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OPA551: Buffer application

Part Number: OPA551

I am using the OPA551 for the purpose of getting an output current in the range of 50-100 mA with a load of 15 or 30 Ohms. My input is a max of +/-10V. With my specifications, the output voltage comes out to be close to 1.5V. The applications information in the datasheet doesn't specify any info about how to achieve the above objective with low noise. The supply is +/-15V. 

  • Hi Abhitji,

    Since your application requires an output voltage (+/-1.5 V) that is smaller than the input voltage (+/-10 V) the OPA551 could be connected as an inverting amplifier attenuator. I show an example of an OPA551 configured as such below. The TINA simulation shows the expected current and voltage outputs for the +/-10 V input.

    The OPA551 is a low noise operational amplifier rated with a 14 nV/Hz voltage spectral density at 1 kHz. If you observe high noise in your circuit it is likely being coupled into the circuit from an external source.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    OPA551_Atten_01.TSC


     

  • Some more questions about the amplifier:

    The data sheet says that pins 1 and 5 should not be connected to negative supply. But the layout for both PDIP and SOIC shows connection between pins 1-4-5. How much of a factor can this be with my performance issue?

    Also if I intent to use the OPA551 as a voltage follower, I plan to connect the output directly to the inverting input in my non inverting configuration. How practical do you think this technique will be?
  • Hi Abhitji,

    The OPA551 datasheet indicates that for the PDIP package pins 1 and 5 are NC, or have no internal connection. Therefore, they can be connected to pin 4 (V-) trace if desired, or just a neutral trace to help get rid of heat.

    The datasheet states, "The SOIC-8 package outline shows three negative supply (V–) pins. These pins are internally connected for improved thermal performance. NOTE Pin 4 must be used as the primary current carrier for the negative supply. It is recommended that pins 1 and 5 are not directly connected to V–. Instead, connect pins 1 and 5 to a thermal mass. DO NOT lay out the printed-circuit-board (PCB) to use pins 1 and 5 as feedthroughs to the negative supply. Such a configuration results in a performance reduction." If you decided to use the SOIC-8 package just follow these instructions.

    A standard op amp buffer amplifier configuration has the output connected directly to the inverting input, while the non-inverting input is driven by the source. Keep in mind that the gain is +1 V/V so the output voltage level will be the same as the input voltage level. You would need to reduce the input level if you want the output maximum volatge to be 1.5 V. A simple resistive voltage divider could be applied at the non-inverting input to reduce th einput voltage level.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • Hi Abhitji,

    Has the information we provided resolved your OPA551 application questions? If so, please close this E2E thread.

    Regards, Thomas
    Precision Amplifiers Applications Engineering