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LM234: Fine tuning set point

Cameron Perl
Expert 4230 points
Part Number: LM234
Other Parts Discussed in Thread: LM4051-N, LM4041-N

Hey team,

I have a customer who is looking for any other tips / tricks for maintaining the setpoint for the current. They have a target value of 1.00 mA and are getting about 0.989 mA on the prototype using the 0 tempco enhancement shown in the data sheet.  They are already using a resistor with tolerances at 0.1%.  

The target circuit is a 100-ohm RTD and they're getting about 7 degrees C of error which they think is mostly due to the current being slightly off.

Any advice?

Thanks!

  • 0
    TI__Mastermind 19420 points

    Hi Cameron,

    Is the "7 degrees C error" the drift error of the error of the device at 25C?

    This device is sadly a bit old so its not the best solution but it is a flexible one. What is the total error that you want in the current source?

    Is the current sinking or sourcing?

  • 0 in reply to Marcoo Z
    TI__Expert 4230 points

    Hey Marcoo, thanks for the reply. If you think a newer device could fit the design better let me know and I'll steer them in that direction as a possible option. With regards to your questions here's the response I got:

    1) I calculate from measurements a current of 0.9888 mA at room temperature, which is about 22 C I would guess.  In my circuit I put that current through an RTD, amplify the signal by 5.016, then run it through an A/D to get the RTD resistance.  Then I plug that value into an equation to get temperature from resistance for the RTD.  After all that, I am reading high by about 7 degrees for the RTD resistance.  I actually use fixed known resistors in place of the RTD so that I can be sure of the value and keep it constant w/o having to change the temperature.  So far I have measure the resistor and measure the input and output of the amplifier, and then I print out the resistance and the calculated temperature.  The resistance is a little too high, which results in the 7 degree error, but the amplifier gain looks right.  However, I could still use more precise equipment and resistors to get the tolerances even less to be sure they are not contributing.  I am calculating the current = 0.9888 mA by reading the input voltage to the amplifier and dividing by the resistor value.

    2) sourcing current out of the LM234 to drive the RTD, which has its other end connected to 0V (GND)

    I appreciate the help!

  • 0 in reply to Cameron Perl
    TI__Mastermind 19420 points

    Hi Cameron,

    The issue with the LM234 is that you are fighting with typical parameters to get an accurate stable output. You have the RTD which will change with temp, then you have the LM234 which will change with temp. This is why sometimes you get these inconsistencies. At the same time the device has a Vset (64mV ish) that is 2% accurate at 25C which can easily give 6C of error by itself. This wont include the deviation of the device over temperature.

    I would prefer to use the LM4041-N ADJ or LM4051-N ADJ for current sourcing such as in fig 8. It is much easier to calibrate and control the output.

  • 0 in reply to Marcoo Z
    TI__Expert 4230 points

    Hey Marcoo,

    So do you think the LM@#4 would be unable to hit the following requirement? 1mA +/- 0.5% over the range of 10 degrees C to 150 degrees C on the current source, in order to maintain an RTD temp reading error of no more than +/- 2 degrees C.  If the error can be kept within +/-3 degrees that would work as well. 

    Is the issue here that the newer technology in the LM4041-N simply makes it easier to calibrate?

  • 0 in reply to Cameron Perl
    TI__Expert 4230 points

    Whoops that should be 50C for the LM234 - the RTD reaches 150C

  • 0 in reply to Cameron Perl
    TI__Mastermind 19420 points

    1mA +/-0.5% is very difficult for the LM234.

    The LM234 has a 2% slope error on Vset which is effectively a 2% error across temperature which is greater than 0.5%.

    The LM4041-N family is easier to calibrate due to the output current being Iout = Vref/R and you can tune the R.

    The LM4051 ADJ I believe can do this. The initial accuracy error is 0.2% and the tempco error is around 0.33% from -40C to 85C. This allows a budget for the resistor temp co error and initial accuracy to reach 0.5% over temp or close to it. The order able I would use is the one below.

    LM4051BIM3X-ADJ/NOPB