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LM35 behavior

Hassan Kelley
Prodigy 210 points
Other Parts Discussed in Thread: OPA2188, LM35

My LM35CZ is exhibiting some strange behavior.  The LM35 is mounted to a small PCB with a decoupling capacitor across the power pins and a 75 Ohm, 1 uF series damping network on the output.  The small PCB with the LM35 is connected to a main PCB via a harness that is approximately 2 feet.  The signal is filtered on the main PCB with an 10k-1uF filter before it is amplified with a non inverting OPA2188.  At room temperature the output from the LM35 is only 180 mV (ie 18 C) when the room temperature is 22 C.  In addition the output from the LM35 appears to be oscillating or very, very noisy.  When A disconnect the harness and power it locally and measure it locally the output voltage is about 220 mV corresponding to 22C and the signal looks clean on a scope.

Any ideas as to what my problem my be?

--hk

  • 0
    TI__Expert 7220 points

    Hi Hassan,

    I can help you with the LM35 problem by answering these questions:

    What type of cable are you using and what is it's capacitance?

    Also, can you attach pictures of the LM35 output at the small PCB end and on the receiver end?

    -Michael Wong

  • 0 in reply to Michael Wong
    Prodigy 210 points

    The cable is a simple 3 wire 22AWG harness.  I estimate the capacitance of the cable is about 100-200 pF between the output and common.

    I attached the schematic of the reference junction board which is remotely located.

    I cannot figure out how to attach multiple files so I will send the remainder in the replies that follow.

    963090-1 Ref Jct.pdf
  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    This is the bare board.  I am unable to photograph the board in question because it is enclosed in the copper isothermal reference junction.  I will send that picture in the next reply.

  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    This is the picture of the reference junction board inside the copper isothermal block.  The brown-black-yellow harness carries the power and output to the front end board were this signal is amplified and digitized.

  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    This picture shows the length of the run from the reference junction to the front end.  Notice the brown-black-yellow wire disappearing on the left side and terminating on the right side of the picture.

  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    This picture shows the brown-black-yellow LM35 connections terminating on the front end board.

  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    This last picture is a better picture showing the LM35 lines terminating on the front end board.  The yellow wire carries the output of the LM35.  Originally the signal was low pass filtered with  10KOhm-1uF.  In the course of troubleshooting I replaced the 10kOhm with a zero ohm resistor and replaced the 1uF capacitor with a 4.99kOhm resistor but the results were the same--incorrect output.  I will include the LM35 amplifier circuit on the next reply.

  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    The LM35 in question is connected via brown-yellow-black harness to the upper circuit in the drawing. 

    U13 has been replaced with an OPA2188.

    R28 has been replace is a zero ohm resistor and C54 has been replaced with a 4.99kOhm resistor.  The voltage at TP16 is inconsistent with the temperature and the transfer function of the LM35.  When probe the signal at TP16 it appears to be oscillating.  When I disconnect the signal line and measure the output signal on the reference junction PCB where the LM35 is located the voltage seems reasonable.

    FE-Alpha.pdf
  • 0 in reply to Hassan Kelley
    Prodigy 210 points

    This additional information may be helpful.


    The output signal (yellow wire) reads correctly with a multimeter when I remove it  from the connector while leaving the power and common intact to provide power to the remote sensor.  The signal is incorrect only when it is in the circuit.

  • 0 in reply to Hassan Kelley
    TI__Expert 7220 points

    Hi Hassan,

    For oscillation problems at the output please put a 200ohm resistor at the output as shown below:

    Also, are there any bypass capacitors on the OPA2188 (U13) on the receive side?

    If there is a noise problem try running the 3 wire harness on the outside of the metal chassis and see if the noise has improved. If this does not work try using a hardness with a shielded twisted pair on the inside of the chassis.

    We can help you more if we have oscilloscope pictures at the VDD of the LM35 part (Point A), at the VOUT node (Point B) with the output disconnected to the receiver circuit, and at the VOUT node (Point B) with the output connected to the receiver circuit.

    Please let me know if these do not solve the issues you are seeing.

    -Michael Wong