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FEATURES Low Cost, 3.3 V-CMOS, Mixed Signal, Front End Converter for Broadband Modems 10-Bit D/A Converter (TxDAC+(R)) 50 MSPS Input Word Rate 2 Interpolating Low-Pass Transmit Filter 100 MSPS DAC Output Update Rate Wide (21 MHz) Transmit Bandwidth Power-Down Modes 10-Bit, 50 MSPS A/D Converter Fourth Order LPF with Selectable Cutoff Frequency Dual Mode Programmable Gain Amplifier Internal Clock Multiplier (PLL) Two Auxiliary Clock Outputs 48-Lead LQFP Package APPLICATIONS Powerline Networking Home Phone Networking
TXEN RXEN TXCLK RXCLK
Broadband Modem Mixed-Signal Front End AD9975
FUNCTIONAL BLOCK DIAGRAM
AD9975
10 K 10 TX+ TxDAC+ TX-
CLK1 ADIO[9:0] CLK-GEN CLK2 OSCIN XTAL
10
ADC
PGA
LPF
PGA
RX+ RX-
AGC[2:0]
3 3
SPORT
REGISTER CONTROL
GENERAL DESCRIPTION
The AD9975 is a single-supply, broadband modem, mixed signal, front end (MxFETM) IC. The device contains a transmit path interpolation filter and DAC and a receive path PGA, LPF, and ADC required for a variety of broadband modem applications. Also on-chip is a PLL clock multiplier that provides all required clocks from a single crystal or clock input. The TxDAC+ uses a digital 2x interpolation low-pass filter to oversample the transmit data and ease the complexity of analog reconstruction filtering. The transmit path bandwidth is 21 MHz when sampled at 100 MSPS. The 10-bit DAC provides differential current outputs. The DAC full-scale current can be adjusted from 2 to 20 mA by a single resistor, providing 20 dB of additional gain range. The receive path consists of a PGA, LPF, and ADC. The programmable gain amplifier (PGA) has two modes of operation. One mode allows programming through the serial port and provides a gain range from -6 dB to +36 dB in 2 dB steps. The other mode allows the gain to be controlled through an asynchronous 3-pin port and offers a gain range from 0 dB to 48 dB in 8 dB steps with the use of an external gain stage. The receive path LPF cutoff frequency can be selected to either 12 MHz or 26 MHz.
TxDAC+ is a registered trademark and MxFE is a trademark of Analog Devices, Inc.
The filter cutoff frequency can also be tuned or bypassed where filter requirements differ. The 10-bit ADC uses a multistage differential pipeline architecture to achieve excellent dynamic performance with low power consumption. The digital transmit and receive ports are multiplexed onto a 10-bit databus and have individual TX/RX clocks and TX/RX enable lines. This interface connects directly to Homelug 1.0 PHY/MAC chips from Intellon and Conexant. The AD9975 is available in a space-saving 48-lead LQFP package. The device is specified over the commercial (-40C to +85C) temperature range.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2002
AD9975-SPECIFICATIONS
Parameter OSC IN CHARACTERISTICS Frequency Range Duty Cycle Input Capacitance Input Impedance CLOCK OUTPUT CHARACTERISTICS CLKA Jitter (FCLKA Derived from PLL) CLKA Duty Cycle TX CHARACTERISTICS 2x Interpolation Filter Characteristics TX Path Latency, 2x Interpolation Pass-Band Flatness 0 MHz to 20.7 MHz Stop-Band Rejection @ 29.3 MHz TxDAC Resolution Conversion Rate Full-Scale Output Current Voltage Compliance Range (TX+ or TX- AVSS) Gain Error Output Offset Differential Nonlinearity Integral Nonlinearity Output Capacitance Phase Noise @ 1 kHz Offset, 10 MHz Signal Signal-to-Noise and Distortion (SINAD) 5 MHz Analog Out (20 MHz BW) Wideband SFDR (to Nyquist, 50 MHz max) 5 MHz Analog Out Narrowband SFDR (3 MHz Window) 5 MHz Analog Out IMD (f1 = 6.25 MHz, f2 = 7.8125 MHz) RX PATH CHARACTERISTICS (LFP Bypassed) Resolution Conversion Rate Pipeline Delay, ADC Clock Cycles Dynamic Performance (AIN = -0.5 dBFS, f = 5 MHz) @ FOSCIN = 50 MHz, RX LPF Bypassed Signal-to-Noise and Distortion Ratio (SINAD) Effective Number of Bits (ENOB) Signal-to-Noise Ratio (SNR) Total Harmonic Distortion (THD) Spurious-Free Dynamic Range (SFDR) RX PATH GAIN/OFFSET Minimum Programmable Gain Maximum Programmable Gain Narrow Band Rx LPF or Rx LPF Bypassed
Wideband Rx LPF
(VS = 3.3 V 10%, FOSCIN = 50 MHz, FDAC = 100 MHz, Gain = -6 dB, RSET = 4.02 k , 100 DAC Load.)
Temp Full 25C 25C 25C 25C 25C Test Level I II III III II III Min 10 40 Typ Max 50 60 Unit MHz % pF M ps rms %
50 3 100 14 50 5
Full Full Full Full Full Full Full 25C 25C 25C 25C 25C 25C 25C 25C 25C 25C 25C N/A Full N/A Full Full Full Full Full 25C 25C 25C 25C 25C Full Full Full 25C 25C 25C 25C 25C 25C
II II II II II II II II II III II III III II III III III III N/A II N/A III III III III III I I I I II II II III III III III III III III -2- 10 2 -0.5 -5.5 0
30 0.8 35 10 10 2 2 0.5 5 -100 -60.6 -76.2 -77.9 -77 10 10 5.5 -56.6 9.1 -59.2 -60.1 -66 -6 +36 +30 2 0.4 1.0 0.8 4 4 270 50 16 684 40 50 100 20 +1.5 +5.0 TBD 1
FDAC Cycles dB dB Bits MHz mA V %FS A LSB LSB pF dBc/Hz dB dBc dBc dBFS Bits MHz Cycles dB Bits dB dB dB dB dB dB dB dB dB dB Vppd pF MHz V rms V rms dB REV. 0
Gain Step Size Gain Step Accuracy Gain Range Error Absolute Gain Error, PGA Gain = 0 dB RX PATH INPUT CHARACTERISTICS Input Voltage Range (Gain = -6 dB) Input Capacitance Differential Input Resistance Input Bandwidth (-3 dB) (Rx LPF Bypassed) Input Referred Noise (at +36 dB Gain with Filter) Input Referred Noise (at -6 dB Gain with Filter) Common-Mode Rejection
AD9975
