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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 2771
Maxim > Design Support > Technical Documents > Application Notes > Basestations/Wireless Infrastructure > APP 2771
Maxim > Design Support > Technical Documents > Application Notes > High-Speed Signal Processing > APP 2771
Keywords: WCDMA, cellular base station, w-cdma, high speed adc, analog digital converter,
APPLICATION NOTE 2771
High-Speed ADC Sets Input Common-Mode
Range
Dec 29, 2003
Abstract: The input common-mode voltage range (V
CM
) is important in the design of communication
receivers that include baseband-sampling, high-speed ADCs. A V
CM
is especially important for single-
supply, low-voltage circuits with DC-coupled inputs. For single-supply circuits, the input signal that feeds
the drive amplifier and ADC should be biased at a DC level well within the V
CM
range. This arrangement
removes a performance hurdle for the amplifier and ADC, because they do not need to maintain low
distortion and linearity at 0V. The application note presents a circuit that features the DC-coupled input
configuration to the MAX1196 through an RF quadrature demodulator front-end circuit.
The input common-mode voltage range (V
CM
) is important in the
design of communication receivers that include baseband-sampling,
high-speed ADCs. It's especially important for single-supply, low-
voltage circuits with dc-coupled inputs.
For single supply circuits, the input signal feeding the drive amplifier
and ADC should be biased at a DC level well within the V
CM
range.
This arrangement removes a performance hurdle for the amplifier
and ADC, because they don't have to maintain low distortion and
linearity at 0V.
Differential, DC-coupled connections to a high-speed ADC are typically found in radio receivers with direct
down-conversion. Such circuits have a Zero-IF (ZIF) architecture that features an RF quadrature
demodulator and dual baseband ADC. ZIF circuits are popular because they eliminate multiple IF down-
conversions and SAW filters. DC-coupled connections are desirable in a ZIF architecture for several
reasons: they accept in-phase (I) and quadrature (Q) baseband data with an information bandwidth
extending close to dc, they eliminate bulky coupling capacitors between the RF down-converter and high-
speed ADC, and they eliminate power-sequencing delays caused by the coupling capacitor's discharge
time.
The importance of V
CM
for the ADC is apparent when you consider the following:
With variation in the supply voltage (VDD), signals sourced by the RF quadrature demodulator
present a wide range of common-mode voltages to the ADC.
Input common-mode levels extending beyond the ADC's V
CM
range generate harmonic distortion
that reduces the dynamic range. A proper V
CM
dc bias therefore optimizes the amplifier and ADC
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