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OPA2188 - Gain peaking at 200kHz

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: OPA188

Hello,

In the customer evaluation, they measured gain peaking at 200kHz.
Is this device has peaking at 200kHz?

Conditions are the fllowing.

Application : Voltage folower
V+ =15V
V- = 15V
RL=2kohm
CL=100pF and 1000pF, ( the peak is increased with large capacitive drive)
RF=100kohm (connect between output and negative input)
CIn=1uF (connect between negative input and GND)

I can send the customer measurement wave and schematics if you contact to the following e-mail address.
asaka-r@clv.macnica.co,jp

Best Regards,
Ryuji Asaka

  • 0
    TI__Expert 6275 points

    I sent an email for you to send me your schematic directly for analysis.

  • 0
    TI__Genius 15410 points

    Hello Ryuji,

    I would expect some gain peaking with the circuit you have described. There are three causes to the gain peaking; a capacitive load of 100pF and 1000pF, large feedback resistor (100kohm), and a capacitor on the inverting input of the op amp.

    In the OPA188 datasheet Figure 25 displays the % overshoot vs. capacitive load. % overshoot and gain  peaking are directly related with the below image. As you can see in Figure 25 of the datasheet, having a 1000pF load causes approximately 35% overshoot which corresponds to approximately 35 degrees of phase margin and therefore the circuit is unstable and will cause some of the gain peaking you are seeing.

    Stability issues are related to delay in the feedback network, too much delay and the circuit becomes unstable creating gain peaking. The large feedback resistor (100kohm) and capacitor on the inverting input (1uF) cause delay in the feedback network. This delay will cause more degradation of the phase margin causing even more gain peaking.

    I recommend removing the 1uF capacitor on the inverting input of the op amp and changing the feedback resistor to 0 ohms. This should reduce the gain peaking. Additionally, I recommend adding an isolation resistor of 60ohms or greater between the output of the op amp and the capacitive load. This will keep the phase margin above 45 degrees for a capacitive load of 1000pF.

    For more information on stability, check out TIPD128 and TI Precision Labs which includes a series of videos on stability.

    -Tim Claycomb