Parameter RX PATH LPF (Low Cutoff Frequency) Cutoff Frequency Cutoff Frequency Variation Attenuation @ 22 MHz Pass-Band Ripple Group Delay Variation Settling Time (to 1% FS, Min to Max Gain Change) Total Harmonic Distortion at Max Gain (THD) RX PATH LPF (High Cutoff Frequency) Cutoff Frequency Cutoff Frequency Variation Attenuation @ 35 MHz Pass-Band Ripple Group Delay Variation Settling Time (to 1% FS, Min to Max Gain Change) Total Harmonic Distortion at Max Gain (THD) RX PATH DIGITAL HPF Latency (ADC Clock Source Cycles) Roll-Off in Stop Band -3 dB Frequency POWER-DOWN/DISABLE TIMING Power-Down Delay (Active-to-Power-Down) DAC Interpolator Power-Up Delay (Power-Down-to-Active) DAC PLL ADC PGA LPF Interpolator Minimum RESET Pulsewidth Low (tRL) ADIO PORT INTERFACE Maximum Input Word Rate TX-Data Setup Time (tSU) TX-Hold Hold Time (tHD) RX-Data Valid Time(tVT) RX-Data Hold Time (tHT) Temp Full Full Full Full Full 25C Full Full Full Full Full Full 25C Full Test Level III III III II II II I III III III II II II I Min Typ 12 7 20 1.0 30 150 -61 26 7 20 1.2 15 80 -61 1 6 fADC /400 Max Unit MHz % dB dB ns ns dBc MHz % dB dB ns ns dBc Cycle dB/Octave Hz
25C 25C 25C 25C 25C 25C 25C 25C Full 25C 25C 25C 25C 25C
II II II II II II II II III I II II II II 100 3.0 0
200 200 10 10 1000 1 1 200 5
ns ns s s s s s ns fOSCIN Cycles MHz ns ns ns ns
3.0 1.5
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AD9975 SPECIFICATIONS (continued)
Parameter SERIAL CONTROL BUS Maximum SCLK Frequency (fSCLK) Clock Pulsewidth High (tPWH) Clock Pulsewidth Low (tPWL) Clock Rise/Fall Time Data/Chip-Select Setup Time (tDS) Data Hold Time (tDH) Data Valid Time (tDV) CMOS LOGIC INPUTS Logic "1" Voltage Logic "0" Voltage Logic "1" Current Logic "0" Current Input Capacitance CMOS LOGIC OUTPUTS (1 mA Load) Logic "1" Voltage Logic "0" Voltage Digital Output Rise/Fall Time POWER SUPPLY All Blocks Powered Up IS_TOTAL (Total Supply Current) Digital Supply Current (IDRVDD + IDVDD) Clock Supply Current (ICLKVDD) Analog Supply Current (IAVDD) Power Consumption of Functional Blocks Rx LPF ADC and SPGA Rx Reference Interpolator DAC PLL-A All Blocks Powered Down IS_TOTAL (Total Supply Current) Digital Supply Current (IDRVDD + IDVDD) Clock Supply Current (ICLKVDD) Analog Supply Current (IAVDD)
Specifications subject to change without notice.
Temp Full Full Full Full Full Full Full 25C 25C 25C 25C 25C Full 25C Full
Test Level II II II II II II II II II II II III II II II
Min 25 18 18
Typ
Max
Unit MHz ns ns s ns ns ns V V A A pF V V ns
10 25 0 20
VDRVDD - 0.7
0.4 12 12 3
VDRVDD - 0.6
1.5
0.4 2.5
25C 25C 25C 25C 25C 25C 25C 25C 25C 25C 25C 25C 25C 25C
I III III III III III III III III III I III III III
210 22.5 5.5 182 110 55 2 20 18 22 21 10 0 11
227
mA mA mA mA mA mA mA mA mA
27
mA mA mA mA
ABSOLUTE MAXIMUM RATINGS*
EXPLANATION OF TEST LEVELS
Power Supply (VS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 V Digital Output Current . . . . . . . . . . . . . . . . . . . . . . . . . . 5 mA Digital Inputs . . . . . . . . . . . . . . . . -0.3 V to DRVDD + 0.3 V Analog Inputs . . . . . . . . . . . . . . . . . -0.3 V to AVDD + 0.3 V Operating Temperature . . . . . . . . . . . . . . . . . . -40C to +85C Maximum Junction Temperature . . . . . . . . . . . . . . . . . 150C Storage Temperature . . . . . . . . . . . . . . . . . . -65C to +150C Lead Temperature (Soldering 10 sec) . . . . . . . . . . . . . . 300C
*Absolute Maximum Ratings are limiting values to be applied individually and beyond which the serviceability of the circuit may be impaired. Functional operability under any of these conditions is not necessarily implied. Exposure to absolute maximum rating conditions for extended periods of time may affect device reliability.
I. Devices are 100% production tested at 25C and guaranteed by design and characterization testing for the commercial operating temperature range (-40C to +85C). II. Parameter is guaranteed by design and/or characterization testing. III. Parameter is a typical value only.
THERMAL CHARACTERISTICS Thermal Resistance
48-Lead LQFP JA = 57C/W JC = 28C/W
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AD9975
ORDERING GUIDE
Model AD9975ABST AD9975ABSTEB AD9975ABSTRL
Temperature Range -40C to +85C -40C to +85C -40C to +85C
Package Description 48-Lead LQFP AD9975 EVAL Board AD9975ABST Reel
Package Option ST-48
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD9975 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
PIN FUNCTION DESCRIPTION
PIN CONFIGURATION
Pin No. 1 2 3 4 5, 38, 47 6, 9, 39, 42, 43, 46 7 8 10 11 12 13 14 15-17 18 19-28 29 30 31 32 33 34 35 36 37 40, 41 44 45 48
Mnemonic OSC IN SENABLE SCLK SDATA AVDD AVSS Tx+ Tx- FS ADJ
Function Crystal Oscillator Inverter Input Serial Bus Enable Input Serial Bus Clock Input Serial Bus Data I/O Analog 3.3 V Power Supply Analog Ground
37 RESET 40 REFB 47 AVDD 38 AVDD 41 REFT 46 AVSS 43 AVSS 42 AVSS 39 AVSS 48 XTAL 44 RX+ 45 RX-
OSC IN 1 SENABLE 2 SCLK 3 SDATA 4 AVDD 5
36 DRVSS 35 DRVDD 34 RXBOOST/SDO 33 CLKOUT2
Transmit DAC + Output Transmit DAC - Output DAC Full-Scale Output Current Adjust with External Resistor REFIO DAC Band Gap Decoupling Node CLKVDD Power Supply for CLKOUT1 DVSS Digital Ground DVDD Digital 3.3 V Power Supply AGC[2:0] AGC Control Inputs CLKOUT1 Auxiliary Clock Output ADIO[9:0] Digital Data I/O Port RXEN ADIO Direction Control Input TXEN TX Path Enable TXCLK ADIO Sample Clock Input RXCLK ADIO Request Clock Input CLKOUT2 Auxiliary Clock Output RXBOOST/ External Gain Control Output/ SDO Serial Data Output DRVDD Digital I/O 3.3 V Power Supply DRVSS Digital I/O Ground RESET Reset Input REFB, REFT ADC Reference Decoupling Node Rx+ Receive Path + Input Rx- Receive Path - Input XTAL Crystal Oscillator Inverter Output
AD9975
48-PIN LQFP TOP VIEW (Not to Scale)
32 RXCLK 31 TXCLK 30 TXEN 29 RXEN 28 ADIO0 27 ADIO1 26 ADIO2 25 ADIO3
AVSS 6 TX+ 7 TX- 8 AVSS 9 FS ADJ 10 REFIO 11 CLKVDD 12
DVSS 13 DVDD 14 AGC2 15 AGC1 16
AGC0 17
CLKOUT1 18
ADIO9 19
ADIO8 20
ADIO7 21
ADIO6 22
ADIO5 23
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ADIO4 24
AD9975
DEFINITIONS OF SPECIFICATIONS Clock Jitter Offset Error
The clock jitter is a measure of the intrinsic jitter of the PLL generated clocks. It is a measure of the jitter from one rising edge of the clock with respect to another edge of the clock nine cycles later.
