OPA688M

UNITY GAINSTABLE WIDEBAND

VOLTAGE LIMITING AMPLIFIER

SGLS144B − APRIL 2003 − DECEMBER 2006

21

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

frequency response compensation

The OPA688 is internally compensated to be unity-gain stable, and has a nominal phase margin of 60

° at a gain

of +2. Phase margin and peaking improve at higher gains. Recall that an inverting gain of −1 is equivalent to

a gain of +2 for bandwidth purposes (i.e., noise gain = 2).

Standard external compensation techniques work with this device. For example, in the inverting configuration,

the bandwidth may be limited without modifying the inverting gain by placing a series RC network to ground on

the inverting node. This has the effect of increasing the noise gain at high frequencies, which limits the

bandwidth.

To maintain a wide bandwidth at high gains, cascade several op amps, or use the high gain optimized OPA689.

In applications where a large feedback resistor is required, such as photodiode transimpedance amplifier, the

parasitic capacitance from the inverting input to ground causes peaking or oscillations. To compensate for this

effect, connect a small capacitor in parallel with the feedback resistor. The bandwidth will be limited by the pole

that the feedback resistor and this capacitor create. In other high gain applications, use a three resistor ”Tee”

network to reduce the RC time constants set by the parasitic capacitances. Be careful to not increase the noise

generated by this feedback network too much.

pulse settling time

The OPA688 is capable of an extremely fast settling time in response to a pulse input. Frequency response

flatness and phase linearity are needed to obtain the best settling times. For capacitive loads, such as an ADC,

use the recommended RS in the typical performance curve ”RS vs Capacitive Load”. Extremely fine-scale

settling (0.01%) requires close attention to ground return current in the supply decoupling capacitors.

The pulse settling characteristics when recovering from overdrive are very good.

distortion

The OPA688’s distortion performance is specified for a 500-

Ω load, such as an ADC. Driving loads with smaller

resistance will increase the distortion as illustrated in Figure 41. Remember to include the feedback network

in the load resistance calculations.

−40

−45

−50

−55

−60

−65

−70

−75

−80

−85

−90

HARMONIC DISTORTION vs LOAD RESISTANCE

Load Resistance (

Ω )

50

100

1000

V O = 2 Vp-p

f 1 = 5MHz

HD2

HD3

Figure 41. 5 MHz Harmonic Distortion vs Load Resistance