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OPA4277: OPA4277 input pulse signal and the charging process occurs

Qi Liu
Intellectual 2560 points
Part Number: OPA4277
Other Parts Discussed in Thread: TLC2254, TL064, OPA4132, OPA4188

Hi, support team

My customer has the questions as follow:

The supply voltage is ±5V.
Input voltage is 3V pulse signal, duration of high level is 1.6ms and the cycle is 1.2s.

When IC46 uses OPA4277, there will be a charging phenomenon (as shown in the picture below),

When remove the high pass filter capacitor C476, the charging process disappears, and the circuit is normal,

When replace OPA4277 with TLC2254 or TL064 (the high pass filter capacitor of C476 is not removed), the charging process disappears and the circuit is normal.

I tried to replace OPA4277 with OPA4132 or OPA4188. The problem remains.

I don't understand why it's OK to replace it with TLC2254, but not with OPA4277?

Pictures with problems:

Normal picture (yellow line in the figure):

Thanks so much.

Best regards,
Yuki

  • 0
    TI__Guru* 93105 points

    Hello Yuki,

    Your 3 V input pulse with +/-5 V supplies places the OPA4277 right at its common-mode voltage (Vcm) limit. It is not a rail-to-rail input op amp and the OPA4132 and OPA4188 are not as well. If you can reduce the input level to maybe 2 or 2.5 V and see if the charging still occurs that would be useful information. If it still occurs with the lower input pulse level, then the Vcm range isn't the issue.

    The other possibility is the input bias current of the OPA4277. The OPA4277 is a bipolar input op amp and its input current will be substantially higher than that of the other op amps. The input bias current return for the non-inverting input is 10 Megohms and that with the C476 100 nF capacitor may be resulting in a long charge time constant. Try reducing the 10 Megohm resistor down to 1 Meghom or even lower, and increase the coupling capacitor proportionately. If the time constant drops, then the bias current is the cause for the charge characteristic.

    The OPA4188 is a CMOS input op amp that relies on input switching technology for its low voltage offset. Even though its average input current may be low, it will have momentary peak currents associated with the switching functions. Therefore, it may show a charging behavior as well. The OPA4132 is JFET input op amp with low, relatively constant input current. I am not sure why it wouldn't work unless it is a Vcm issue. It s maximum positive Vcm is (V+) - 2.5 V, or in this case +2.5 V maximum.

    Regards, Thomas

    Precision Amplifiers Applications Engineering