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OPA1622: Output Voltage Swing

Part Number: OPA1622

Hello E2E,

I am working with the OPA1622

I've configured it in the lab and am seeing approximately 1.5V between the peak output and the supply rails (0 & 13.5VDC)

 

Below is the test circuit that i designed to help verify the OPA1622's fit in our design.

I am intentionally saturating the opamp, and see it railing within 1.5V of the supply.

I feel like I'm probably missing something obvious, but why is there about 1.5V of headroom?  The "Voltage Output Swing from Rail" parameter within the datasheet is listed as between 800-900mV depending on load.  Isn't that the "headroom" of the opamp? What's causing the extra 600-700mV of cutoff?  The input signal is only 1kHz, so that is far away from cutting out Vout.  

 

ch1-yel: input to amps (not applicable)

ch2-blu: VCC rail

ch2-ppl: output 1 within the OPA1622

ch2-grn: output 2 within the OPA1622

cursors used to show headroom.

(CH2,3,4 all centered on same spot,  ch1 centered in middle of screen)

R_LOAD is approximately 80ohms at the moment 

 

Thanks for the help!

Regards,

Russell

 

  • Russell,

    There are at least two issues with your thinking.

    First, for the linear output swing you must look at the conditions of AOL test where the output is undistorted and NOT output swing which refers to slaming the output against the rail; therefore, Vout must be 2V from the rails for 32ohm load (Iout~97mA) or 1.5V from the rail for 600ohm load (Iout~28mA).

    Second problem is that with 13.5V output and 80ohm load, Iout~169mA and thus above short-circuit protection limit, which will limit the output swing - see below.

  • Thank you for the quick response Marek!

    Russell