintendo ;)

I know once again, i will make 'friends' and e-mail breeze with this comment...

[Tobias]

Here is what I have and it seems to work great in the simulation software. I am now bench testing and have a few questions. I can add voltage into the circuit on the bench yet it seems very noisy. For example Pot1 is outputting average 1.33v with a VPP of 240mv. The output of Op-Amp1 is average 1.48v with a VPP of 1.12v.

I am using an AD5220 10k digital pot and a L272M Op-Amp. Here is a link to the basic info regarding the Op-Amp.

http://search.digikey.com/scripts/Dk...=497-1387-5-ND

This is my first attempt using Op-Amps. There seems to be a hell of alot of variations to choose from. What would cause the noise? If I am off on the correct Op-Amp, please suggest a replacement. Thanks

[mister_e]

I haven't place your circuit on a breadboard, but i have the following feeling.

Noise can come from each Opamp, and then mixed & doubled with the last stage. Higher your resistor values are, greater the chances to pickup noise... and then you also have the PCB layout, ground loops, PSU filtering etc etc etc.

You want to smooth everything from the port wiper with a capacitor, at the mix point (last stage), and probably at the output of the last one. Some op-amp may react weird with capacitive load so far. RC filter would help, while a bit useless in few cases. Software noise rejection is a common use.

I stick to the idea of a dual rail voltage. This can be easily produced with a simple PWM signal and a capacitive inverter. Match this to a low quiescent current OP-AMP and you're in business.

I have those AD5220 in stock, but not this specific op-amp.. got a load of others though.

I still not 100% understand why you can't trick the whole thing in software instead... i miss the whole idea. Anyways, you know where to reach me ;)