THS4501CDG4 datasheet(29/48 Pages) TI1

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10 20

30 40

50 60

f − Frequency − MHz

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

OIP

+ PO )

IMD

where

+ 10 log

Pdiff

0.001

NOTE: Po is the output power of a single tone, RL is the

differential load resistance, and VP(diff) is the differential

peak voltage for a single tone.

THS4500

THS4501

www.ti.com

SLOS350F

– APRIL 2002 – REVISED OCTOBER 2011

design decisions. Traditionally, these systems use

primarily class-A, single-ended RF amplifiers as gain

blocks. These RF amplifiers are typically designed to

operate in a 50-

Ω environment, just like the rest of

the receiver chain. Since intercept points are given in

dBm, this implies an associated impedance (50

Ω).

However, with a fully differential amplifier, the output

does not require termination as an RF amplifier

would. Because closed-loop amplifiers deliver signals

to the outputs regardless of the impedance present, it

important

comprehend

this

feature

when

evaluating the intercept point of a fully differential

amplifier. The THS4500 series of devices yields

optimum distortion performance when loaded with

200

Ω to 1 kΩ, very similar to the input impedance of

Figure 110. Equivalent 3rd-Order Intercept Point

an analog-to-digital converter over its input frequency

for the THS4500

band. As a result, terminating the input of the ADC to

Ω

can

actually

detrimental

system

Comparing specifications between different device

performance.

types becomes easier when a common impedance

This

discontinuity

between

open-loop,

class-A

level is assumed. For this reason, the intercept points

amplifiers

and

closed-loop,

class-AB

amplifiers

on the THS4500 family of devices are reported

becomes apparent when comparing the intercept

normalized to a 50-

Ω load impedance.

points of the two types of devices. Equation 10 gives

the definition of an intercept point, relative to the

AN ANALYSIS OF NOISE IN FULLY

intermodulation distortion.

DIFFERENTIAL AMPLIFIERS

Noise

analysis

fully

differential

amplifiers

(10)

analogous

noise

analysis

single-ended

amplifiers; the same concepts apply. Figure 111

shows 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

(11)

figures indicate the appropriate scaling factor for each

As can be seen in the equations, when a higher

of the noise sources in two different cases. The first

impedance is used, the same level of intermodulation

case includes the termination resistor, and the

distortion performance results in a lower intercept

second, simplified case assumes that the voltage

point. Therefore, it is important to understand the

source is properly terminated by the gain-setting

impedance seen by the output of the fully differential

resistors. With these scaling factors, the amplifier

amplifier when selecting a minimum intercept point.

input noise power (NA) can be calculated by summing

Figure 110 shows the relationship between the strict

each individual noise source with its scaling factor.

definition of an intercept point with a normalized, or

The noise delivered to the amplifier by the source (NI)

equivalent, intercept point for the THS4500.

and input noise power are used to calculate the noise

factor and noise figure as shown in Equation 23

through Equation 27.

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