AN-11410 pages

AN-1141
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Powering a Dual Supply Precision ADC with Switching Regulators
by Rui Du

INTRODUCTION
Compared with an LDO device, a switching regulator always
dissipates much less heat and provides higher efficiency. Therefore, a switching regulator is suitable for powering different
kinds of portable devices or the nodes in a wireless sensor
network to lengthen battery time. Unfortunately, a switching
regulator intrinsically generates ripple and noise at both the
output rail and the ground. At the same time, a switching
regulator brings electromagnetic radiation. These interferences
are almost inevitable due to the continuously on-off operation
of the power switch. With the parasitic parameters involved, the
noise will be present at unexpected frequency points besides the
integer multiple of the switching frequency.

10481-001

NOISE

RIPPLE

Figure 1. Typical Ripple and Noise at the Output of a Switching Regulator

Although damaged from the noise issues, if the interference of
the switching regulator can be effectively controlled while the
circuit is powered by a switching regulator, which has a strong
anti-interference capacity, then the degrading of the system
performance will be slight.
For a typical application, when a data converter is powered by a
switching mode power supply (SMPS), keep the total noise over
the band of interest lower than the noise floor to prevent it from
being seen by the converter. Although the in-band noise from
the switching regulator is normally greater than the noise floor,
because of the power rejection ratio of the ADC, the noise will
be sharply attenuated before entering the signal path; as a result,
the switching noise will not degrade the performance of the
ADC.

There are two options to power the ADC with switching
regulators:
A. Select a low noise switching regulator and then use
carefully designed filtering and shielding methods
to remove as much of the noise as possible.
B. Estimate the noise suppressing capability of the data
converter, and then select the product, which has good
noise-immunity performance.
In practice, both options can be used at the same time so
that the power solution of using a switching regulator can
be acceptable in most cases. The switching regulator solution
includes the benefits of high efficiency and low temperature.
In CN-0137, a dual-output synchronous buck switching
regulator, ADP2114, is used to power the 16-bit, 125 MSPS
analog-to-digital converter, AD9268. The outputs of ADP2114
are filters using an extra stage of an LC filter (ferrite bead).
Compared with the linear supplies solution, the testing result
shows that using a dc-to-dc supply has nearly no influences on
the performance of the ADC (see Table 1).
Table 1. Experiment Results Reported in CN-0137
Analog Input
Frequency (MHz)
10.3
70.0
100.3
140.3
170.3
200.3

Linear Supplies
SNR
SFDR
(dBFS)
(dBc)
79.2
92.2
78.5
91.0
77.8
85.8
76.9
85.0
76.2
84.3
75.0
76.9

DC-to-DC Supply
SNR
SFDR
(dBFS)
(dBc)
79.2
92.3
78.4
90.8
77.7
85.6
76.9
84.8
75.9
84.6
75.0
77.0

The noise performance of the ADP2114 is guaranteed by
multiple technologies implemented in the design. The typical
voltage ripple is less than 1 mV. Using additional filtering, its
noise performance can even align with linear supplies.

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