OPA551 datasheet(13/24 Pages) TI

13 / 24 page

OPA551, OPA552

SBOS100A

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INPUT PROTECTION

The OPA551 and OPA552 feature internal clamp diodes

to protect the inputs when voltages beyond the supply rails

are encountered. However, input current should be limited

to 5mA. In some cases, an external series resistor may be

required. Many input signals are inherently current-limtied,

therefore, a limiting resistor may not be required. Please

consider that a “large” series resistor, in conjunction with

the input capacitance, can affect stability.

USING THE OPA552 IN LOW GAINS

The OPA552 family is intended for applications with

signal gains of 5 or greater, but it is possible to take

advantage of their high slew rate in lower gains using an

external compensation technique in an inverting configu-

ration. This technique maintains low noise characteristics

of the OPA552 architecture at low frequencies. Depending

on the application, a small increase in high frequency

noise may result. This technique shapes the loop gain for

good stability while giving an easily controlled second-

order low-pass frequency response.

Considering only the noise gain (non-inverting signal

gain) for the circuit of Figure 11, the low frequency noise

gain (NG1) will be set by the resistor ratios, while the high

frequency noise gain (NG2) will be set by the capacitor

ratios. The capacitor values set both the transition fre-

quencies and the high frequency noise gain. If this noise

gain, determined by NG2 = 1 + CS/CF, is set to a value

greater than the recommended minimum stable gain for

the op amp and the noise gain pole, set by 1/RFCF, is

placed correctly, a very well controlled, 2nd-order low-

pass frequency response will result.

To choose the values for both CS and CF, two parameters

and only three equations need to be solved. First, the

target for the high frequency noise gain (NG2) should be

greater than the minimum stable gain for the OPA552. In

the circuit in Figure 11, a target NG2 of 10 is used.

Second, the signal gain of –1 shown in Figure 11 sets the

low frequency noise gain to NG1 = 1 + RF/RG (=2 in this

example). Using these two gains, knowing the Gain Band-

width Product (GBP) for the OPA552 (12MHz), and

targeting a maximally flat 2nd-order, low-pass Butterworth

frequency response (Q = 0.707), the key frequency in the

compensation can be found.

For the values shown in Figure 11, the f–3dB will be

approximately 956kHz. This is less than that predicted by

simply dividing the GBP by NG1. The compensation

network controls the bandwidth to a lower value while

providing the full slew rate at the output and an excep-

tional distortion performance due to increased loop gain at

frequencies below NG1 • Z0. The capacitor values shown

in Figure 11 are calculated for NG1 = 2 and NG2 = 10 with

no adjustment for parasitics.

Actual circuit values can be optimized by check the

small-signal step response with actual load conditions.

Figure 12 shows the small-signal step response of this

OPA552, G = –1 circuit with a 500pF load. It is well-

behaved with no tendency to oscillate. If CS and CF were

removed, the circuit would be unstable.

FIGURE 11. Compensation of the OPA552 for G = 1.

FIGURE 12. Small-Signal Step Response for Figure 11.

SMALL-SIGNAL STEP RESPONSE

OPA552, G = –1, CL = 500pF

Time (1

µs/div)

Ω

1.88nF

1 = 1 + RF/RG = 2

2 = 1 + CS/CF = 10

OPA552

+30V

–30V

OUT

208pF

Ω

OPA552

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