An299 – Cirrus Logic AN299 User Manual
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AN299REV1
AN299
power pins. The practice of placing the bypass capacitors on the opposite side of the circuit board and connecting
them to the power pins through vias should be avoided. The inductance of the via degrades the performance of the
capacitor significantly at high frequencies. Also, the capacitor should be placed between the ADC power pins and
the via(s) connecting to the power source. In other words, if the power pins are connected by vias to power or ground
planes, the capacitor should be between the power pins and the vias as illustrated by C3 in
It should also be noted that placement of C4, the 22 nF low-pass filter capacitor, is very important. In
this
component is placed on the opposite side of the PCB and connected with vias. This is acceptable and preferred over
crowding the top side of the PCB with the VA+ bypass capacitor. In fact, traces to the C2 pin and the VA+ pin should
be kept away from each other to avoid leakage currents in high-humidity conditions.
It can be advantageous to have some impedance between the power pin bypass capacitors and the power source.
For example, when using the same power source for VA+ and VD+, it is recommended that a 10-ohm resistor is
placed between the VD+ pin and the power source. This is possible with CS553x devices because the VD+ pin re-
quires very little average current and therefore only a small voltage drop occurs. Since the bypass capacitor on the
VD+ pin has a much lower source impedance than the 10-ohm resistor to the power source, the dynamic current
required by VD+ comes almost exclusively from the capacitor. This prevents digital noise from the VD+ pin from
being introduced on the VA+ pin. The same applies for VD- and VA- in bipolar power supply configurations.
The bypass capacitor also has a second purpose – to act as a power supply noise filter device. All power sources
contribute some amount of noise. It is very important to eliminate as much noise as possible from the VA+ and VA-
supplies. One of the surest ways to degrade the performance of a high-resolution ADC is by powering it with a noisy
power source. Like CMRR, the power supply rejection ratio (PSRR) is good at low frequencies such as 50 or 60 Hz.
However, the PSRR decreases as the frequency increases. The higher the noise frequency, the more likely it is that
it will degrade the performance of the ADC. Therefore, it is wise to provide some means of filtering as much noise
as possible from the power source at the VA+ and VA- pins. One approach is to use inductors or ferrite beads to
introduce an impedance at higher frequencies only. However, care should be taken when using inductors in series
with the power source because, in conjunction with the bypass capacitors, they can resonate. This will result in a
“ringing” effect whenever a transient edge excites the LC circuit at its resonate frequency. This can be minimized by
including a small series resistance to damp the resonance of the LC circuit by lowering its Q factor. Often the resis-
tance of the coil is adequate if the value of the inductance is sufficiently high.
If they are carefully chosen, ferrite beads can also be an effective means of reducing power supply noise. Unlike an
inductor, ferrite beads dissipate high-frequency energy rather than simply introducing a high impedance to it. Ferrite
beads will not resonate with the bypass capacitors. While a ferrite bead does have inductance, it can be thought of
as a frequency-dependent resistor and therefore has a very low Q factor. In selecting a ferrite bead to filter the power
supply, several parameters should be considered. First, the DC bias current characteristics must be determined.
Ferrite beads have a current rating which determines when the bead saturates. When a ferrite bead saturates, its
impedance drops off rapidly causing it to be ineffective at reducing high-frequency noise. Saturation is partially a
factor of the physical size of the bead. therefore, a larger ferrite bead may be more effective with higher currents.
The second characteristic that must be considered in selecting a ferrite bead is its effective frequency range. This
is determined by the composition of the ferrite alloy. For power supply filtering, the lowest-frequency compositions
should be the most effective. For example, the Fair-Rite
®
P/N 2773021447 surface mount bead has an impedance
of 28 ohms at 1 MHz (see
). Many beads, especially surface mount beads, offer very little if any impedance
at this low frequency. Since much of switch-mode power supply harmonics will be in this frequency range, this com-
ponent would be an effective means of attenuating much of that noise. This impedance, in combination with the low
impedance of an X7R ceramic capacitor will attenuate a large portion of the power supply noise with almost no DC
voltage drop. In a bipolar supply configuration, these capacitors should be between each supply pin and ground as
well as the capacitor from VA+ to VA- as mentioned earlier.