OPA688P datasheet(16/18 Pages) TI1

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OPA688

SBOS082A

FIGURE 12. 5MHz Harmonic Distortion vs Load Resistance.

–40

–45

–50

–55

–60

–65

–70

–75

–80

–85

–90

HARMONIC DISTORTION vs LOAD RESISTANCE

Load Resistance (

Ω)

100

1000

O = 2Vp-p

1 = 5MHz

HD2

HD3

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 peak-

ing 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 applica-

tions, 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

12. Remember to include the feedback network in the load

resistance calculations.

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