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Frequency Response Compensation

Pulse Settling Time

Distortion

Load Resistance (

Ω)

100

1k

-55

-60

-65

-70

-75

-80

-85

-90

V

f = 5MHz

3rd-Harmonic

See Figure 47

2nd-Harmonic

G013

Noise Performance

OPA699M

SGLS289A – MAY 2005 – REVISED MARCH 2006

In general, capacitive loads should be minimized for optimum high-frequency performance. The capacitance of

coax cable (29 pF/ft for RG-58) does not load the amplifier when the coaxial cable or transmission line is

terminated in its characteristic impedance.

The OPA699 is internally compensated to be +4 gain stable and has a nominal phase margin of 60° at a gain of

+6. Phase margin and peaking improve at higher gains. Recall that an inverting gain of –5 is equivalent to a gain

of +6 for bandwidth purposes (that is, noise gain = 6). 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.

For unity-gain stable amplifier applications, the OPA698 is recommended.

In applications where a large feedback resistor is required, such as a 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 is 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.

The OPA699 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,

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 extremely good as shown in the typical

characteristics.

The OPA699's distortion performance is specified for a 500-

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

resistance increases the distortion as illustrated in Figure 61. Remember to include the feedback network in the

load resistance calculations.

Figure 61. 5-MHz Harmonic Distortion vs Load Resistance

High slew rate, voltage-feedback op amps usually achieve their slew rate at the expense of a higher input noise

voltage. The 4.1-nV/

√Hz input voltage noise for the OPA699, however, is much lower than comparable

amplifiers. The input-referred voltage noise and the two input-referred current noise terms, combine to give low

output noise under a wide variety of operating conditions. Figure 62 shows the op amp noise analysis model with

all the noise terms included. In this model, all noise terms are taken to be noise voltage or current density terms

in either nV/

√Hz or pA/√Hz.

27

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