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unity gain buffer

av10
Prodigy 20 points
Other Parts Discussed in Thread: OPA551

Hi,

I am trying to supply a square wave pulsed voltage to a resistor. The pulse goes from 4 volts for a few ms to 5 volts for a few ms, then back down to 4 volts and so on.

I am using the DAQ card in my computer to provide this signal but it cannot provide the current drawn by the load. So I was planning to use an op amp that can source high output current (max current of at least 130 mA).   When I search online, I find several that can do this but they have other parameters that I am not familiar with since I am not an electronics engineer.

Could anyone suggest the best op amp to use for my application?

Thanks.

  • 0
    TI__Guru* 93105 points

    Hi Av,

    I think the OPA551 can do what you need. It is a high-current op amp capable of a 200 mA output current. You could set it up as a buffer and apply the +4 V to +5 V pulse to the non-inverting input. The output would be connected through the load resistor to ground and would follow the +4 V to + 5 V input. Since you don't need to swing anywhere near 0 V at the input, or output, the OPA551 could be operated from a single +10 to +30 V supply (V- would be connected to GND).

    You can find the datasheet here:

    http://www.ti.com.cn/cn/lit/ds/symlink/opa551.pdf

    The OPA551 should be fast enough for a signal that is at one level, or the other, for a few milliseconds.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points
    Thanks Thomas.

    I have a couple of questions here.

    The datasheet says (under absolute max ratings) that the input voltage range in -0.5V to +0.5V. Can I still supply a 4 to 5V signal?

    Also, since I need the op amp to have unity voltage gain, should I not be connecting the output to the inverting input?
  • 0 in reply to av10
    TI__Guru* 93105 points

    Hi Av,

    Actually, the Absolute Maximum input voltage range is listed as VIN (V–) - 0.5  to (V+) + 0.5 V. Therefore, for example, if you set V- to ground (0 V) and V+ to +15V, the absolute maximum input range would be -0.5 V to +15.5 V. That isn't the linear input range, but rather the range the input can tolerate without any possibility of damage.

    Yes, for a unity-gain follower the output is connected directly back to the inverting input. The voltage source (+4 V to +5 V) which is the common-mode voltage (VCM) is applied to the non-inverting input.

    You mentioned that the load is resistive. Do note that if any significant amount of capacitance is added from the output to ground that overshoot can occur at the edges of the waveform. This is normal. If the capacitance becomes too high the overshoot can become excessive and lead to ringing, and possibly instability. OPA551 datasheet Fig. 19 provides some helpful information. The load resistance in this case was 3 k-ohms, and the overshoot may differ some from that shown in the graph for a different load resistance. Capacitance can come about from not so obvious sources such as a cable connected to the op amp output.

    Regards, Thomas

    PA - Linear Applications Engineering