Differential Nonlinearity Error (DNL, No Missing Codes)
First transition should occur for an analog value 1/2 LSB above negative full scale. Offset error is defined as the deviation of the actual transition from that point.
Gain Error
An ideal converter exhibits code transitions that are exactly 1 LSB apart. DNL is the deviation from this ideal value. Guaranteed no missing codes to 10-bit resolution indicate that all 1024 codes, respectively, must be present over all operating ranges.
Integral Nonlinearity Error (INL)
The first code transition should occur at an analog value 1/2 LSB above negative full scale. The last transition should occur for an analog value 1 1/2 LSB below the nominal full scale. Gain error is the deviation of the actual difference between first and last code transitions and the ideal difference between first and last code transitions.
Input Referred Noise
Linearity error refers to the deviation of each individual code from a line drawn from "negative full scale" through "positive full scale." The point used as negative full scale occurs 1/2 LSB before the first code transition. Positive full scale is defined as a level 1 1/2 LSB beyond the last code transition. The deviation is measured from the middle of each particular code to the true straight line.
Phase Noise
The rms output noise is measured using histogram techniques. The ADC output code's standard deviation is calculated in LSB and converted to an equivalent voltage. This results in a noise figure that can directly be referred to the RX input of the AD9975.
Signal-to-Noise and Distortion Ratio (SINAD)
Single-sideband phase noise power density is specified relative to the carrier (dBc/Hz) at a given frequency offset (1 kHz) from the carrier. Phase noise can be measured directly on a generated single tone with a spectrum analyzer that supports noise marker measurements. It detects the relative power between the carrier and the offset (1 kHz) sideband noise and takes the resolution bandwidth (rbw) into account by subtracting 10 log(rbw). It also adds a correction factor that compensates for the implementation of the resolution bandwidth, log display, and detector characteristic.
Output Compliance Range
SINAD is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components below the Nyquist frequency, including harmonics but excluding dc. The value for SINAD is expressed in decibels.
Effective Number of Bits (ENOB)
For a sine wave, SINAD can be expressed in terms of the number of bits. Using the following formula, N = ( SINAD - 1.76 ) dB / 6.02 it is possible to get a measure of performance expressed as N, the effective number of bits.
Signal-to-Noise Ratio (SNR)
The range of allowable voltage at the output of a current-output DAC. Operation beyond the maximum compliance limits may cause either output stage saturation, resulting in nonlinear performance, or breakdown.
Spurious-Free Dynamic Range (SFDR)
SNR is the ratio of the rms value of the measured input signal to the rms sum of all other spectral components below the Nyquist frequency, excluding harmonics and dc. The value for SNR is expressed in decibels.
Total Harmonic Distortion (THD)
The difference, in dB, between the rms amplitude of the DAC's output signal (or ADC's input signal) and the peak spurious signal over the specified bandwidth (Nyquist bandwidth, unless otherwise noted).
Pipeline Delay (Latency)
THD is the ratio of the rms sum of the first six harmonic components to the rms value of the measured input signal and is expressed as a percentage or in decibels.
Power Supply Rejection
The number of clock cycles between conversion initiation and the associated output data being made available.
Power supply rejection specifies the converter's maximum full-scale change when the supplies are varied from nominal to minimum and maximum specified voltages.
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Typical Performance Characteristics-AD9975
10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 0 0.1 0.2 0.3 0.4 0.5 0.6 FSAMPLE 0.7 0.8 0.9 1.0 INCLUDING SIN(X)/X INTERPOLATION FILTER
0 -10 -20
MAGNITUDE - dBc
-30 -40 -50 -60 -70 -80 -90 -100
0 5 10 15 20 25 30 35 40 45
MAGNITUDE - dB
FREQUENCY - MHz
TPC 1. 2 x Low-Pass Interpolation Filter
TPC 4. Single Tone Spectral Plot @ fDATA = 50 MSPS, fOUT = 11 MHz, 2 x LPF
10 INTERPOLATION FILTER 0 -10 -20 INCLUDING SIN(X)/X
10 0 -10 -20 -30
MAGNITUDE - dB
-30
dBc
-40 -50 -60 -70 -80 -90 -100 0 0.1 0.2 0.3 0.4 0.6 0.5 FS - MHz 0.7 0.8 0.9 1.0
-40 -50 -60 -70 -80 -90 -100 10.0 1.02 10.4 10.6 10.8 11.0 11.2 11.4 FREQUENCY - MHz 11.6 11.8 12.0
TPC 2. 2 x Band-Pass Interpolation Filter, FS /2 Modulation, Adjacent Image Preserved
TPC 5. Dual Tone Spectral Plot @ fDATA = 50 MSPS, fOUT = 6.7 MHz and 7.3 MHz, 2 x LPF
0 -10 -20
-70 -72 -74
MAGNITUDE - dBc
THIRD ORDER HARMONIC
MAGNITUDE - dBc
-30 -40 -50 -60 -70 -80 -90 -100
0 5 10 15 20 25 30 35 40 45
-76 -78 -80 -82 -84 -86 -88 -90 SECOND ORDER HARMONIC
FREQUENCY - MHz
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 FREQUENCY - MHz
TPC 3. Single Tone Spectral Plot @ fDATA = 50 MSPS, fOUT = 5 MHz, 2 x LPF
TPC 6. Harmonic Distortion vs. fOUT @fDATA = 50 MSPS
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AD9975
10 0 -10 -20 35
FREQUENCY - MHz
-30
30
dBc
-40 -50 -60 -70 -80 -90 -100
10.003 10.004 10.005 10.006 10.007 10.008 10.009 10.010 10.011 10.012 10.013
25
20 192
208
FREQUENCY - MHz
224 240 TUNING TARGET - Decimal
255
TPC 7. Phase Noise Plot @fDATA 50 MSPS, fOUT = 10 MHz, 2 x LPF
TPC 10. FC vs. Tuning Target, FADC = 50 MHz, LPF = Wideband Rx Filter
10 0
20
18
-10 -20
FREQUENCY - MHz
16
dBc
-30 -40 -50
14
12
-60 -70 3 5 7 9 11 13 15 17 FREQUENCY - MHz 19 21 23
10 192
208
224 240 TUNING TARGET - Decimal
255
TPC 8. "In-Band" Multitone Spectral Plot @fDATA = 50 MSPS, fOUT = k x 195 kHz, 2 x LPF
TPC 11. FC vs. Tuning Target, FADC = 50 MHz, LPF = Narrowband Rx Filter
10
0.60
0
0.40
-10 -20
dBc
GAIN ACCURACY - dB
0.20
-30 -40 -50 -60
0
-0.20
-0.40
-0.60
-70 1 11 21 31 41 51 61 71 FREQUENCY - MHz 81 91 101
-0.80 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 VGA GAIN - dB
TPC 9. "Wide-Band" Multitone Spectral Plot @fDATA = 50 MSPS, fOUT = k x 195 kHz, 2 x LPF
TPC 12. PGA Gain Error vs. Gain
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AD9975
2.5 2.4 0 2.3 2.2
OFFSET - LSBs
LOG MAG
5dB/Div
REF 0dB
-3.0dB 24.1MHz
2.1 2.0 1.9 1.8 1.7 1.6 1.5 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 VGA GAIN - dB
1MHz
10MHz
100MHz
TPC 13. PGA Gain Step vs. Gain
