How to use LM34 to full range with PBP

[polymer52]

How can I go about reading in an LM34 at full range with a 12F683 and PBP?
It outputs -500mv at -50deg anf +3000mv at 300 deg.
Can a pic read a negative voltage and how would PBP tell the the difference between -500 and +500?

[Charles Linquis]

You can do this several ways.

But probably the best way is the method given in a National Semi application note: Put two diodes between the "-" terminal of the LM34 and GND. This offsets the voltage by about 1V. You will also note that - because the diodes have a temperature coefficient of their own, you should also measure the voltage at the junction of the diodes and the LM34 and take a differential reading. Of course, this will take two channels of A/D.

Look for National Semi Application Note AN-460, and check out Figure 10.

[Charles Linquis]

And yet another method that I have used:

Your system probably has a precision reference voltage (for example: I use a 4.096V reference for the A/Ds). Use a resistive divider off that reference to give you about 1.0V, and then buffer that voltage with an op-amp. Use the output of the buffer for the negative supply of the LM34.

[spcw1234]

Can a pic read a negative voltage and how would PBP tell the the difference between -500 and +500?

No a pic can not read negative voltages. However the voltage may be shifted to a range the pic can read.

One simple way I see of doing this to offset the voltage is by using a voltage divider to great a "fake" ground for the LM34. If you set this fake ground to +1VDC in refference to your pic's ground that will shift the output of the lm34 up 1000mv. The new range would be +500mv for -50*F and +4000mv for 300*F. Then in PBP remember to offset this value for the correct temp.

[polymer52]

No a pic can not read negative voltages. However the voltage may be shifted to a range the pic can read.

One simple way I see of doing this to offset the voltage is by using a voltage divider to great a "fake" ground for the LM34. If you set this fake ground to +1VDC in refference to your pic's ground that will shift the output of the lm34 up 1000mv. The new range would be +500mv for -50*F and +4000mv for 300*F. Then in PBP remember to offset this value for the correct temp.

How do I connect the floating ground to the pic? Does it need a separate isolated supply? I can't ground the floating ground correct?

[Charles Linquis]

The link I gave you from National Semi handles all the voltage offsets. It is all you need.

[polymer52]

The link I gave you from National Semi handles all the voltage offsets. It is all you need.

I read that but that basically gives me 2 grounds. The supply ground and the new floating ground. Do I have to use 2 analog channels and read the difference or use a separate isolated supply? I was hoping to get away with just a single supply but that may not be possible.

[Pic_User]

I read that but that basically gives me 2 grounds. The supply ground and the new floating ground. Do I have to use 2 analog channels and read the difference or use a separate isolated supply? I was hoping to get away with just a single supply but that may not be possible.

Hi polymer52,

I was confused by the same thing. Think this is how it goes:
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