OPAX170 datasheet(24/64 Pages) TI2

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AOL

100

10k

100k

100

120

10M

100M

Frequency (Hz)

20 dB

ROC

dec

40 dB

1 dec

ISO

LOAD

ISO

LOAD

ISO

LOAD

1 + C

× R

× s

T(s) =

1 + R + R

× C

× s

OPA170, OPA2170, OPA4170

SBOS557E – AUGUST 2011 – REVISED APRIL 2018

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Product Folder Links: OPA170 OPA2170 OPA4170

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Typical Application (continued)

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9.2.2.2 Unity-Gain Buffer

Figure 41 shows a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the

circuit in Figure 41. Not shown in Figure 41 is the open-loop output resistance of the operational amplifier, Ro.

(1)

The transfer function in Equation 1 has a pole and a zero. The frequency of the pole (fp) is determined by (Ro +

RISO) and CLOAD. Components RISO and CLOAD determine the frequency of the zero (fz). A stable system is

obtained by selecting RISO such that the rate of closure (ROC) between the open-loop gain (AOL) and 1/β is 20

dB/decade. Figure 42 depicts the concept. The 1/β curve for a unity-gain buffer is 0 dB.

Figure 42. Unity-Gain Amplifier With RISO Compensation

ROC stability analysis is typically simulated. The validity of the analysis depends on multiple factors, especially

the accurate modeling of Ro. In addition to simulating the ROC, a robust stability analysis includes a

measurement of overshoot percentage and ac gain peaking of the circuit using a function generator,

oscilloscope, and gain and phase analyzer. Phase margin is then calculated from these measurements. Table 3

shows the overshoot percentage and ac gain peaking that correspond to phase margins of 45° and 60°. For

more details on this design and other alternative devices that can be used in place of the OPA170, see the

Precision Design, Capacitive Load Drive Solution Using an Isolation Resistor.

Table 3. Phase Margin versus Overshoot and AC Gain

Peaking

PHASE

MARGIN

OVERSHOOT

AC GAIN PEAKING

45°

23.3%

2.35 dB

60°

8.8%

0.28 dB

OPAX170 Datasheet(PDF) 24 Page - Texas Instruments

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