TPC 16. Rx LPF Group Delay, LPF = Narrowband Rx Filter, FADC = 50 MHz, Tuning Target = 255
DELAY 5 ns/Div REF 0s 33.9ns 22.1MHz 0
LOG MAG
5dB/Div REF 0dB
-3.0dB 12.1MHz
0
1MHz
10MHz
100MHz
1MHz
10MHz
100MHz
TPC 14. Rx LPF Frequency Response, LPF = Narrowband Rx Filter, FADC = 50 MHz, Tuning Target = 255
TPC 17. Rx LPF Group Delay, LPF = Wideband Rx Filter, FADC = 50 MHz, Tuning Target = 255
DELAY
10ns/Div
REF 0s
70.1ns 9.9MHz
LOG MAG 123.5 kHz
5dB/Div
REF 0dB 0
-3.00dB
0
1MHz
10MHz
100MHz
10kHz
100kHz
1MHz
TPC 15. Rx LPF Frequency Response, LPF = Wideband Rx Filter, FADC = 50 MHz, Tuning Target = 255
TPC 18. Rx HPF Frequency Response, FADC = 50 MHz
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AD9975
4000 3800 59 3600
ADC OUTPUT CODE
60
3400 3200 3000 2800
MAGNITUDE - dB
58
57
56 2600 2400 1 5 9 13 17 21 25 29 ADC CLOCK CYCLES 33 37 55 20 25 30 35 FS - MHz 40 45 50
TPC 19. Rx Path Settling, 1/2 Scale Rising Step with Gain Change, LPF FC = 26 MHz, FADC = 50 MHz
TPC 22. Rx Path SNR vs. FADC, FIN = 5 MHz, Gain = -6 dB, Rx LPF Bypassed
4000 3800 3600
ADC OUTPUT CODE
-55
3400 3200 3000 2800 2600 2400 1 5 9 13 17 21 25 29 ADC CLOCK CYCLES 33 37
MAGNITUDE - dB
-60
-65 20 25 30 35 FS - MHz 40 45 50
TPC 20. Rx Path Settling, 1/2 Scale Falling Step with Gain Change, LPF FC = 26 MHz, FADC = 50 MHz
TPC 23. Rx Path THD vs. FADC, FIN = 5 MHz, Gain = -6 dB, Rx LPF Bypassed
9.50
9.5
9.3
9.25
9.1
ENOBs
9.00
ENOBs
8.9 8.7 8.5
8.75
8.50 20 25 30 35 FS - MHz 40 45 50
0
2
4
6
8
10 12 FIN - MHz
14
16
18
20
TPC 21. Rx Path ENOB vs. FADC, FIN = 5 MHz, Gain = -6 dB, Rx LPF Bypassed
TPC 24. Rx Path ENOB vs. FIN, FADC = 50 MHz, Gain = -6 dB, Rx LPF Bypassed
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AD9975
58 57 -55 56
MAGNITUDE - dB
-50
55 54 53 52
MAGNITUDE - dB
-60
-65
-70 51 50 0 2 4 6 8 10 12 FIN - MHz 14 16 18 20 -75 0 2 4 6 8 10 12 FIN - MHz 14 16 18 20
TPC 25. Rx Path SNR vs. FIN, FADC = 50 MHz, Gain = -6 dB, Rx LPF Bypassed
TPC 26. Rx Path THD vs. FIN, FADC = 50 MHz, Gain = -6 dB, Rx LPF Bypassed
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AD9975
TRANSMIT PATH
The AD9975 transmit path consists of a digital interface port, a bypassable 2x interpolation filter, and a transmit DAC. The clock signals required by these blocks are generated by the internal PLL. The block diagram below shows the interconnection between the major functional components of the transmit path.
INTERPOLATION FILTER
of L is programmed through the serial interface port and can be set to 1, 2, 4, or 8. The transmit path expects a new input sample at the ADIO interface at a rate of fDAC/2 if the interpolation filter is being used. If the interpolation filter is bypassed, the transmit path expects a new input sample at the ADIO interface at a rate of fDAC.
D/A CONVERTER
The interpolation filter can be programmed to run at a 2x upsampling ratio in either a low-pass filter or band-pass filter mode. The transfer functions of these two modes are shown in TPC 1 and TPC 2, respectively. The y-axes of the figures show the magnitude response of the filters in dB, and the x-axes show the frequency normalized to FDAC. The top trace of the plot shows the discrete time transfer function of the interpolation filter. The bottom trace shows the TX path transfer function including the sin(x)/x transfer function of the DAC. In addition to the two upsampling modes, the interpolation filter can be programmed into a pass-through mode if no interpolation filtering is desired. The table below shows the following parameters as a function of the mode in which it is programmed. Latency - The number of clock cycles from the time a digital impulse is written to the DAC until the peak value is output at the TX+ and TX- Pins. Flush - The number of clock cycles from the time a digital impulse is written to the DAC until the output at the TX+ and TX- Pins settles to zero. FPASS - The frequency band over which the pass-band ripple is less than the stated magnitude (i.e., 0.1 dB or 1.0 dB). FSTOP - The frequency band over which the stop-band attenuation is greater than the stated magnitude (i.e., 40 dB or 50 dB).
Table I. Interpolation Filters vs. Mode
The AD9975 DAC provides differential output current on the TX+ and TX- pins. The values of the output currents are complementary, meaning they will always sum to IFS, the full-scale current of the DAC. For example, when the current from TX+ is at full scale, the current from TX- is zero. The two currents will typically drive a resistive load that will convert the output currents to a voltage. The TX+ and TX- output currents are inherently ground seeking and should each be connected to matching resistors, RL, that are tied directly to AGND. The full-scale output current of the DAC is set by the value of the resistor placed from the FS ADJ pin to AGND. The relationship between the resistor, RSET, and the full-scale output current is governed by the following equation:
IFS = 39.4 / RSET
The full-scale current can be set from 2 to 20 mA. Generally, there is a trade-off between DAC performance and power consumption. The best DAC performance will be realized at an IFS of 20 mA. However, the value of IFS adds directly to the overall current consumption of the device. The single-ended voltage outputs appearing at the TX+ and TX- nodes are:
VTX + = ITX + x RL VTX - = ITX - x RL
Register 7[7:4] Mode Latency, FDAC Clock Cycles Flush, FDAC Clock Cycles FPASS, 0.1 dB FPASS, 1.0 dB FSTOP, 40 dB FSTOP, 50 dB
0x1 2x LPF 30 48 <0.204 <0.207 <0.296 <0.302
0x5 2x BPF, Adj. Image 30 48 >0.296, <0.704 >0.293, <0.707 >0.204, <0.796 <0.198, >0.802
Note that the full-scale voltage of VTX+ and VTX- should not exceed the maximum output compliance range of 1.5 V to prevent signal compression. To maintain optimum distortion and linearity performance, the maximum voltages at VTX+ and VTX- should not exceed 0.5 V. The single-ended full-scale voltage at either output node will be:
VFS = IFS x RL
The differential voltage, VDIFF, appearing across VTX+ and VTX- is:
VDIFF = ( ITX + - ITX - ) x RL
and
VDIFF _ FS = IFS x RL
DPLL-A CLOCK DISTRIBUTION
Figure 1 shows the clock signals used in the transmit path. The DAC sampling clock, fDAC, is generated by DPLL-A. fDAC has a frequency equal to L x fOSCIN, where L is the PLL clock multiplier value and fOSCIN is the frequency of the input to PLL-A. The value
It should be noted that the differential output impedance of the DAC is 2 x RL and any load connected across the two output resistors will load down the output voltage accordingly.
