RF Quadrature Transceiver / RF Quadrature Receiver
CMX991/CMX992
4
External Components
4.1
Power Supply Decoupling
The CMX991/CMX992 has separate supply pins for the analogue and digital circuitry: a 3.3V nominal
supply is recommended for all circuits but a different voltage for VDDIO may be used (see section 5.6).
DVDD
DVDD
R5
VDDIO
R4
VCCRF
R3
VCCIF
R2
AVDD
VCCSYNTH
R1
C5
C4
C3
C2
C1
GND Plane for: AGND
GND for: DGND
GND
Figure 5 Recommended Power Supply Connections and Decoupling
C1 10nF
C2 10nF
C3 10nF
C4 10nF
C5 10nF
R1 3.3 Ω
R2 3.3 Ω
R3 3.3 Ω
R4 10 Ω
R5 10 Ω
Table 3 Decoupling Components
Notes:
1. Maximum Tolerances: Resistors 5%, capacitors 20% unless otherwise stated
2. It is expected that any low frequency interference on the 3.3 Volt supply will be removed by active
regulation; a large capacitor is an alternative but may require more board space and so may not
be preferred. It is particularly important to ensure that there is no interference from the VDDIO
(which supplies the digital I/O) or from any other circuit that may use the DVDD supply (such as a
microprocessor), to sensitive analogue supplies (AVDD). It is therefore advisable to use separate
power supplies for the digital and analogue circuitry.
3. The supply decoupling shown is intended for RF noise suppression. It is necessary to have a
small series impedance prior to the decoupling capacitor for the decoupling to work well; this may
be cost effectively done with the resistor and capacitor values shown. The use of resistors results
in small dc voltage drops (up to approx 0.1V). Choosing resistor values approximately inversely
proportional to the dc current requirements of each supply ensures the dc voltage drop on each
supply are reasonably matched. In any case, the dc voltage change that results is well within the
design tolerance of the device. If higher impedance resistors are used (not recommended) then
greater care will be needed to ensure the supply voltages are maintained within tolerance, even
when parts of the device are enabled or disabled.
4. It is advisable to have separate ground planes for the analogue and digital circuits.
2012 CML Microsystems Plc
10
D/991_992/18
CMLMICRO [ CML MICROCIRCUITS ]
