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TMP422 with diode connected PNP transistor

massimo ganzetti
Intellectual 350 points
Other Parts Discussed in Thread: TMP422, DXP

Hi

i've problem with the TMP422 having two 2N3906 connected in diode configuration.

I can read the internal sensor temperature, but on the two remote i get always on the first -64 or -52 or -48

and on the second i got always -1

Where i'm wrong?

regards

Massimo

  • 0
    TI__Mastermind 23170 points

    Massimo,

    Question/Suggestions:

    1. Could you provide a schematic? 
    2. Are you using any filtering? If so please try removing the filter and directly connect the transistor instead.
    3. Can you try connecting the other configuration in the datasheet known as the "Ground Collector Connected" configuration?
    4. How is the transistor connected? Is it close the the TMP422 pins, or is it far away and connected with wires? The device should be no further than 8 inches from the TMP422 connection pins. 
  • 0 in reply to Chris Featherstone
    Intellectual 350 points

    GoodMorning Chris

    I'm not filtering the transistor emitter is directly connected to the DX1 pin 1 and the collector tied to the base and the DX2 pin 2 same for the second transistor

    emitter on pin 3 and collector+base on pin 4.

    I check the circuit both connecting the transistor near to the TMP422 or by a 30cm cable but always the same result

    i'have also connected in other configuration but no result.

    Why You say no more than 8 inches? the datasheet says that the device can compesate till 3kOhm of resistance so a cable can be longer than that.

    hi

    massimo

  • 0 in reply to massimo ganzetti
    TI__Mastermind 23170 points

    Massimo,

    I believe the issue is with the length of cable you are using. Please try lowering this length to a few inches and see if this problem goes away.

    When using a connection exceeding 8 inches the following considerations should be made:

    Please see page 24 of the datasheet for further material regarding layout considerations. 

    A filter may also be needed in order to filter out any noise pickup on the DXP and DXN lines. This is application specific and characterization of your system will have to be performed in order to achieve the best set of values. 

  • 0 in reply to Chris Featherstone
    Intellectual 350 points

    GoodMorning Chris,

    well the problem was due to a wrong register counter increment in our software, now with the transistor tied near the board all is working fine,

    we will connect the transistor trought the cable using Your suggestion, anyway we do not need a very precise measurement, so i hope that what

    we need will be what we get from the field.

    regards

    massimo

  • 0 in reply to massimo ganzetti
    TI__Mastermind 23170 points

    Massimo,

    I am glad to hear this was resolved. Please let me know if I can be of further assistance.