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4 Amplifiers vs. Baluns: The Disadvantages

A balun is more like a window than a door, and provides little to no isolation. Therefore, RF signal-chain lineups sometimes need to include isolation from the balun’s passing standing waves, impedance mismatches and kickback. Kickback is a common term used to describe the existence of charge injection from the opening and closing of the converter’s internal sampling switch capacitor if the ADC is unbuffered.

Unlike amplifiers, passive baluns can be lossy; and a high-frequency balun needs a wide-band matching pad (3 to 6dB) to help stiffen the broadband impedance across multi-gigahertz bands. What this means is that the balun is more gain-dependent of the output impedance, unlike an amplifier. Therefore, adding a matching or attenuation pad adds more loss into the RF signal-chain lineup and can increase the overall noise figure of an analog receiver design but provide increased BW. Finally, because of the broadband match approach, standing waves add and subtract from the passband flatness, causing ripple throughout the passband. See The 3rd dB: Why a Lossy Attenuation Network Pad Works Well With RF ADCs, application brief for more information on matching pads with baluns.

Also, as an amplifier is inherently active, the amplifier also inherently outputs noise and spurious distortion as well. The noise can vary depending on the amplifier’s design, but all amplifiers can have some amount of noise and spurious output, which ultimately the ADC can see. For example, if a particular amplifier has a gain of 12dB, an output-referred noise of 5nV/√Hz and a 12bit, 10Gsps ADC with an 8GHz input bandwidth, 1Vpp differential full scale has a signal-to-noise ratio (SNR) of roughly 60dB. These two devices can effectively add together and therefore, worsen the ADC’s noise floor and a lower the dynamic range, in this case worsening SNR by 5.45dB or...

Equation 1.

S N R_{t o t a l} = 20 \times \log (\frac{\frac{\frac{1}{2}}{\sqrt{2}}}{\sqrt{N o i s e A D C^{2} + N o i s e A M P^{2}}}) = 54.5 d B

Where,

Equation 2.

N o i s e A D C = (\frac{\frac{\frac{1}{2}}{\sqrt{2}}}{10^{(\frac{60}{20})}}) = 354 u V r m s

and

Equation 3.

N o i s e A M P = 5 n V \times \sqrt{1.57 \times 8000 M} = 560.4 u V r m s

Amplifiers not only have noise, but are prone to linearity as well. This linearity effectively adds to the ADC’s linearity, making this worse overall. For example, if an amplifier’s worst spurious output is –80dB and the ADC’s worst spurious is also –80dB, the best effective linearity in an amplifier-plus-ADC interface design at this particular frequency is –77dB or...

Equation 4.

S p u r i o u s = 20 \times \log (\sqrt{{(10^{(\frac{- 80}{20})})}^{2} + {(10^{(\frac{- 80}{20})})}^{2}}) = - 77 d B

To combat against any noise or spurious output in the band of interest, an anti-aliasing filter (AAF) between the two devices can help. How much this can help depends on how narrow or broad the filter design is and on the AAF’s roll-off criteria. Extra supportive components also need to be added to support this interface between the amplifier and ADC interface. Please see How anti-aliasing filter design techniques improve active RF converter front ends on how to properly design AAFs between amplifiers and ADCs.