Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

positive/negative voltage from positive regulator

Status
Not open for further replies.
I really appreciate all of your input... thanks again to everyone who responded. I think maybe just adding a negative rail will suffice the needs for the +/- opamp, if that don't bring the range of the sensor to within needed parameters I will rethink the process . perhaps another type of sensor . I know the accuracy is .5 but trying to stay away frm load cell as the are so bulky, with the inaccuracy is why I need the range on the adc. Everybody here is awesome with their input, Thanks again henry
 
I'll add:
a) You may be using the wrong OP-amp for single supply rail to rail operation.
b) many times you could do better with a ratiometric converter. e.g. The reference is based on the current power supply voltage, hence variations are normalized out. Automotive pressure sensors are usually ratiometric. The sensor and the A/D use the 5 V supply as the reference.
c) Capacitive converters are available to get negative supplies.
d) A Rail splitter IC.
 
I know the accuracy is .5 but trying to stay away frm load cell as the are so bulky, with the inaccuracy is why I need the range on the adc.

There are ways to increase accuracy. It depends on what you need.

One of the best ways to increase accuracy is oversampling. Say, if you need a value every 20 ms, and if you sample at 100kHz, each of your readings will be an average of 2000 samples. If the goal is to detect changes, you may be able to detect small changes even with inaccurate sensor. All depends on the nature of the noise produced by the sensor - won't work very well if the noise has considerable low-frequency component.

If the goal is to compare values hours or days appart then oversamping will not help, but there are other ways too, such as re-calibration.
 
wow that is something I never thought of.. need sampling in milliseconds real time.. avoid the wall... can clock the adc at kHz. given me a new thought to look at thanks......
 
Hi,

Unfortunately you can not use just capacitors to split the DC power supply into a plus and minus supply, you can only do that for AC. You need resistors for DC.
Using two resistors (they do not have to be small valued, 1k or even 5k should be ok) as a voltage divider you can get plus and minus 6 volts. The voltage follower op amp has it's input connected to the junction of the two resistors, and the output of the op amp becomes the new ground.
There was a typo in my previous post that i had to fix this morning too that made that post a little unclear, but to make it clear the output of the op amp becomes the new virtual ground. So between that virtual ground and the old ground we get -6 volts, and between that virtual ground and +12v we get +6v now.

Battery voltage changes though so a regulator might also be needed.
 
I didn't completely read all the replies so perhaps this was mentioned, but if you just need to offset the voltage from the sensor then you can configure the op amp as a differential amp with the signal going to the positive input and a positive offset voltage going to the negative input (which gives a negative going output). No negative voltage required.
 
Last edited:
If you want -12V in your circuit, you can simply use a "charge pump" IC for it.
Here is an example of obtaining -5V from +5V:
uDIZBPK.png

You can search a charge pump IC here.
 
I really appreciate all of your input... thanks again to everyone who responded. I think maybe just adding a negative rail will suffice the needs for the +/- opamp, if that don't bring the range of the sensor to within needed parameters I will rethink the process . perhaps another type of sensor . I know the accuracy is .5 but trying to stay away frm load cell as the are so bulky, with the inaccuracy is why I need the range on the adc. Everybody here is awesome with their input, Thanks again henry
These work very nicely for generating a - rail from a single source. Not much current available but as a - source for an opamp that's just being asked to respond with an output that is well and trully ground (or less) it's aces:
TL081 + - Rail Supply.JPG

Obviously, it would work with either a 12 or 5Vdc source.
 
thank you for your input... ive taken 2 weeks away to let my brain rest.. I will prototype this. all im looking for is to increase the range of my fsr, to accomotate the tare weight of my hand so I can still get almost 1000 steps to fed the adc on my arduino, with the weight of my hand I only get about half scale that is not enough resolution to to what im adsking of the microcontroller, still I thank you....
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top