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MC1458: Problem for oscillation by any MC1458

Satoshi
Guru 19615 points
Part Number: MC1458

A few MC1458 occur oscillation and ADC at output side become stop.

I want to judge the reason is circuit problem with variation spec on MC1458, or defect.

TIJ CQE requested that request schematic review to E2E.

Would you check and advice about the reason below?

【Schematic】

Excerpted schematic is attached below, MC1458 are using 4pcs and all input and output are the same external components.

【Problem】

Waveforms are attached below, oscillation occur into -1V～-2V.

（ch1: input ±4V, ch2: output）

When NG parts, oscillation occur also all output channels.

※Normal case

・Additional information

When negative current exceed 1.7A, NG parts occur ripple. (60μs period)

（ch1: output, ch2: current, ch3: input）

→Vcc+, Vcc-, input side, and 5V supply (to ADC) are not find ripple.

→When OK parts replaced from NG parts, ripple was improved. (OK ⇒ NG is occur ripple)

If there need detailed information, please contact offline or directly e-mail.

Best regards,

Satoshi

  • 0
    TI__Guru**** 156616 points

    Hello Satoshi-san,

    Are the outputs directly tied to ADC or is there some series resistance (or other circuitry)?

    What is the capacitance of the ADC in pF?

  • 0 in reply to Ron Michallick
    Guru 19615 points

    Hi Ron san

    Thank you for reply,

    Yes, MC1458's outputs is directly tied to ADC.

    The capacitance of the ADC in is 10pF, the information of the ADC is below.

    https://www.analog.com/media/en/technical-documentation/data-sheets/AD7891.pdf

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Guru**** 156616 points

    Satoshi-san,

    All of the AC parameters in MC1458 have only typical values; there are no limits. There is no specified maximum for AC variance.

    I am unable to link these observations with any specified parameter that has minimum or maximum values.

    For design robustness, it is recommended to have some capacitor in parallel with feedback resistors R502, R504, R506. 

    Minimum value = NON_INV_GAIN/(6.28*GBWP*R) = 2pF

    Minimum value = 2/(6.28*1e6*20000) = 2pF

  • 0 in reply to Ron Michallick
    Guru 19615 points

    Hi Ron-san

    Thank you for your advice, customer is confirming to add capacitor. 

    Would you let me know two points below? 

    ・Above advice mean that improve the transient, phase margin, and other internal compensation, is it correct?  

    ・These oscillation occur only MC1458 manufactured by certain FAB.

    　Is there a possibility that the characteristic variation or Min/Max value will change depending on the FAB?

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Guru**** 156616 points

    Satoshi-san,

    I'll need all of the label information to research this path of thought.

  • 0 in reply to Ron Michallick
    TI__Guru**** 156616 points

    Satoshi-san,

    The above advice is meant to improve phase margin in the application to make it less sensitive to product performance variances .

    There was recent PCN 20210720001.1 that has changes to design and wafer process and other less important factors. This will potentially increase the amount of performance variance observed. Post PCN material is fully data sheet compliant.