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AD9975
RECEIVE PATH DESCRIPTION
The receive path consists of a two stage PGA, a continuous time, 4-pole LPF, an ADC, and a digital HPF. Also working in conjunction with the receive path is an offset correction circuit and a digital phase-locked loop. Each of these blocks will be discussed in detail in the following sections.
PROGRAMMABLE GAIN AMPLIFIER
There are two pass band settings for the LPF. Within each pass band, the filters are tunable over about a 15% frequency range. The formula for the cutoff frequency is:
FC = FADC x 64 / (64 + Target )
Where Target is the decimal value programmed as the tuning target in Register 5. This filter may also be bypassed. In this case, the bandwidth of the RX path will be gain dependent and will be around 50 MHz at the highest gain settings.
ADC
The PGA has a programmable gain range from -6 dB to +36 dB if the narrower (approximately 12 MHz) LPF bandwidth is selected, or if the LPF is bypassed. If the wider (approximately 29 MHz) LPF bandwidth is selected, the gain range is -6 dB to +30 dB. The PGA is comprised of two sections, a continuous time PGA (CPGA), and a switched capacitor PGA (SPGA). The CPGA has possible gain settings of 0, 6, 12, 18, 24 and 30. The SPGA has possible gain settings of -6 dB, -4 dB, -2 dB, 0 dB, +2 dB, +4 dB, and +6 dB. Table II shows how the gain is distributed for each programmed gain setting. The CPGA input appears at the device RX+ and RX- input pins. The input impedance of this stage is nominally 270 differential and is not gain dependent. It is best to ac-couple the input signal to this stage and let the inputs self-bias. This will lower the offset voltage of the input signal, which is important at higher gains, since any offset will lower the output compliance range of the CPGA output. When the inputs are driven by direct coupling, the dc level should be AVDD/2. However, this could lead to larger dc offsets and reduce the dynamic range of the RX path. There are two modes for selecting the RX path gain. The first mode is to program the PGA through the serial port. A 5-bit word determines the gain with a resolution of 2 dB per step. More detailed information about this mode is included in the Register Programming Definitions section of this data sheet. The second mode sets the gain through the asynchronous AGC[2:0] pins. These three pins set the PGA gain and state of the RXBOOST pin according to Table II.
Table II. AGC[2:0] Gain Mapping
The AD9975's analog-to-digital converter implements pipelined multistage architecture to achieve high sample rates while consuming low power. The ADC distributes the conversion over several smaller A/D subblocks, refining the conversion with progressively higher accuracy as it passes the results from stage to stage. As a consequence of the distributed conversion, ADCs require a small fraction of the 2n comparators used in a traditional n-bit flashtype A/D. A sample-and-hold function within each of the stages permits the first stage to operate on a new input sample while the remaining stages operate on preceding samples. Each stage of the pipeline, excluding the last, consists of a low resolution Flash A/D connected to a switched capacitor DAC and interstage residue amplifier (MDAC). The residue amplifier amplifies the difference between the reconstructed DAC output and the flash input for the next stage in the pipeline. One bit of redundancy is used in each one of the stages to facilitate digital correction of flash errors. The last stage simply consists of a Flash A/D.
SHA AINP A/D AINN A/D D/A A/D D/A GAIN SHA GAIN
CORRECTION LOGIC
AGC [2:0] 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07
Rx Path Gain -6 -6 2 10 2 10 18 26
CPGA Gain -6 -6 -6 0 -6 0 12 18
SPGA Gain 0 0 8 10 8 10 6 8
Figure 1. ADC Theory of Operation
RXBOOST 0 0 0 0 1 1 1 1
The digital data outputs of the ADC are represented in straight binary format. They saturate to full scale or zero when the input signal exceeds the input voltage range. The maximum value will be output from the ADC when the RX+ input is 1 V or more greater than the RX- input. The minimum value will be output from the ADC when the RX- input is 1 V or more greater than the RX+ input. This results in a full-scale ADC voltage of 2 Vppd. The data can be translated to straight binary data format by simply inverting the most significant bit. The timing of the interface is fully described in the Digital Interface Port Timing section.
LOW-PASS FILTER
The low-pass filter (LPF) is a programmable, three-stage, fourth order low-pass filter. The first real pole is implemented within the CPGA. The second filter stage implements a complex pair of poles. The last real pole is implemented in a buffer stage that drives the SPGA.
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AD9975
DIGITAL HPF AGC PROGRAMMING
Following the ADC, there is a bypassable digital HPF. The response is a single pole IIR HPF. The transfer function is approximately:
H ( z ) = ( Z - 0.99994 ) / ( Z - 0.98466 )
where the sampling period is equal to the ADC clock period. This results in a 3 dB frequency approximately 1/400th of the ADC sampling rate. The transfer function of the digital HPF with an ADC sample rate of 50 MSPS is plotted in TPC 23. The digital HPF introduces a 1 ADC clock cycle latency. If the HPF function is not desired, the HPF can be bypassed and the latency will not be incurred.
CLOCK AND OSCILLATOR CIRCUITRY
The gain in the receive path can be programmed in two ways. The default method is through the AGC[2:0] pins. In this mode, the gain is achieved using a combination of internal and external gain. The external gain is controlled by the RXBOOST output pin, which is determined by the decode of the 3-bit AGC gain value.
DIGITAL INTERFACE PORT OPERATION
The AD9975 generates all internally required clocks from a single clock source. This source can be supplied in one of two ways. The first method uses the on-chip oscillator by connecting a fundamental frequency quartz crystal between the OSC IN (Pin 1) and XTAL (Pin 48) with parallel resonant load capacitors as specified by the crystal manufacturer. Alternatively, a TTL-level clock applied to OCS IN with the XTAL pin left unconnected can overdrive the internal oscillator circuit. The PLL has a frequency capture range between 10 MHz and 50 MHz.
AGC TIMING CONSIDERATIONS
The digital interface port is a 10-bit bidirectional bus shared in burst fashion between the transmit path and receive path. The MxFE acts as a slave to the digital ASIC, accepting two input enable signals, TXEN and RXEN, as well as two input clock signals, TXCLK and RXCLK. Because the sampling clocks for the DAC and ADC are derived internally from the OSC IN signal, it is required that the TXCLK and RXCLK signals are exactly the same frequency as the OSC IN signal. The phase relationships between the TXCLK, RXCLK, and OSC IN signal are arbitrary. In order to add flexibility to the digital interface port, there are several programming options available. The data input format is straight binary by default. It is possible to independently change the data format of the transmit path and receive path to twos complement. Also, the clock timing can be independently changed on the transmit and receive paths by selecting either the rising or falling clock edge as the validating/sampling edge of the clock. The digital interface port can also be programmed into a threestate output mode allowing it to be connected onto a shared bus. The timing of the interface is fully described in the Digital Interface Port Timing section.
