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Adding offset voltage to MAX4304 VF op-amp

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Hi,

I am trying to add offset voltage adjustment to MAX4304 voltage feedback op-amp and couldn't get it to work. I was using the same schematic I was using for AD811 by connecting the offset voltage adjustment to the inverting input. But with MAX4304, I don't see any change in the offset voltage of the output. If I take out R17, I can see voltage change at VR1 viper from ±1.2V I may be doing something wrong here which I am not able to get around it.

Any suggestions?

thanks
 

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ronsimpson

Well-Known Member
Most Helpful Member
Pin 2, the voltage will not change when you turn VR1.
Pin 6, the voltage will change when you turn VR1.
upload_2017-9-28_18-48-30.png
 
Hi Ron,

Is this specific to this IC or it should be connected to Pin 6 for any op-amp? Because this same design works with other op-amps that I use.

thanks
 

ronsimpson

Well-Known Member
Most Helpful Member
If the op-amp is working AND if the output is not pushed against the supplies, all amps work the same. If the output is almost +5V to almost -5V then the op-amp input can be not equal.

Op-Amp function is to keep (+input) and (-input) at the same voltage.

Your circuit:
R14 makes (+input) at 0 volts.
If VR1 is at 0V then R17 tries to make (-input) at 0 volts.
The Output will be at 0 volts.

If VR1 is turned and pulls up (plus) a small amount the the inputs are not equal. Because (-input) is higher than (+input) the output must go down. Now R17 & VR1 are pulling up and R15 pulls down, to get (-input) back to 0 volts.

upload_2017-9-29_7-15-38.png
 
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