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Schematic: Op_Amp_Cload.asc
Sch Open Loop: Op_Amp_Cload - OL.asc
Op Amp Symbol: Opamp_2.asy
Op Amp Shematic: Opamp_2.asc
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Intro One of the big challenges in analog design is driving a capacitive load! You may need to
• drive a coax cable.
• buffer an ADC's ref pin having a huge bypass cap.
So, what's the big deal?
Cload (and Ro) form a low-pass filter which can lead to ringing and oscillations!Definitions Ro - the output resistance of the op amp (typically 20 to 50 ohms.)
Cload - the cap load at the op amp's output (typically 10s of pF to 10s of uF).Design Goal A stable amplifier output (minimial overshoot and ringing). Design Spec Overshoot < 10% given Ro = 50 ohms, Cload = 22nF and 0.1V step voltage input.
Step Response Phase (deg) Oscillations - UNSTABLE -360 (or 0) 40% Overshoot -330 (or +30) 25% Overshoot -315 (or +45) 10% Overshoot -300 (or +60) No Overshoot - Optimal Response -270 (or +90)
Final Note: Riso might cause an unacceptable voltage drop if the load draws significant current. In that case, check out driving a cap load with a feedback point at the capacitor.
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