CLOCK DISTRIBUTION
When implementing the AGC timing loop, it is important to consider the delay and settling time of the RX path in response to a change in gain. Figure 2 shows the delay the receive signal experiences through the blocks of the RX path. Whether the gain is programmed through the serial port or via the AGC[2:0] pins, the gain takes effect immediately with the delays shown in Figure 2. When gain changes do not involve the CPGA, the new gain will be evident in samples after about 7 ADC clock cycles. When the gain change does involve the CPGA, it takes an additional 45 ns to 70 ns due to the propagation delays of the buffer, LPF and PGA. Table VI in the Register Programming section details the PGA programming map.
GAIN REGISTER
The DAC sampling clock, fDAC, is generated by the internal digital phase-locked loop (DPLL). fDAC has a frequency equal to L x fOSCIN, where fOSCIN is the internal signal generated either by the crystal oscillator when a crystal is connected between the OSC IN and XTAL pins or by the clock that is fed into the OSC IN pin, and L is the multiplier programmed through the serial port. L can have the values of 1, 2, 4, or 8. When the interpolation filter is enabled (either 2x LPF or 2x BPF is selected), the data rate is upsampled by a factor of two. In this case, the transmit path expects a new data input word at the rate of fDAC/2. When the interpolation filter is bypassed, the transmit path expects a new input word at the same frequency as DAC sampling clock, fDAC. Therefore, in terms of fOSCIN, the TXCLK frequency should be:
5ns
DECODE LOGIC
fTXCLK = L x fOSCIN / K
DIGITAL HPF A/D SHA BUFFER LPF PGA
where K is the interpolation factor. The interpolation factor, K, is equal to 2 when the interpolator is enabled and is equal to 1 when the interpolator is bypassed. The ADC sampling clock is derived from fOSCIN and a new output sample is available every fOSCIN clock cycle. The ADC sampling lock can be programmed to be equal to fOSCIN if desired. The timing of the digital interface port is illustrated in the Figures 3 and 4.
1 CLK CYCLE
5 CLK CYCLE 1/2 CLK CYCLE
10ns
25ns OR 50ns
10ns
Figure 2. AGC Loop Timing
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DIGITAL INTERFACE PORT TIMING Table III. Instruction Byte Bit Definitions
The ADIO[9:0] bus accepts input data-words into the transmit path when the TXEN pin is high, the RXEN pin is low, and a clock is present on the TXCLK pin. Figure 3 illustrates the transmit path input timing.
tSU
TXCLK TXEN
MSB I7 R/W I6 N1 I5 N0 I4 A4 I3 A3 I2 A2 I1 A1
LSB I0 A0
tHD
ADIO[9:0] TX0 TX1 TX2 TX3 TX4
Bit I7 - R/W This bit determines whether a read or a write data transfer will occur after the instruction byte write. Logic high indicates a read operation, and Logic 0 indicates a write operation. Bits I6:I5 - N1:N0 These two bits determine the number of bytes to be transferred during the data transfer cycle. The bit decodes are shown in Table IV.
Table IV. N1:N0 Bit Map
RXEN
Figure 3. Transmit Data Input Timing Diagram
It should be noted that to clear the transmit path input buffers, an additional six clock cycles on the TXCLK input are required after TXEN goes low. The interpolation filters will be "flushed" with zeros if the clock signal into the TXCLK pin is present for 48 clock cycles after TXEN goes low (the data on the ADIO bus being irrelevant over this interval). The output from the receive path will be driven onto the ADIO[9:0] bus when the RXEN pin is high, and a clock is present on the RXCLK pin. When both TXEN and RXEN are low, the ADIO[9:0] bus is three-stated. Figure 4 illustrates the receive path output timing.
tVT
RXCLK
N1:N0 0:0 0:1 1:0 1:1
Description Transfer 1 Byte Transfer 2 Bytes Transfer 3 Bytes Transfer 4 Bytes
Bits I4:I0 - A4:A0 These bits determine which register is accessed during the data transfer portion of the communications cycle. For multibyte transfers, this address is the starting byte address. The remaining register addresses are generated by the AD9975.
Serial Interface Port Pin Description
tHT
RXEN
ADIO[9:0]
RX0
RX1
RX2
RX3
Figure 4. Receive Data Output Timing Diagram
SERIAL INTERFACE FOR REGISTER CONTROL
SCLK--Serial Clock The serial clock pin is used to synchronize data transfers to and from the AD9975 and to run the internal state machines. SCLK maximum frequency is 25 MHz. All data transmitted to the AD9975 is sampled on the rising edge of SCLK. All data read from the AD9975 is validated on the rising edge of SCLK and is updated on the falling edge. SENABLE--Serial Interface Enable The SENABLE Pin is active low. It enables the serial communication to the device. SENABLE select should stay low during the entire communication cycle. All input on the serial port is ignored when SENABLE is inactive. SDATA--Serial Data I/O The signal on this line is sampled on the first eight rising edges of SCLK after SENABLE goes active. Data is then read from or written to the AD9975 depending on what was read. Figures 5 and 6 show the timing relationships between the three SPI signals.
tDS
SENABLE
The serial port is a 3-wire serial communications port consisting of a clock (SCLK), chip select (SENABLE), and a bidirectional data (SDATA) signal. The interface allows read/write access to all registers that configure the AD9975 internal parameters. Single or multiple byte transfers are supported as well as MSB first or LSB first transfer formats.
General Operation of the Serial Interface
Serial communication over the serial interface can be from 1 byte to 5 bytes in length. The first byte is always the instruction byte. The instruction byte establishes whether the communication is going to be a read or write access, the number of data bytes to be transferred, and the address of the first register to be accessed. The instruction byte transfer is complete immediately upon the eighth rising edge of SCLK after SENABLE is asserted. Likewise, the data registers change immediately upon writing to the eighth bit of each data byte.
Instruction Byte
tSCLK tPWH tPWL
SCLK
tHT tDS
SDATA INSTRUCTION BIT 7 INSTRUCTION BIT 6
The instruction byte contains the information shown in Table III.
Figure 5. Timing Diagram Register Write to AD9975
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AD9975
SCLK
tDV
SDATA INSTRUCTION BIT n INSTRUCTION BIT n-1
Figures 7a and 7b show how the serial port words are built for each of these modes.
SENABLE SCLK SDATA
R/W I6(n) I5(n) I4 I3 I2 I1 I0 D7n D6n D6n D7n
INSTRUCTION CYCLE
DATA TRANSFER CYCLE
Figure 6. Timing Diagram Register Read from AD9975
MSB/LSB Transfers
The AD9975 serial port can support both most significant bit (MSB) first or least significant bit (LSB) first data formats. The bit order is controlled by the SPI LSB First Bit (Register 0, Bit 6). The default value is 0, MSB first. Multibyte data transfers in MSB format can be completed by writing an instruction byte that includes the register address of the last address to be accessed. The AD9975 will automatically decrement the address for each successive byte required for the multibyte communication cycle. When the SPI LSB First Bit (Register 0, Bit 6) is set high, the serial port interprets both instruction and data bytes LSB first. Multibyte data transfers in LSB format can be completed by writing an instruction byte that includes the register address of the first address to be accessed. The AD9975 will automatically increment the address for each successive byte required for the multibyte communication cycle.
