OPA690IDBVT datasheet(22/37 Pages) TI1

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100

CapacitiveLoad(pF)

100

1000

NG=2

NG=3

NG=4

4kT

OPA690

4kT=1.6E

20J

at290 K

4kTR

OPA690

SBOS223F – DECEMBER 2001 – REVISED FEBRUARY 2010

www.ti.com

This gain of +2 circuit includes a noise gain tuning

In most op amps, increasing the output voltage swing

resistor across the two inputs to increase the noise

increases harmonic distortion directly. The new

gain, increasing the unloaded phase margin for the

output stage used in the OPA690 actually holds the

op amp. Although this technique will reduce the

difference between fundamental power and the 2nd-

required RS resistor for a given capacitive load, it

and 3rd-harmonic powers relatively constant with

does increase the noise at the output. It also will

increasing output power until very large output swings

decrease the loop gain, slightly decreasing the

are required ( > 4VPP). This also shows up in the

distortion performance. If, however, the dominant

2-tone,

3rd-order

intermodulation

spurious

(IM3)

distortion mechanism arises from a high RS value,

response curves. The 3rd-order spurious levels are

significant

dynamic

range

improvement

can

moderately low at low output power levels. The

achieved using this technique. Figure 45 shows the

output stage continues to hold them low even as the

required RS versus CLOAD parametric on noise gain

fundamental power reaches very high levels. As the

using this technique. This is the circuit of Figure 44

Typical

Characteristics

show,

the

spurious

with RNG adjusted to increase the noise gain

intermodulation powers do not increase as predicted

(increasing the phase margin) then sweeping CLOAD

by a traditional intercept model. As the fundamental

and finding the required RS to get a flat frequency

power level increases, the dynamic range does not

response. This plot also gives the required RS versus

decrease significantly. For two tones centered at

CLOAD for the OPA690 operated at higher signal

20MHz, with 10dBm/tone into a matched 50

Ω load

gains.

(that is, 2VPP for each tone at the load, which requires

8VPP for the overall two-tone envelope at the output

pin),

the

Typical

Characteristics

show

47dBc

difference between the test tone powers and the

3rd-order

intermodulation

spurious

powers.

This

performance improves further when operating at

lower frequencies.

NOISE PERFORMANCE

High slew rate, unity-gain stable, voltage-feedback op

amps usually achieve their slew rate at the expense

of a higher input noise voltage. The 5.5nV/

√Hz input

voltage noise for the OPA690 is, however, 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 46

Figure 45. Required RS vs Noise Gain

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

DISTORTION PERFORMANCE

in either nV/

√Hz or pA/√Hz.

The OPA690 provides good distortion performance

into a 100

Ω load on ±5V supplies. Relative to

alternative

solutions,

provides

exceptional

performance into lighter loads and/or operating on a

single +5V supply. Generally, until the fundamental

signal reaches very high frequency or power levels,

the 2nd-harmonic dominates the distortion with a

negligible 3rd-harmonic component. Focusing then on

the 2nd-harmonic, increasing the load impedance

improves distortion directly. Remember that the total

load

includes

the

feedback

network;

the

noninverting configuration (see Figure 36), this is sum

of RF + RG, while in the inverting configuration it is

just

RF.

Also,

providing

additional

supply-decoupling capacitor (0.1µF) between the

supply pins (for bipolar operation) improves the

Figure 46. Op Amp Noise Analysis Model

2nd-order distortion slightly (3dB to 6dB).

Product Folder Link(s): OPA690

OPA690IDBVT Datasheet(PDF) 22 Page - Texas Instruments

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