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WEBENCH® Tools/OPA1678: OPA1678 BPF issue

Tseng Roger
Intellectual 500 points
Part Number: OPA1678
Other Parts Discussed in Thread: OPA1612-Q1

Tool/software: WEBENCH® Design Tools

Hi Sir,

The attachment is the BPF+OPAMP's schematic for TINA. In real board when switch on, we can measure the voltage of VF2 about 4V VPP, but after 1 day, the voltage will become 3V VPP.

We found that VF1&VF2 output voltage decrease with time. What is the problem for schematic?

Meanwhile, my application is that when using a variable resistor(R18) to adjust the required VF3 voltage. I hope the voltage drifts within 1V on VF3 and 0.2V on VF2.

15-2K_BPF.TSC

Regards,

Roger

  • 0
    TI__Guru* 93105 points

    Hello Roger,

    Upon inspection of your OPA1678 filter/amplifier circuit I do not see anything about the design that would explain the huge decrease in the gain. It would seem if the OPA1678 op amps had a problem that they would have stopped functioning altogether rather than cause a sever gain shift.

    The first thing to do is check the dc voltages around the op amps to make sure they are correct. If they are, then apply a 15.4 kHz sine wave at the input adjusted so that you obtain 4 Vpp at VF2 as measured with a DSO. Check VF1 to make sure the level is correct at that point. See if there is any evidence of oscillation or other unexpected distortion. Move the input frequency around 15.4 kHz and make sure the band-pass center frequency is still the same, or if it has changed from the original 15.4 kHz.

    If the dc voltages and ac waveforms look okay except for gain I would make sure that the PC board is clean. If there is solder flux residue on the board it can cause problems.

    If all checks okay at this point except for the gain replace each OPA1678 one at a time. As each OPA1678 is replaced check to see if the gain returns to normal. If that happens the device may have been ESD damaged with handling.

    The other possibility is a shift in the passive RC components used in the band-pass filter stages. It looks like you are using 1 % resistors, but the capacitors may be an issue - especially if they are low cost, X7R dielectric ceramic capacitors. If you find the band-pass frequency has shifted and you are able to obtain the 4 Vpp again but at a different frequency, then I would suspect the capacitors.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 500 points

    Hi  Thomas,

    Thank you for your quick reply.

    I also noticed the problem of capacitance, I will use 1% capacitor for testing later. In addition, I found that the temperature of the OPA1678IDGKT will be 15 degrees C higher than room temperature, but the SOIC package IC (XOPA462IDDA, gain=15) only rises 5 degrees. The temperature factor is considered if OPA1612-Q1 is more stable.

    Regards,

    Roger

  • 0 in reply to Tseng Roger
    Intellectual 500 points

    Hi Thomas,

    Your analysis is correct. After changing the 1% capacitors, the output voltage is stable at 50 degrees centigrade.

    Thanks a lot.

    Regards,

    Roger