Figure 7a. Serial Register Interface Timing MSB First
SENABLE SCLK SDATA
I0 I1 I2 I3 I4 I5(n) I6(n) R/W D00 D10 D20 D6n D7n
INSTRUCTION CYCLE
DATA TRANSFER CYCLE
Figure 7b. Serial Register Interface Timing LSB First
Notes on Serial Port Operation
The serial port is disabled and all registers are set to their default values during a hardware reset. During a software reset, all registers except Register 0 are set to their default values. Register 0 will remain at the last value sent, with the exception that the Software Reset Bit will be set to 0.
Table V. Register Layout
Address (hex) Bit 7 00 01
Bit 6
Bit 5 Software Reset PowerDown PLL-A
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Default (hex) Type 00 R/W R/W
Select 4-Wire LSB/MSB SPORT First
PowerDown DAC
Power-Down Power-Down PowerPower-Down 00 Interpolators RX Reference Down ADC Receive Filter and SPGA and CPGA 00
02 03 ADC Clock Source OSC IN/2 RX LPF Tuning Update Disable RX LPF Tuning Update in Progress RX Path RX Digital Fast ADC DC Offset HPF Sampling Correction Bypass PLL-A (xL) Multiplier [1:0] Wideband RX Enable LPF 1-Pole RX LPF RX LPF Bypass
R/W R/W
01
04
01
R/W
05 06
RX LPF Filter Tuning Target [7:0] RXBOOST Active Low RXBOOST PGA Gain Setting through Register RX PGA Gain [4:0]
80 00
R/W R/W
07
TX Interpolation Filter Select
Sample TX TX Data 10 on Falling Input Twos TXCLKIN Complement CLK-A Output Disable Three-State RX Port ADC Output RX Data 00 on Falling Output Twos RXCLK Complement Version [3:0] 00
R/W
08
CLK-B Equal CLK-A CLK-B to OSC IN/4 Equal Output to OSC IN Disable
R/W
0F
R
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AD9975
The serial port is operated by an internal state machine and is dependent on the number of SCLK cycles since the last time SENABLE went active. On every eighth rising edge of SCLK, a byte is transferred over the SPI. During a multibyte write cycle, this means the registers of the AD9975 are not simultaneously updated but occur sequentially. For this reason, it is recommended that single byte transfers be used when changing the SPI configuration or performing a software reset.
REGISTER PROGRAMMING DEFINITIONS Register 0, RESET/SPI Configuration Bit 1,0: PLL-A Multiplier
Bits 1 and 0 determine the multiplication factor (L) for PLL-A and the DAC sampling clock frequency, FDAC. FDAC = L x FCLKIN. Bit 1,0 0,0: L = 1 0,1: L = 2 1,0: L = 4 1,1: L = 8
Bit 6: ADC Clock Source OSC IN/2
Bit 5: Software Reset Setting this bit high resets the chip. The PLLs will relock to the input clock and all registers (except Register 0x0, Bit 6) revert to their default values. Upon completion of the reset, Bit 5 is reset to 0. The content of the interpolator stage is not cleared by software or hardware resets. It is recommended to "flush" the transmit path with zeros before transmitting data.
Bit 6: LSB/MSB First
Setting Bit 6 high selects the the OSC IN clock signal divided by 2 as the ADC sampling clock source. Setting Bit 6 low selects the OSC IN clock to be used directly as the ADC sampling clock source. The best ADC performance is achieved by using an external crystal or by driving the OSC IN pin with a low jitter clock source.
Register 4, Receive Filter Selection
Setting this bit high causes the serial port to send and receive data least significant bit (LSB) first. The default low state configures the serial port to send and receive data most significant bit (MSB) first.
Bit 7: Select 4-Wire SPORT
The AD9975 receive path has a continuous time 4-pole LPF and a 1-pole digital HPF. The 4-pole LPF has two selectable cutoff frequencies. Additionally, the filter can be tuned around those two cutoff frequencies. These filters can also be bypassed to different degrees as described below. The continuous time 4-pole low-pass filter is automatically calibrated to one of two selectable cutoff frequencies. The cutoff frequency, Fcutoff, is described as a function of the ADC sampling frequency FADC and can be influenced 15% by the RX filter tuning target word in Register 5. Fcutoff _ high = FADC x 158 / (64 + Target ) Bit 0: RX LPF Bypass Setting this bit high bypasses the 4-pole LPF. The filter is automatically powered down when this bit is set.
Bit 1: Enable 1-Pole RX LPF
Setting this bit high puts the serial port into a four-line mode. The SCLK and SENABLE retain their normal functions, SDATA becomes an input only line, and the RXBOOST/SDO pin becomes the serial port output. When in 4-wire mode, the data on the RXBOOST/SDO pin will change on the falling edge of SCLK and should be sampled on the rising edge of SCLK.
Register 1, Power-Down
Fcutoff _low = FADC x 64 / (64 + Target )
Bit 0: Power-Down Receive Filter and CPGA Setting this bit high powers down and bypasses the RX LPF and continuous time programmable gain amplifier. Bit 1: Power-Down ADC and SPGA Setting this bit high powers down the ADC and the switched capacitor programmable gain amplifier (SPGA). Bit 2: Power-Down RX Reference Setting this bit high powers down the ADC reference. This bit should be set if an external reference is applied.
Bit 3: Power-Down Interpolators
The AD9975 can be configured with a 1-pole filter when the 4-pole receive low-pass filter is bypassed. The 1-pole filter is untrimmed and subject to cutoff frequency variations of 20%. Bit 2: Wideband RX LPF This bit selects the nominal cutoff frequency of the 4-pole LPF. Setting this bit high selects a nominal cutoff frequency of 28.8 MHz. When the wideband filter is selected, the RX path gain is limited to 30 dB. Bit 3: Fast ADC Sampling Setting this bit increases the quiescent current in the SVGA block. This may provide some performance improvement when the ADC sampling frequency is greater than 40 MSPS. Bit 4: RX Digital HPF Bypass Setting this bit high bypasses the 1-pole digital HPF that follows the ADC. The digital filter must be bypassed for ADC sampling above 50 MSPS. Bit 5: RX Path DC Offset Correction Writing a 1 to this bit triggers an immediate receive path offset correction and reads back 0 after the completion of the offset correction. Bit 6: RX LPF Tuning Update in Progress This bit indicates when receive filter calibration is in progress. The duration of a receive filter calibration is about 500 s. Writing to this bit has no effect.
Setting this bit high powers down the transmit digital interpolator. It does not clear the content of the data path.
Bit 4: Power-Down DAC
Setting this bit high powers down the transmit DAC. Bit 5: Power-Down PLL-A Setting this bit high powers down the on-chip phase-locked loop that generates the transmit path clocks and the auxiliary clock CLK-A. When powered down, the CLK-A output goes to a high impedance state.
