ANALOG INPUT
INPUT COMMON-MODE VOLTAGE
Depending on the application and the desired level of perfor-
mance, the analog input of the ADS5220 can be configured
in various ways and driven with different circuits. In any case,
the analog interface requirements should be carefully exam-
ined before selecting the appropriate circuit configuration.
The circuit definition should include considerations on the
input frequency band and amplitude, as well as the available
power supplies.
The ADS5220 operates from a single +3.3V supply, and
requires an external common-mode voltage. This allows a
symmetrical signal swing while maintaining sufficient head-
room to the supply rails. The common-mode voltage can be
generated from an external DC voltage source (for example,
an analog +3.3V supply with a simple resistor divider), or
from the input signal source with DC-coupling. For a single-
ended input configuration, the common-mode voltage is
typically +1.25V. When the input configuration is differential,
the common-mode voltage is +1.5V.
INPUT IMPEDANCE
The input impedance of the ADS5220 is capacitive due to the
input stray and sampling capacitors. These capacitors effec-
tively result in a dynamic input impedance that is a function
of the sampling and input frequency. Figure 2 depicts the
differential input impedance of the ADS5220 as a function of
the signal input frequency. For applications that use op amps
to drive the ADC, it is recommended that a series resistor be
added between the amplifier output and the converter inputs.
This will isolate the capacitive input of the converter from the
driving source and avoid gain peaking, or instability; further-
more, it will create a 1st-order, low-pass filter (LPF) in
conjunction with the specified input capacitance of the
ADS5220. The cutoff frequency of this LPF can be further
adjusted by adding an external shunt capacitor. In any case,
the use of the RC network is optional, but optimizing the
values to adapt to the specific application is encouraged.
INPUT FULL-SCALE RANGE
The input full-scale range (FSR) of the ADS5220 is select-
able from 1VPP to 2VPP and any value within this range, by
the configuration of the reference select pin RSEL and
reference voltage pin VREF (see Table I). The input FSR
(differential) is always twice VREF (the voltage at the VREF pin)
for all reference modes.
By choosing different signal input ranges, trade-offs can be
made between noise and distortion performance. For ex-
ample, applications requiring the maximum signal-to-noise
performance (SNR) will benefit from the 2VPP input range
while lower distortion may be obtain with the reduced input
range of 1VPP. Depending on the input driver configuration
the 1VPP range may also relax the requirements for the
driver, particularly for single-ended, single supply applica-
tions.
DIFFERENTIAL INPUTS
The ADS5220 input structure is designed to accept both a
single-ended or differential analog signal. However, the
ADS5220 will achieve its optimum performance when the
analog inputs are driven differentially.
DIFFERENTIAL INPUT IMPEDANCE
50k
40k
30k
20k
10k
100
Differential operation of the ADS5220 requires that an input
signal at the inputs (IN, IN) has the same amplitude and is
180 degrees out-of-phase. Differential signals offer a number
of advantages:
• The signal amplitude is half 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 also allows a wider selec-
tion of the most suitable driver amplifier.
1M
10M
100M
Input Frequency (Hz)
• Minimization of even-order harmonics.
• Improved noise immunity based on the common-mode
input rejection of the converter.
FIGURE 2. Differential Input Impedance vs Input Frequency.
ADS5220
10
SBAS261A
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