AN-1296 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Optimizing Power Supplies for the AD9129 by Corey Birdsall INTRODUCTION improved close-in phase noise at 20 Hz offset by approximately 5 dB, as well as single-tone IMD by up to 5 dB. Most of the capacitors proved to be redundant; these are shown as Xs in Figure 1. The decoupling capacitors, C1 and C2, encircled in Figure 1, improved the ACLR for 6 MHz carriers by 5 dB. The capacitor arrays, CN1 and C25, also encircled in Figure 1, improved the ACLR for 6 MHz carriers by approximately 6 dB and the NSD by approximately 1 dB. The AD9129 evaluation board features filtering on the power supplies to guarantee optimal performance of the digital-toanalog converter (DAC). This application note explores the effects of removing most of the filtering components. All ferrite beads on the board were removed, as well as the majority of the capacitors on the power supplies. Phase noise, noise spectral density (NSD), spurious-free dynamic range (SFDR), intermodulation distortion (IMD), and adjacent channel leakage ratio (ACLR) performance were then all measured to demonstrate the effect of removing the filtering components. The measurement results showed that the ferrite beads Removing all of the capacitors crossed out with an X in Figure 1 did not affect the performance of the DAC. JP3 1 DVDD CN1 2 1 C25 2 3 4 3 4 3 4 3 4 5 6 5 6 5 6 5 6 7 7 8 47000pF GND 8 7 47000pF 8 7 VSSA 3 1 2 571-0500 GND P4 1 2 DVDD IN OUT GND OUT 1 4 C62 1µF C0603 8 47000pF GND 47000pF GND A2 ADP3339AKCZ-1.8-RL +5V P3 CN4 2 1 TP2 RED E3 1 JPR0805 AVDD CN3 2 1 1 P E4 1 2 C6 22µF C7343 N 2 C12 0.001µF C0402 C58 0.1µF C0603 C68 0.001µF C0402 C60 0.1µF C0603 P N GND 1 2 1 TP12 RED C50 0.1µF C0603 C53 0.1µF C0603 P C37 0.001µF C0402 N C38 P 4.7µF N C3528 U6 ADP3339AKCZ-1.8-RL 3 C39 47µF C7343 C61 0.1µF C0603 C40 0.001µF C0402 N C41 P 4.7µF N C3528 C42 47µF C7343 OUT GND C13 0.1µF C0603 C14 0.1µF C0603 P N 1 C19 22µF C7343 2 OUT IN 7 SD_N GND C21 3 1µF C0603 GND GND 2 DVDD C74 1µF C0603 C56 0.1µF C0402 C51 0.1µF C0402 GND E6 1 N TP21 BLK 1 GND 2 AVDD 115Ω C75 22µF C7343 P C77 0.1µF C0402 C76 0.1µF C0402 GND U4 C16 4.7µF C3528 TP20 RED 2 ADP3333ARMZ-2.5-RL VSSA TP9 BLK JPR0805 GND VDD_USB AVDD C59 0.1µF C0306 IN 1 JP6 2 GND OUT 1 4 C73 1µF C0603 GND P E1 1 1 GND C9 0.1µF C0402 C8 0.1µF C0402 115Ω GND AVDD C43 0.1µF C0306 1 JPR1206 571-0100 C104 47µF C7343 TP3 BLK GND JP1 GND GND N C45 P 4.7µF N C3528 VDD_USB 115Ω C17 1µF C0603 P C10 0.001µF C0402 2 115Ω 1 JP4 1 1 TP5 RED E9 2 1 JPR0805 115Ω C22 1µF C0603 GND P N C23 22µF C7343 GND 1 TP6 BLK C30 0.1µF C0402 2 VCC2.5 C31 0.1µF C0402 AVDD –5V 1 2 571-0500 C11 1µF C0603 E5 P5 U1 LM337IMP/NOPB 4 IN1 2 IN2 2 1 C18 C7343 22µF 115Ω N P C1 0.1µF C0402 C2 0.1µF C0402 N GND GND TP1 RED E2 1 2 GND 2 VSSA JPR0805 115Ω R1 120Ω C15 1µF C0603 C7343 N 22µF C20 P 1 P TP10 BLK C3 0.1µF C0402 C7 0.1µF C0402 AVDD GND GND 1 3 ADJ 1 C24 C7343 22µF 1 JP2 OUT R4 200Ω AVDD AVDD Figure 1. Capacitors Removed from the Power Supplies Rev. 0 | Page 1 of 3 12113-001 1 DVDD AVDD 2 AN-1296 Application Note Removal of Ferrite Beads –60 The E1 to E6 and E9 ferrite beads were all replaced with shorts. Because the E7 and E11 ferrite beads are parallel to the E8 and E10 ferrite beads, the E7 and E11 ferrite beads were deemed unnecessary and were removed. The E8 and E10 ferrite beads were left in place to ensure that the AVDD and the DVDD domains power up simultaneously. Inductors can be used in place of ferrite beads; however, one or the other is needed to give a dc short, while providing radio frequency (RF) isolation. Because AVDD and DVDD are at the same voltage, the same low dropout (LDO) regulator can power them; however, separate filtering networks and power planes are needed to ensure that the digital noise does not couple back onto the DAC output. Replacing the ferrite beads with shorts degraded the close-in phase noise performance slightly, as shown in Figure 2. This measurement was taken with a maximum full-scale current of 33 mA and no digital backoff. The phase noise was measured across