User's Manual

VZ22Q LTE Module
V0.1
USI
Confidential Page 18 of 23
5.3 Decoupling and Bypassing Capacitors
Bypass capacitors on the PCB are used to short digital noise into ground. Commonly,
codec generates noise when its internal digital circuitry turns currents on and off. These current
changes arise in the power and ground pins for the related section of the codec. The goal is to
force AC currents to flow in the shortest possible loop from the supply pin through the bypass
cap and back into the codec through the nearby ground pin. A bypassing circuit is supposed to be
a low lead inductance between the codec and the bypass capacitors when in the operating
frequency of the codec. The trace is longer, and the inductance is larger. To avoid long-trace
inductance effects, use the shortest possible traces for bypass capacitors, with wide traces to
reduce impedance. For best performance, use supply bypass leads of less than one-half inch.
In Table 5-1, it shows the priorities of series codec capacitor placemen.
i. Pins with a first – “A” priority components placed around the codec are the bypass caps,
which are located as close as possible to the power supply pins. The capacitors must have low
inductance and low equivalent series resistance (ESR). Tantalum 10μF surface mount devices
are good if they are used in conjunction with 0.1μF ceramics.
ii. The filter capacitors with “B” priority stabilize the reference voltage for internal Ops should
be placed close to codec. A good reference voltage is relative to good analog performance.
iii. These decoupling capacitors (“C” priority) should be close to the codec pins, or positioned
for the shortest connections to pins, with wide traces to reduce impedance. The Table 1 also
point out the distribution of codec capacitor locations and placement priorities.
Signal Description Package
Pins
Priority of Close Proximity to Codec Pin Placement
of Filter and Decoupling Capacitors
Digital Supply Voltage , +3.3DVDD 13,14 A
Analog Supply Voltage, +3.3AVDD 31 A
Speaker amp SPKVDD , +5V 26 A
Voltage Reference Filter (V
REF
) 27 B
Analog Signal Inputs & Output (Decouple) 1~6,19~25 C
Table 5-1. Series codec capacitor placement priorities
5.4 The Trace Routing
5.4.1 Power Input Trace
For the better loading, the power trace width is better to correspond to the suggestion as below.
i. AVDD, DCVDD, DBVDD≧15mils
5.4.2 Analog Input & Output Signal Trace
For reaching the best audio quality, some guideline should be obeyed.