3dB down compared to the 4Vp-p range, while an improve-
ment in the distortion performance of the driver amplifier may
be realized due to the reduced output power level required.
The third option, 2Vp-p full-scale range, may be considered
mainly for applications requiring DC-coupling and/or single-
supply operation of the driver and the converter.
ANALOG INPUTS
TYPES OF APPLICATIONS
The analog input of the ADS5421 can be configured in various
ways and driven with different circuits, depending on the appli-
cation and the desired level of performance. Offering an ex-
tremely high dynamic range at high input frequencies, the
ADS5421 is particularly well suited for communication systems
that digitize wideband signals. Features on the ADS5421, like
the input range selector, or the option of an external reference,
provide the needed flexibility to accommodate a wide range of
applications. In any case, the analog interface/driver require-
ments should be carefully examined before selecting the appro-
priate circuit configuration. The circuit definition should include
considerations on the input frequency spectrum and amplitude,
as well as the available power supplies.
INPUT BIASING (VCM
)
The ADS5421 operates from a single +5V supply, and
requires each of the analog inputs to be externally biased to
a common-mode voltage of typically +2.5V. This allows a
symmetrical signal swing while maintaining sufficient head-
room to either supply rail. Communication systems are usu-
ally AC-coupled in between signal processing stages, mak-
ing it convenient to set individual common-mode voltages
and allow optimizing the DC operating point for each stage.
Other applications, such as imaging, process mainly unipolar
or DC-restored signals. In this case, the common-mode
voltage may be shifted such that the full input range of the
converter is utilized.
DIFFERENTIAL INPUTS
The ADS5421 input structure is designed to accept the applied
signal in differential format. Differential operation of the
ADS5421 requires an input signal that consists of an in-phase
and a 180° out-of-phase component simultaneously applied to
the inputs (IN, IN). Differential signals offer a number of
advantages, which in many applications will be instrumental in
achieving the best harmonic performance of the ADS5421:
It should be noted that the CM pin is not internally buffered,
but ties directly to the reference ladder. Therefore, it is
recommended to keep loading of this pin to a minimum
(< 100µA) to avoid an increase in the nonlinearity of the
converter. Additionally, the DC voltage at the CM pin is not
precisely +2.5V, but is subject to the tolerance of the top and
bottom references, as well as the resistor ladder. Further-
more, the common-mode voltage typically declines with an
increase in sampling frequency. This, however, does not
affect the performance.
•
The signal amplitude is half of that required for the single-
ended operation and is, therefore, less demanding to
achieve while maintaining good linearity performance from
the signal source.
•
The reduced signal swing allows for more headroom of
the interface circuitry and, therefore, a wider selection of
the best suitable driver amplifier.
INPUT IMPEDANCE
The input of the ADS5421 is capacitive, and the driving source
needs to provide the slew current to charge or discharge the
input sampling capacitor while the track-and-hold amplifier is
in track mode (see Figure 1). This effectively results in a
dynamic input impedance that is a function of the sampling
frequency. Figure 2 depicts the differential input impedance of
the ADS5421 as a function of the input frequency.
•
•
Even-order harmonics are minimized.
Improves the noise immunity based on the common-
mode input rejection of the converter.
Both inputs are identical in terms of their impedance and
performance with the exception that by applying the signal to
the complementary input (IN) instead of the IN input will invert
the orientation of the input signal relative to the output code.
INPUT FULL-SCALE RANGE VERSUS PERFORMANCE
1000
100
10
Employing dual-supply amplifiers and AC-coupling will usually
yield the best results. DC-coupling and/or single-supply ampli-
fiers impose additional design constraints due to their head-
room requirements, especially when selecting the
4Vp-p input range. The full-scale input range of the ADS5421
is defined either by the settings of the reference select pins
(SEL1, SEL2) or by an external reference voltage
(see Table I). By choosing between the different signal input
ranges, trade-offs can be made between noise and distortion
performance. For maximizing the SNR—important for time-
domain applications—the 4Vp-p range may be selected. This
range may also be used with low-level (–6dBFS to –40dBFS)
but high-frequency inputs (multi-tone). The 3Vp-p range may
be considered for achieving a combination of both low-noise
and distortion performance. Here, the SNR number is typically
1
0.1
0.01
0.1
1
10
100
1000
f
IN (MHz)
FIGURE 2. Differential Input Impedance vs Input Frequency.
ADS5421
SBAS237D
9
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