ADC and LM35

[Thirdium]

Dear all,
I'm trying to interface an LM35 sensor(-55C to 150C) to ADC0808 (4-analoge Channels), since i want the temp range between -10C to 60C, which is an output (-0.1V to 0.6V) , and i'm looking for an accuracy of at least 0.5C, can anyone help me with the conditioning circuit?
i've designed one but i got some errors, can anyone help with a diagram?

Thanks all

 

[ericgibbs]

Dear all,
I'm trying to interface an LM35 sensor(-55C to 150C) to ADC0808 (4-analoge Channels), since i want the temp range between -10C to 60C, which is an output (-0.1V to 0.6V) , and i'm looking for an accuracy of at least 0.5C, can anyone help me with the conditioning circuit?
i've designed one but i got some errors, can anyone help with a diagram?

Thanks all

hi,
Do you have the two diodes from the LM35's 0V pin to 0V and the 18K from the LM35 output to 0V..??

 

[Thirdium]

Thanks for the reply, but could you explain more?

 

[ericgibbs]

Thanks for the reply, but could you explain more?

hi,
Using the LM35 for tempr less than +2C require either a -V supply [to the bottom end of a output resistor] or the two diodes in the LM35 ground pin and the LM35 output pin connected to 0V via a 18K.

The 'dual' output is then taken from the LM35 [0v pin] and LM35 output pin as per the datasheet applications.

Post a drawing on how you have test connected the LM35.

 

[Thirdium]

well this gives me full range?, also what range of voltages do i get? -0.55v to 1.5v? what i wanted is first of all to get voltage range to start from 0, and if i possibly could narrow the temp range since most of it wont be used.....smthn like -15 to 60 or anything like that inorder to get more accuracy from the ADC.

As for my design, it had nothing to do with the sensor, it assumes the sensor is connected like in ur 2nd diagram, and shift,amplifies the output using op-amps, but i found out that its not a good idea to do that, u'll get more errors !

Thanks

 

[ericgibbs]

well this gives me full range?, also what range of voltages do i get? -0.55v to 1.5v?
You would get min/max voltages of -0.55v to +1.5v, which would represent -55C thru +150C

what i wanted is first of all to get voltage range to start from 0, and if i possibly could narrow the temp range since most of it wont be used.....smthn like -15 to 60 or anything like that inorder to get more accuracy from the ADC.
This part of your requirement is achieved by using a voltage level shifter/amp
to give 0V == to -15C and +5V == +60C


As for my design, it had nothing to do with the sensor, it assumes the sensor is connected like in ur 2nd diagram, and shift,amplifies the output using op-amps, but i found out that its not a good idea to do that, u'll get more errors !
This statement is not correct
Thanks

So how do you want to proceed.?

 

[Thirdium]

well it is correct for my design, maybe it sucked also coz cant get accurate resistor values that i come up using the equations, how about i take some time to design a crkt and post it here and help out
i'll do a simulation for the conditiong crkt and see how it will behave


Thanks

 

[ericgibbs]

well it is correct for my design, maybe it sucked also coz cant get accurate resistor values that i come up using the equations, how about i take some time to design a crkt and post it here and help out
i'll do a simulation for the conditiong crkt and see how it will behave

Thanks

That sounds a good idea.

In your design use a medium quality OPA something like a CA3140/CA3240 not a 741.!
Remember to allow at least +8V for the OPA supply rail as you want +5Vout.

You will require also multiturn variable resistors to set the exact OFFSET and SPAN.

OK.?

 

[Thirdium]

Well im done with my design,
a few notes, my design measures -25C (-0.25 V) to 65C(0.65V).
Also i simulated the circuit putting a Voltage source instead of the LM35, but i've replaced it with the LM35 and the 100k resistor, as i understood from the datasheet, i hope im not mistaken.
The resistor values are the calculated values, im not sure which close values are available on the market, might some need help with that.
The equation i solved on : Vout = (-5.55)Vin + 3.61075 (Vin is the assumed output of the sensor crkt).

using ADC0808 with this, 5v reference i get for every 0.01953V a bit change)
since depending on my simulation , the difference in the output between 2 tempretures (1C step) i get nearly 0.056 Volts, meaning the resoulotion of the temp reading is less than 0.5 C, around 0.349 C.

can please comment on my design, since im not sure if its gonna work, also resistor values, and will the resistor values affect the current i need ?

i know this is too long, but this LM35 is givin me a real headache!

 

[Thirdium]

oh forgot to include the crkt!
i'll upload it and post it in a couple of minutes

 

[Thirdium]

here it is xD The output of the op-am is connected to the ADC input xD (forgot to put that in the diagram)
**broken link removed**

 

[ericgibbs]

using ADC0808 with this, 5v reference i get for every 0.01953V a bit change)
since depending on my simulation , the difference in the output between 2 tempretures (1C step) i get nearly 0.056 Volts, meaning the resoulotion of the temp reading is less than 0.5 C, around 0.349 C.

With the ADC0808, which is 8 bit ie: 255 numeric so

my design measures -25C (-0.25 V) to 65C(0.65V)

Which is a range of 90 degrees, so you have 90/255 = 0.35Cdeg/bit!!!

This compares with your results

EDIT:
Can you say what you believe the gain for the LM324 is, for the NI and INV inputs relative to the output of the LM324.???

The circuit you have posted will not work.

 

[ericgibbs]

here it is xD The output of the op-am is connected to the ADC input xD (forgot to put that in the diagram)

hi,
Would you like me to post a circuit.?

 

[shruti jain]

thanks a lot .this really helped me in my project because i was stuck up in this portion of my project . thanks a lot for the circuit diagram you have put up .