New To A/D Need Some Guide

[kchriste]

So you are saying that the ambient temperature is 25C and the LM35 is outputting 0.28V? If so, it is possible that the LM35 really is at 28C if it is mounted on a PCB / breadboard that has components generating heat. Also, are you measuring the voltage right at the LM35s pins Vout & Gnd? If not, it is possible that there is a voltage drop in the leads due to current draw from other devices.

 

[Suraj143]

I looked at the display & it shows 25 C suddenly I measured the voltage Vout to GND in the LM35DZ.It shows 0.28V.

In the real calculation it must show 28 C.

My circuit is in a vero board I never use bread boards.

 

[Suraj143]

Hi kchriste I'm trying my best to get a smooth value count in the segments without adjusting or adding any hardware changes.

I'm going to try in different ways.Something like calling AD interrupts etc....

Last night I didn't sleep even.

 

[kchriste]

You'll have to do some more tests then. If the LM35 reads 0.28V and your display reads 25 then it is probably due to the fact that your 5V supply, which I assume is used for your ADC reference, is a little high. What is the voltage reading on Vdd of the PIC?

Shall I add 3 to the result of the ADC every time?

Only if the reading is 3 degrees low across the temperature range you'll be using.
It won't work if it reads 3 degrees low at 25C but 6 degrees low at 50C. Then you'd have to adjust the reference/supply voltage.

 

[Suraj143]

Hi kchriste thanks for being with me through out the thread.
last night I measured all the voltages.

Vdd = 5V
LM35DZ output voltage = 0.28V at 25 C

I replaced another LM35DZ but same.That of course ok.The bubble thing is the main headache.
I'm trying various adjustments in my main code to get a smooth stable count up without bubbles.

 

[Suraj143]

Am I the only person who gets this bubble thing error?I have never seen a similar thread like this.How others managed their LM35 temperature sensors? Didn't they get such a effect?

I'm really really upset.

 

[kchriste]

Am I the only person who gets this bubble thing error?

If you read this article you'll see on page 5 a description of "Last-digit bobble":
**broken link removed**

LM35DZ output voltage = 0.28V at 25 C

What do you get at other temperatures? Try boiling water at 100C as another calibration point.

 

[Suraj143]

Ok this weekend I'm going to work out on fully about this in the weekend I'll tell the progress kchriste.

Thanks for the support.

 

[blueroomelectronics]

What sort of displays are you using, what type of regulator, how fast are you running your PIC? Your sensor must be located away from any heat source as even a few LEDs or a fast PIC can heat that LM34 up.
**broken link removed**

 

[Suraj143]

Hi blueroomelectronics I’m using CC seven segments. I’m using two segments.

My PIC is running 4MHZ. My regulator is 7805 I measured the VDD & its 5.0V

Did you get a bobble error in your last digit? Here is the place I got stucked.

 

[blueroomelectronics]

Nope solid as a rock and well within spec. I sample during sleep, run at 32KHz and use an LCD display. The power supply is remote or 3xAA
I've hooked the LM35 directly to RA0 and no cap.
Not in the photo I've got an HS1101 humidity sensor too and still rock solid.
Your LED display will add about 2C if the sensor is close. Also keep it vertical and the sensor on the bottom. (heat rises)

 

[Suraj143]

Oh I see did you use any interrupts? I mean AD interrupts.I haven't try interrupts yet.

 

[blueroomelectronics]

Yes one to wake up the PIC. Timer1 overflows every 2 seconds, increments the RTCC, samples the A/D, toggles the TLC555 (humidity) or read the humidity counter. The next version will have RS485 communications.

Interrupts won't fix the error with thermal heat your design has, try turning off the display with a pushbutton, lower your PICs speed, move the LM35 off the PCB.

 

[Suraj143]

try turning off the display with a pushbutton

You mean to switch off the display then what I can see from my Display?

lower your PICs speed

This is one of my concern.I was thinking that.I'll try to do that.