THS4501CD datasheet(28/37 Pages) TI

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THS4500

THS4501

SLOS350D − APRIL 2002 − REVISED JANUARY 2004

www.ti.com

As can be seen in the equation, when a higher impedance

is used, the same level of intermodulation distortion

performance results in a lower intercept point. Therefore,

it is important to comprehend the impedance seen by the

output of the fully differential amplifier when selecting a

minimum intercept point. The graphic below shows the

relationship between the strict definition of an intercept

point with a normalized, or equivalent, intercept point for

the THS4502.

Figure 109

1020

3040

5060

f − Frequency − MHz

THIRD-ORDER OUTPUT INTERCEPT POINT

FREQUENCY

70 80

90 100

Normalized to 200

Ω

Gain = 1

Rf = 392 Ω

VS = ± 5 V

Tone Spacing = 200 kHz

OIP3 RL= 800 Ω

Normalized to 50

Ω

Comparing specifications between different device types

becomes easier when a common impedance level is

assumed. For this reason, the intercept points on the

THS4500 family of devices are reported normalized to a

50-

Ω load impedance.

AN ANALYSIS OF NOISE IN FULLY

DIFFERENTIAL AMPLIFIERS

Noise analysis in fully differential amplifiers is analogous

to noise analysis in single-ended amplifiers. The same

concepts apply. Below, a generic circuit diagram

consisting of a voltage source, a termination resistor, two

gain setting resistors, two feedback resistors, and a fully

differential amplifier is shown, including all the relevant

noise sources. From this circuit, the noise factor (F) and

noise figure (NF) are calculated. The figures indicate the

appropriate scaling factor for each of the noise sources in

two different cases. The first case includes the termination

resistor, and the second, simplified case assumes that the

voltage source is properly terminated by the gain-setting

resistors. With these scaling factors, the amplifier’s input

noise power (NA) can be calculated by summing each

individual noise source with its scaling factor. The noise

delivered to the amplifier by the source (NI) and input noise

power are used to calculate the noise factor and noise

figure as shown in equations 23 through 27.

Figure 110. Noise Sources in a Fully

Differential Amplifier Circuit

ini

iii

−

fully-diff

amp

egef

)

RsRt

2 Rs)Rt

(eni)2

(ini)2

(iii)2

4kTRt

2RsRG

)2Rg

Rt )

2RsRg

)2Rg

4kTRf

4kTRg

)

RsRt

2 Rs

NA: Fully Differential Amplifier

Noise

Source

Scale Factor

(12)

(13)

(14)

(15)

(16)

(17)

Rg2

Figure 111. Scaling Factors for Individual Noise

Sources Assuming a Finite Value Termination

Resistor

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