LTC6409 datasheet(13/24 Pages) AD

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LTC6409

6409fb

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Finally, noise figure can be obtained as:

NF = 10log 1+

eno2

eno2(RS)

















Figure 7 specifies the measured total output noise (eno),

excluding the noise contribution of source resistance, and

noise figure (NF) of LTC6409 configured at closed loop

gains (AV = RF/RI) of 1V/V, 2V/V and 5V/V. The circuits

in the left column use termination resistors and trans-

formers to match to the 50Ω source resistance, while the

circuits in the right column do not have such matching.

For simplicity, DC-blocking and bypass capacitors have

not been shown in the circuits, as they do not affect the

noise results.

Relationship Between Different Linearity Metrics

Linearity is, of course, an important consideration in many

amplifier applications. This section relates the inter-mod-

ulation distortion of fully differential amplifiers to other

linearity metrics commonly used in RF style blocks.

Intercept points are specifications that have long been

used as key design criteria in the RF communications

world as a metric for the intermodulation distortion

performance of a device in the signal chain (e.g., ampli-

fiers, mixers, etc.). Intercept points, like noise figures,

can be easily cascaded back and forth through a signal

chain to determine the overall performance of a receiver

chain, thus resulting in simpler system-level calculations.

Traditionally, these systems use primarily single-ended RF

amplifiers as gain blocks designed to operate in a 50Ω

environment, just like the rest of the receiver chain. Since

intercept points are given in dBm, this implies an associ-

ated impedance of 50Ω.

However, for LTC6409 as a differential feedback amplifier

with low output impedance, a 50Ω resistive load is not

required (unlike an RF amplifier). This distinction is impor-

tant when evaluating the intercept point for LTC6409. In

fact, the LTC6409 yields optimum distortion performance

when loaded with 200Ω to 1kΩ (at each output), very

similar to the input impedance of an ADC. As a result,

terminating the input of the ADC to 50Ω can actually be

detrimental to system performance.

APPLICATIONS INFORMATION

Figure 6. A More General Noise Model Including

Source and Termination Resistors

calculation, a termination resistor (RT) is included and

its noise contribution is taken into account.

Now, the total output noise power (excluding the noise

contribution of RS) is calculated as:

eno 2 = eni • 1+

RI +

RT ||RS









































+ 2 • in •RF

(

)2 +

2 • enRI •

RI +

RT ||RS





























+ 2 • enRF 2 +

enRT •

•

2RI ||RS

RT + 2RI ||RS

(

)





























Meanwhile, the output noise power due to noise of RS is

given by:

eno 2 (RS) = enRS •

•

2RI ||RT

RS + 2RI ||RT

(

)





























–

eno2

VOCM

enRI2

enRF2

enRS2

enRT2

enRI2

eni2

enRF2

in+2

in–2

6409 F06

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