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LM35: Which Circuit in the data sheet provides the correct reference temperature of the LM35 TO-220 without a cable harness

Part Number: LM35

I am using the LM35DT TO-220 package .

My first question is which circuit on the system design examples in the data sheet provides the correct temperature readings as per your manufactured design specification.

When I connect it to a 4.07 v supply and use figure 12 (with 2 core screened cable of 1 meter length using the screen as the output) I get a different reading to figure 16 with the same cable . If I connect without resistors as per figure 14 it is different again with only the supply + and -.connected and the output leg not connected to a cable. 

If I use the above cable with figure 12 and take a measurement at the pin with the output disconnected at the output pin leg (no resistor) I get 114mv when I connect the cable and take a reading at the end of the 1 meter cable with the resistor 2k I get 131mv. When both are connected the readings are 131mv on the output pin and 133mv at the end of the resistor at the other end of the cable. 



If your design team can recommend the simplest and most accurate connection method for a 1 meter length of cable and what cable type that would be great and which system design circuit was the unit designed for (so I can use this as a  reference check for a second unit) so I can ensure that I am not introducing errors with my cable connections and other designs using this part. 

Thanks.

  • Hi Seamus,

    Thank you for your post.

    My first question is which circuit on the system design examples in the data sheet provides the correct temperature readings as per your manufactured design specification.

    Per the notes at the start of the electrical characteristics table, the specifications apply from 2C to TMAX when using Figure 14, and from -40C to 110C for LM35C & LM35CA, and from 0C to 100C for LM35D when using the circuit on the first page of the datasheet.

    When I connect it to a 4.07 v supply and use figure 12 (with 2 core screened cable of 1 meter length using the screen as the output) I get a different reading to figure 16 with the same cable . If I connect without resistors as per figure 14 it is different again with only the supply + and -.connected and the output leg not connected to a cable. 

    If I use the above cable with figure 12 and take a measurement at the pin with the output disconnected at the output pin leg (no resistor) I get 114mv when I connect the cable and take a reading at the end of the 1 meter cable with the resistor 2k I get 131mv. When both are connected the readings are 131mv on the output pin and 133mv at the end of the resistor at the other end of the cable. 



    If your design team can recommend the simplest and most accurate connection method for a 1 meter length of cable and what cable type that would be great and which system design circuit was the unit designed for (so I can use this as a  reference check for a second unit) so I can ensure that I am not introducing errors with my cable connections and other designs using this part. 

    Figures 12 or 13 would be our recommendation for connecting to any significant length of cable due the capacitive loading.

    At the moment I'm not certain there is a problem with any of the setups you have described.  To help clarify, what temperature is your environment at, and are you testing all of these configurations in the same environment? A difference of less than 20mv like you are seeing could be due to something as simple as the heat from an operator's hand warming the LM35, or normal changes of temperature inside a room or chamber. 

    Best Regards,
    Brandon Fisher

  • Hi Brandon, Thanks for the update . Let me do it all again using the same device using figure 14 as a reference (without cable) while comparing design circuits figure 12 ,13 and figure 16 on the same device.

    Just one clarification can one put the 2 k resistor at the other end of the cable in figure 12 rather than at the LM35 end and the same for figure 13 with the bypass capacitor and R-C damper?.  Thanks

  • Hi Seamus,

    The 2kOhm Isolation resistor will perform best when placed closed to the LM35, on the other side of a meter of cable is likely too far to see much benefit. 

    Best Regards,
    Brandon Fisher

  • Hi ,

    I got everything working as designed. 

  • I attached two sensors to a solid brass mass -one with a meter lenght of cable from the recommendations above with a 2k resistor and the other as a reference unit. I can not figure out how to attach images that would help but all is fine now. What was the cause of the initial errors- When I did an auto set up of the scope the scope stabilized. The second issue I think was with the cable I was using I think it was a 2 pair cable with the screen as the output and connected to negative (this apears to have have introduced more errors.