multiple clock frequencies, at a low output frequency of 51 MHz, and a high output frequency of 991 MHz. The phase noise degraded by approximately 5 dB within 20 Hz of the 991 MHz signal when the ferrite beads were removed. Otherwise, performance with and without the ferrite beads was similar. fS =1800MHz fS = 2200MHz WORST IMD3 (dBc) –65 FERRITES: FALSE FERRITES: TRUE –70 –75 –80 –90 0 400M 600M 800M FREQUENCY (Hz) 1.0G 1.2G Figure 3. IMD With and Without Ferrite Beads Removal of C1 and C2 The removal of C1 and C2 degraded the ACLR performance with 6 MHz carriers by about 5 dB, as shown in Figure 4. The ACLR performance was measured with an eight carrier, 256 quadrature amplitude modulation (QAM) signal, full-scale current of 33 mA, no digital backoff, FIR40, and a clock rate of 2.3 GHz. The performance of the 5.25 MHz bandwidth channel on both sides of the eight carriers is plotted in Figure 4 and Figure 5. –70 –56 fOUT = 991MHz fS = 2305MHz –58 –80 –90 fS = 2000MHz fS = 2200MHz fS = 2600MHz fS = 2800MHz fOUT = 51MHz FERRITES: FALSE FERRITES: TRUE –100 C1, C2: TRUE C1, C2: FALSE –60 LOWER 5.25MHz (dBc) PHASE NOISE (dBc/Hz) 200M 12113-003 –85 –110 –120 –62 –64 –66 –68 –70 –130 10 100 FREQUENCY (Hz) 1k 10k –74 0 200M 400M 600M OUTPUT FREQUENCY (Hz) 800M 1.0G Figure 4. ACLR Without C1 and C2, Lower 5.25 MHz Figure 2. Phase Noise With and Without Ferrite Beads –56 Replacing the ferrite beads with shorts degraded single-tone IMD by up to 5 dB. The IMD is shown at two different clock rates, with and without the ferrite beads, in Figure 3. This measurement was taken with a full-scale current of 28 mA and no digital backoff. fS = 2305MHz –58 C1, C2: TRUE C1, C2: FALSE UPPER 5.25MHz (dBc) –60 –62 –64 –66 –68 –70 –72 –74 0 200M 400M 600M OUTPUT FREQUENCY (Hz) 800M 1.0G Figure 5. ACLR Without C1 and C2, Upper 5.25 MHz Channel Rev. 0 | Page 2 of 3 12113-005 1 12113-002 –140 12113-004 –72 Application Note AN-1296 –62 Removal of Capacitor Arrays fS = 2305MHz Removing capacitor arrays, CN1 and C25, degraded the NSD performance by about 1 dB, except at lower output frequencies, as shown in Figure 6. The NSD was tested with a band-pass filter centered at 70 MHz. Figure 6 shows the performance at a full-scale current of 28 mA, no digital backoff, and a clock rate of both 2200 MHz and 2800 MHz. The performance was also verified to be similar at other clock rates. CAPACITOR ARRAYS: FALSE CAPACITOR ARRAYS: TRUE LOWER 2.5MHz (dBc) –64 –154 –66 –68 –70 –74 –158 200M 400M 600M 800M 1.0G OUTPUT FREQUENCY (Hz) Figure 7. ACLR Without Capacitor Arrays, Lower 5.25 MHz Channel –162 –60 –164 –62 fS = 2305MHz CAPACITOR ARRAYS: TRUE CAPACITOR ARRAYS: FALSE –168 0 200M 400M 600M 800M 1.0G OUTPUT FREQUENCY (Hz) 1.2G 1.4G Figure 6. NSD Without Capacitor Arrays The removal of the capacitor arrays degraded the ACLR performance with 6 MHz, 256 QAM carriers by about 6 dB, as shown in Figure 7 and Figure 8. The ACLR performance was measured with an eight carrier, 256 QAM signal, full-scale current of 33 mA, no digital backoff, FIR40, and a clock rate of 2.3 GHz. The performance of the 5.25 MHz bandwidth channel on both sides of the eight carriers is plotted in Figure 7 and Figure 8. CAPACITOR ARRAYS: FALSE CAPACITOR ARRAYS: TRUE –64 –66 –68 –70 –72 –74 0 200M 400M 600M 800M 1.0G OUTPUT FREQUENCY (Hz) 12113-008 –166 UPPER 5.25MHz (dBc) fS = 2200MHz fS = 2800MHz 12113-006 NSD (dBm/Hz) 0 –160 12113-007 –72 –156 Figure 8. ACLR Without Capacitor Arrays, Upper 5.25 MHz Channel Conclusion Removing the ferrite beads degraded the phase noise by about 5 dB, and the IMD by about 5 dB. Most of the decoupling capacitors on the AD9129 evaluation board did not demonstrate any noticeable contribution to performance and, therefore, it was concluded that they were not needed. Removing the decoupling capacitors on the negative supply, C1 and C2, degraded the ACLR by about 5 dB. Removing the capacitor arrays, CN1 and C25, degraded the NSD by about 1 dB, and the ACLR by about 6 dB. ©2014 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN12133-0-6/14(0) Rev. 0 | Page 3 of 3