OPA690 datasheet(21/33 Pages) TI2

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OPA699

SBOS261D

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In general, capacitive loads should be minimized for optimum

high-frequency performance. The capacitance of coax cable

(29pF/ft for RG-58) will not load the amplifier when the

coaxial cable, or transmission line, is terminated in its char-

acteristic impedance.

FREQUENCY RESPONSE COMPENSATION

The OPA699 is internally compensated to be unity-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 frequen-

cies, which limits the bandwidth.

If a unity-gain stable amplifier is needed, 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 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.

PULSE SETTLING TIME

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, use the recom-

mended RS in the typical performance curve Recommended

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 extremely good as shown in the typical char-

acteristics.

DISTORTION

The OPA699 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 15. Re-

member to include the feedback network in the load resis-

tance calculations.

NOISE PERFORMANCE

High slew rate, voltage-feedback op amps usually achieve

their slew rate at the expense of a higher input noise voltage.

The 4.1nV/

√Hz input voltage noise for the OPA699, how-

ever, 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 16 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.

OPA699

L is optional

Load Resistance (

Ω)

100

–55

–60

–65

–70

–75

–80

–85

–90

O = 2VPP

f = 5MHz

3rd-Harmonic

See Figure 1

2nd-Harmonic

4kT

OPA699

4kT = 1.6E –20J

at 290

°K

4kTR

√

4kTR

√

FIGURE 14. Driving Capacitive Loads.

FIGURE 15. 5MHz Harmonic Distortion vs Load Resistance.

FIGURE 16. Op Amp Noise Analysis Model.

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