Maker Pro
Maker Pro

Simple offset voltage question

gelpack

Jul 7, 2022
13
Joined
Jul 7, 2022
Messages
13
OK, so my scenario is that I have two 10K joysticks and they don't exactly go to zero or 10k between the slider and the end point. something like 200ohms at the bottom and about 9.7/9/8 at the top end.

The requirement is that I have a reference voltage of 1.8 volts and a VCC of 5v available, but no negative voltage unless I add a small camera battery in.

The aim is to be able to output 0v - ref voltage ( 1.8 ), so I think I might need a -ve supply, but not sure if it can be done without on.

I was thinking about using an op amp, but I don't really know too much about them, and was wondering if anyone can suggest a circuit which will enable me to do this.

Many thanks.
 

CircutScoper

Mar 29, 2022
290
Joined
Mar 29, 2022
Messages
290
OK, so my scenario is that I have two 10K joysticks and they don't exactly go to zero or 10k between the slider and the end point. something like 200ohms at the bottom and about 9.7/9/8 at the top end.

The requirement is that I have a reference voltage of 1.8 volts and a VCC of 5v available, but no negative voltage unless I add a small camera battery in.

The aim is to be able to output 0v - ref voltage ( 1.8 ), so I think I might need a -ve supply, but not sure if it can be done without on.

I was thinking about using an op amp, but I don't really know too much about them, and was wondering if anyone can suggest a circuit which will enable me to do this.

Many thanks.

This should fill the bill, and no negative supply required. The other half of the 6482 would be used for your other joystick channel. Most any opamp with an output span that includes ground would work.

JoyStick.GIF
 

gelpack

Jul 7, 2022
13
Joined
Jul 7, 2022
Messages
13
Excellent, I never thought of doing it that way at all, but I knew there was likely a solution and you have come up trumps here.

So, thank you very much, I will go get myself a couple of these as there will have to be one for each axis and there are two joystick ports.

:D << Happy camper !
 

gelpack

Jul 7, 2022
13
Joined
Jul 7, 2022
Messages
13
This should fill the bill, and no negative supply required. The other half of the 6482 would be used for your other joystick channel. Most any opamp with an output span that includes ground would work.

Actually.

I replied above your your original post ( thank you ) , then I had thought below.....

I was wondering is there an online tool you can use to model these circuits which allows you to put the components in and adjust the values to monitor a test point. That would be very useful !?

Cheers
 

Martaine2005

May 12, 2015
4,304
Joined
May 12, 2015
Messages
4,304
Ltspice should do exactly what you want. It’s software though.
Check it out here.


Martin
 

CircutScoper

Mar 29, 2022
290
Joined
Mar 29, 2022
Messages
290
Actually.

I replied above your your original post ( thank you ) , then I had thought below.....

I was wondering is there an online tool you can use to model these circuits which allows you to put the components in and adjust the values to monitor a test point. That would be very useful !?

Cheers

Probably, but I just do it in my noggin.
 

AnalogKid

Jun 10, 2015
2,665
Joined
Jun 10, 2015
Messages
2,665
There are several online simulation sites that do not require any software to be downloaded and installed, but I've found them to have relatively low performance and limited capabilities; to the point that I no longer track them to see if they have improved.

Linear Technology Spice (LTS) is the gold standard for free circuit simulation. It takes up a lot of disk space, and certainly has its personality quirks, but the performance is excellent and the higher-end features, like plotting a power spectrum, are outstanding. I find it's biggest limitation is that the library has only Linear Tech and Analog Devices ICs. Still, that is a huge library and you probably can find a close-enough equivalent to almost any other devices on the market.

Separate from that, how close does the low end output voltage have to be to 0.000 V? No rail-to-rail opamp output stage can go to true 0 V, because there always is some voltage across the lower output transistor. How close any particular device can get is spelled out in the datasheet, along with the maximum output current before the output voltage increases. For example, the LMC6482 mentioned above can swing down to within 20 mV of the negative rail, but only if the output load is 100 K ohms or more. If that is good enough, then you do not need a negative voltage supply; the device's negative rail can be GND.

ak
 
Last edited:

CircutScoper

Mar 29, 2022
290
Joined
Mar 29, 2022
Messages
290
For example, the LMC6482 mentioned above can swing down to within 20 mV of the negative rail, but only if the output load is 100 K ohms or more. If that is good enough, then you do not need a negative voltage supply; the device's negative rail can be GND.

ak

Since the circuit above requires the amp to sink only 1 or 2uA, if unloaded it can do a bit better than that, and if necessary, a 1k resistor from output to ground would improve that by another factor of 10x.

temp.GIF
 
Last edited:
Top