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LM324: CS2957866: LM324DR

WEI XIAO
Prodigy 30 points
Part Number: LM324


Tool/software:

Adverse phenomenon: PCBA test reports fault 16: AO current signal abnormality. After replacing 3908A000/LM324, the test is OK. After replacing the removed 3908A000/LM324 with an OK board, the defect still exists. The fixture measurement on the defective board found that the AO current range of 0mA corresponds to 0.11/0.14V, and the standard requirement value cannot exceed 0.01V

Defective quantity: 4/22544=177DPPM

Application products: Inverter

  • 0
    TI__Guru* 77541 points

    Hi Wei, 

    WEI XIAO said:
    The fixture measurement on the defective board found that the AO current range of 0mA corresponds to 0.11/0.14V, and the standard requirement value cannot exceed 0.01V

    You have to show me the schematic how 0.11/0.14V signal is generated. Is this V-to-I converter? I need more information. I do not know what 0.11/0.14 mean based on a piece of requirement. 

    If you prefer to keep the conversion in private, please post "friendship" request over E2E and we can discuss the issues in the E2E private messaging system or I can provide you with our internal contact information once the communication is established. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 30 points

    The input of 1AO1 is 0mV (static input), and the normal AO output should be 0V. However, the actual output is 0.11V/0.14V. After replacing U15, it became normal. The analysis suggests that there is zero drift in U15 LM324
    Refer to the circuit diagram below

  • 0 in reply to WEI XIAO
    TI__Guru* 77541 points

    Hi Xiao Wei,

    WEI XIAO said:
    The input of 1AO1 is 0mV (static input), and the normal AO output should be 0V. However, the actual output is 0.11V/0.14V.

    Here is the Sallenkey LPF response, where there is a Q near the cutoff frequency, where Q is too high. 

    If you want to fix the Q or lower the gain peaking, please modify the circuit below.

    LM324 Sallenkey Filter.TSC

    Regarding to the possible offset error, it may cause by the op amp you used and Sallenkey filter above. The Sallenkey filter as is should be operated in dual supply rail. If you are using a single supply rail, the active 2nd order LPF may not be biasing properly. I do not know if dual supply rails are used. 

    Please modify the circuit as follows:

    1. remove C34, 100nF capacitor --> likely that is the source of the issue. Op amp can not drive the capacitive directly. 

    2. If you want to keep C34, then you may leave R178, 100ohm resistor has to be outside of the U15C's feedback loop. As it is right now, the 100ohm is not doing any op amp stability compensation. 

    The LM324s do have some variations, it seems to be working ok in some vs. others (but it could be an illusion as well).  

    If you have other questions, please let us know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 30 points

    According to your suggestion, we have tested and found that the AO voltage is 0.12V, which is not good. After removing C128 (circled in red in the circuit below), retesting still shows 0.12V and there is no improvement. Please contact the original factory for further analysis of the cause. Thank you!

  • 0 in reply to WEI XIAO
    Prodigy 30 points

    Please  help me confirm and reply to the above information as soon as possible

  • 0 in reply to WEI XIAO
    TI__Mastermind 25448 points

    Wei,

    1. In your text your mentioned you removed C128 which you indicate is circled in red.  The schematic shows R178 circled.  Did you remove C128 or R128?
    2. In Raymond's response, he suggested remove C34, 100nF capacitor.  Did you try that? I think this is likely the issue.  Op amps cannot directly drive capacitors.  When an op amp is connected to a capacitor load it can oscillate.  You should monitor the signal across C128 with an oscilloscope to see if if has oscillations.  Normally this issue can be resolved by adding a resistor at the output of the op amp outside of the feedback loop.  R128 will not help with stability the resistor needs to be after the feedback loop. See schematic below.

    Best regards, Art