GC3011 Datasheet, PDF (10/35 Pages) Texas Instruments – DIGITAL RESAMPLER

GC3011 DIGITAL RESAMPLER
2.8.4 Selecting External Components for the PLL
The recommended analog supply filter and loop filter is shown in Figure 5.
VCC
AVCC
CVIN
GC3011
CVOUT
AGND
GND
LOOP FILTER
SUPPLY FILTER
Analog Vcc
47 Ohms
Control Voltage
R
10uF 0.1uF
C₂
C₁
Analog Gnd
47 Ohms
Digital V_CC
Digital GND
Figure 5. Supply and Loop Filters
All components shown in figure 5 should be placed as close as possible to the package. CVIN and
CVOUT should be shorted together. Separate CVIN and CVOUT pins are provided for specialized
applications that need an active loop filter.
The supply filter is used to isolate the analog power pins from the noise on the digital supply. This
filter will cause a slight drop in the analog VCC voltage. At 5 volts the drop is about 0.2 volts. At 3.3 volts
the drop is about 0.1 volts.
The PLL bandwidth and damping are set by the resistor R and the capacitors C₁ and C₂. The
resistor R sets the loop gain K (which should be 1-10% of F_OUT) using the formula:
Where the pump current Ip is:
and the VCO gain K₀ is:
R = -_K^2----π₀----_××-----_IK--_p--
Ip = 0.16 × (V-1) mA
K₀ = 0.785 F_out (at 5V) or K₀ = 1.047 F_out (at 3.3V)
(EQ 1)
The fastest response is achieved by using critical damping. Underdamping introduces oscillations
that can rapidly increase the response time. Overdamping reduces overshoot but gradually increases
response time. One should select a damping factor that minimizes response time without introducing
significant oscillations. Overdamping is preferred to underdamping. The damping factor should be between
0.7 and 1.4 (critical damping is 0.7). Let ζ be the damping factor.
C₁ = ^4----_K-^×-----^ζ×----^×-_R----^ζ-
(EQ 2)
GRAYCHIP,INC.
- 10 -
JULY 22, 1996
This document contains information which may be changed at any time without notice