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OPA140: The input limitation of OPA140

Part Number: OPA140
Other Parts Discussed in Thread: DIYAMP-EVM
Hello, I would like to ask, when testing the OPA140EVM board, the power supply is a tracking circuit, and Vcc is a plus or minus 5V power supply. When Vin=4.5V, it dose not meet the range of the input common mode voltage, but when the input voltage frequency is relatively low (such as 1kHZ), the output voltage waveform is normal, and the top is cut when the frequency is relatively high. May I ask why? Thank you!
  • Hi Maggie,

    Vcc is a plus or minus 5V power supply. When Vin=4.5V, it dose not meet the range of the input common mode voltage

    You are correct, your input voltage is violating the common-mode input range of the OPA140.

    when the input voltage frequency is relatively low (such as 1kHZ), the output voltage waveform is normal, and the top is cut when the frequency is relatively high.

    It is difficult for me to comment on this as I do not have enough information about your circuit and input signal. Is your input signal a sine wave? a square wave? is it 4.5V peak-to-peak? Or is the 4.5V a DC bias? What do you mean by "the power supply is a tracking circuit?" Can you provide a schematic? Can you provide scope captures of your input and output waveforms?

    With the information that I have, my best guess is that your input signal is coupling directly into the output node and this is what you are measuring. Also, I do not see a OP140EVM on the product page, are you using the DIYAMP-EVM?

    Thanks,

    Zach

  • Hi Zach,

    Thank you for your reply! My test circuit is shown below. 

    And the input signal is a 9V peak-to -peak with no DC bias. When I simulate with TINA, no matter f=1kHZ or 50kHz, the output waveform still has clipping. But the output will not be distorted at low frequencies in the experiment. The experimental output waveforms at low frequency and high frequency are as follows.

     (f=50k)

     (f=1k)

    this point is also mentioned in the input/output limitation of TI precision laboratory OP amplifier series video. The video mentions that the OPA140 is rail-to-rail input and output at low frequencies, I don't quite understand why. Thank you!

  • Hi Maggie,

    Thanks for the additional information. Yes, it is known that the OPA140 behaves as rail-to-rail input at low frequency (~1kHz), although the device spec does not show this. The input-common mode range in the datasheet is specified for the device's entire usable bandwidth. There is no common-mode specification for low frequency vs high frequency signals.

    In practice, most devices are able to operate with some margin outside of the device specifications, although the performance will be reduced. The electrical characteristics in the datasheet are often somewhat conservative to ensure high performance in the operating range. In the case of the OPA140, the device is observed to operate outside of the input common-mode specifications, although with severely reduced bandwidth (1kHz instead of 11MHz). This 1kHz bandwidth limitation not practical for a customer to use in their circuit, so it is not characterized in the datasheet.

    Thanks,

    Zach

  • Hi Zach,

    Thank you sooooo much!!