Voltage conditioning for ADC input

[MrAl]

Hi,

Often people dont want to have to add extra transformers to their projects. This is when we look for other solutions.

For example, if the original PIC board is already line isolated (e.g. wall wart) then another solution is to use a second low end PIC with offline non isolated power supply (cheap as you can get) and use that to sample the current or voltage, then send the data digitally through an opto coupler to the original PIC chip which reads the data every so often. This also means the original chip bandwidth is preserved for doing the more regular tasks while the second chip does all the fast sampling.
A typical low end PIC is the 12F675 which has 10 bit AD converter.

 

[Daniel Wood]

Well I think the original spec is for true power.. Although I will suggest measuring apparent power since I got the capture module to measure the frequency.. It could mean extra points!!
Thanks Ian.. I may take you up on that adapter you're offering. I think im meeting my advisor tomorrow so I will let you guys know how things pan out!

 

[MrAl]

Hi,

It doesnt matter how you sample (transformers, second PIC chip, etc.), as long as you sample both current and voltage and then do the math. Of course you should also test it with several known loads when your done building up the circuits.

 

[Daniel Wood]

Yeah don't worry I will be calibrating the thing when I get a hardware version running.
I'm having a problem with xc8 and receiving multiple values with the usart. Does anyone know a reliable way to parse a comma seperated string into a set of integers? Ive tried using strtok() but im quite sure its bugged :/

 

[Ian Rogers]

Do you want me to post my code...... I'll send you a PM..

 

[Daniel Wood]

Hey guys.. I dont know if i should revive an old thread or not. I'm having a problem with the adc and battery voltage.
One of the design goals is not to use a regulator. As you can see from the picture, the voltage coming into the pic is independent of the voltage supplying the chip (0 - 4v).
If the chip is supplied with 5v, its fine but as soon as the battery starts to drain, the adc input will be greater than VDD and could cause damage.

Is there any way I can voltage shift the adc input (preferably linearly) by using an op amp or some other arrangement and using the VDD as a reference?

 

[NorthGuy]

You can scale your input to 0-4V instead of 0-5V by altering your burden resistor.

However, PIC measures the voltage relative to its reference. If you use VDD as an ADC reference and it varies between 4 and 5.5V, your measurements will be off. You need some sort of stable reference for ADC.

 

[Daniel Wood]

I thought about the same.. its so difficult to achieve without a voltage regulator :/

 

[Nigel Goodwin]

I thought about the same.. its so difficult to achieve without a voltage regulator :/

Choose a PIC with a built-in voltage reference

The 16F1827 is one such example, and as an enhanced 14 bit device has many other advantages.

 

[geko]

The 12F1822 he's using has a built in voltage reference. Section 14 of the datasheet

 

[Daniel Wood]

Nigel.. Ive tried suggesting a pic more functionality like with two Vref pins but they're at least 14 pin devices and anything above 8 pins a no go.
Can this built in voltage reference be higher than the pics Vdd voltage though?

 

[Daniel Wood]

oh i didnt see your post geko.. ill check the datasheet!

 

[Daniel Wood]

Turns out I cant have the reference higher than VDD.. Although setting the reference to 4.096V should provide me with enough linearity with a 5.5V to a 4.1V range.
Maybe 2.1V if noise isnt an issue :/
Thanks guys!

 

[NorthGuy]

I would suggest 2.048 over 4.096. The built-in voltage reference is only about 10% accurate, so you won't get any precision by moving to larger scale.

 

[Nigel Goodwin]

Can this built in voltage reference be higher than the pics Vdd voltage though?

No - but there's absolutely no point in it being so.

If your incoming voltage is too high, simply use an attenuator to reduce it.

As NorthGuy suggests, use a 2.048V reference, and simply attenuate the ADC input to suit.