Register 3, Clock Source Configuration
The AD9975 contains a programmable PLL referred to as PLL-A. The output of the PLL is used to generate the internal clocks for the TX path and the auxiliary clock, CLK-A.
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AD9975
Bit 7: RX LPF Tuning Update Disable Setting this bit high disables the automatic background receive filter calibration. The AD9975 automatically calibrates the receive filter on reset and every few (~2) seconds thereafter to compensate for process and temperature variation, power supply, and long term drift. Programming a 1 to this bit disables this function. Programming a 0 triggers an immediate first calibration and enables the periodic update.
Register 5, Receive Filter Tuning Target Table VI. PGA Programming Map
RX Path Gain [4:0] 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0A 0x0B 0x0C 0x0D 0x0E 0x0F 0x10 0x11 0x12* 0x13* 0x14* 0x15*
RX Path Gain -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30/30 30/32 30/34 30/36
CPGA Gain 0 0 0 0 0 0 6 6 6 12 12 12 18 18 18 24 24 24 24/30 24/30 24/30 24/30
SPGA Gain -6 -4 -2 0 2 4 0 2 4 0 2 4 0 2 4 0 2 4 6/0 6/2 6/4 6/6
This register sets the filter tuning target as a function of FOSCIN. See Register 4 description.
Register 6, RX Path Gain Adjust
The AD9975 uses a combination of a continuous time PGA (CPGA) and a switched capacitor PGA (SPGA) for a gain range of -6 dB to +36 dB with a resolution of 2 dB. The RX path gain can be programmed over the serial interface by writing to the RX path Gain Adjust Register or directly using the GAIN and MSB aligned TX[5:1] Bits. The register default value is 0x00 for the lowest gain setting (-6 dB). The register always reads back the actual gain setting irrespective of which of the two programming modes was used. Bits [4:0]: RX PGA Gain Table VI describes the gains and how they are achieved as a function of the RX path adjust bits. It should be noted that the value of these bits will read back the actual gain value to which the PGA is set. If Bit 5 of this register is low, then the value read back will be that set by the AGC[2:0] Pins. Bit 5: PGA Gain Set through Register Setting this bit high will result in the RX path gain being set by writing to the PGA Gain Control Register. Default is zero, which selects writing the gain through the AGC[2:0] pins in conjunction with the RXBOOST pin. Bit 6: RXBOOST This bit is read-only. It reflects the level of the RXBOOST pin. Bit 7: RXBOOST Active Low Setting this bit high results in the value mapped to the RXBOOST pin by the AGC inputs being inverted.
*When the wideband RX filter bit is set high, the RX path gain is limited to 30 dB. The first of the two values refers to the mode when the lower RX LPF cutoff frequency is chosen, or when the RX LPF filter is bypassed.
Register 7, Transmit Path Settings Bit 0: TX Data Input Twos Complement
Setting this bit high changes the TX path input data format to twos complement. When this bit is low, the TX data format is straight binary. Bit 1: Sample TX Data on Falling TXCLKIN If Bit 1 is set high, the TX path data will be sampled on the falling edge of TXCLKIN. When this bit is low, the data will be sampled on the rising edge of TXCLKIN. Bit 4 to Bit 7: Interpolation Filter Select Bits 4 to 7 define the interpolation filter characteristic and interpolation rate. Bits 7:4; 0x1; see TPC 1. 2 Interpolation, LPF. 0x2; Interpolation Bypass. 0x5; see TPC 2. 2 Interpolation, BPF, Adj image. The interpolation factor has a direct influence on the rate at which the TX path will read the input data-words from the input buffer. When the interpolation filter has been bypassed, the data will be read out of the buffer at a rate of FOSCIN L. When the interpolator is configured to run in either of the 2 interpolation modes, the data will be read out of the buffer at a rate of 0.5 FOSCIN L.
Register 8, Receiver and Clock Output Settings Bit 0: Rx Data Output Twos Complement
Setting this bit high changes the RX path input data format to twos complement. When this bit is low, the RX data format is straight binary.
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Bit 1: ADC Output on Falling RXCLK If Bit 1 is set high, the TX path data will be sampled on the falling edge of RXCLK. When this bit is low, the data will be sampled on the rising edge of RXCLK. Bit 3: Three-State RX Port This bit sets the receive output RX[5:0] into a high impedance three-state mode. It allows for sharing the bus with other devices. Bit 4: CLK-A Output Disable Setting Bit 4 high fixes the CLK-A output to a Logic 0 output level. Bit 5: CLK-B Output Disable Setting Bit 5 high fixes the CLK-A output to a Logic 0 output level. Bit 6: CLK-A Equal to OSC IN Setting Bit 6 high sets the CLK-A output signal frequency equal to the OSC IN signal frequency. Otherwise, the CLK-A output frequency is equal to FOSCIN x L. Bit 7: CLK-B Equal to OSC IN/4 Setting Bit 7 high sets the CLKB output signal frequency equal to the OSC IN/4 signal frequency. Otherwise, the CLKB output frequency is equal to OSC IN/2.
Register F, Die Revision Power Planes and Decoupling
The AD9975 evaluation board demonstrates a good power supply distribution and decoupling strategy. The board has four layers; two signal layers, one ground plane, and one power plane. The power plane is split into a 3VDD section, which is used for the 3 V digital logic circuits; a DVDD section, which is used to supply the digital supply pins of the AD9975; an AVDD section, which is used to supply the analog supply pins of the AD9975; and a VANLG section, which supplies the higher voltage analog components on the board. The 3VDD section will typically have the highest frequency currents on the power plane and should be kept the furthest from the MxFE and analog sections of the board. The DVDD portion of the plane brings the current used to power the digital portion of the MxFE to the device. This should be treated similar to the 3VDD power plane and be kept from going underneath the MxFE or analog components. The MxFE should largely sit on the AVDD portion of the power plane. The AVDD and DVDD power planes may be fed from the same low noise voltage source; however, they should be decoupled from each other to prevent the noise generated in the DVDD portion of the MxFE from corrupting the AVDD supply. This can be done by using ferrite beads between the voltage source and DVDD and between the source and AVDD. Both DVDD and AVDD should have a low ESR, bulk decoupling capacitor on the MxFE side of the ferrite as well as a low ESR, ESL decoupling capacitors on each supply pin (i.e., the AD9975 requires five power supply decoupling caps, one each on Pins 5, 38, 47, 14, and 35). The decoupling caps should be placed as close to the MxFE supply pins as possible. An example of the proper decoupling is shown in the AD9975 evaluation board schematic.
Ground Planes
This register stores the die revision of the chip. It is a read-only register.
PCB DESIGN CONSIDERATIONS
Although the AD9975 is a mixed signal device, the part should be treated as an analog component. The digital circuitry on-chip has been specially designed to minimize the impact that the digital switching noise will have on the operation of the analog circuits. Following the power, grounding, and layout recommendations in this section will help you get the best performance from the MxFE.
Component Placement
If the three following guidelines of component placement are followed, chances for getting the best performance from the MxFE are greatly increased. First, manage the path of return currents flowing in the ground plane so that high frequency switching currents from the digital circuits do not flow on the ground plane under the MxFE or analog circuits. Second, keep noisy digital signal paths and sensitive receive signal paths as short as possible. Third, keep digital (noise generating) and analog (noise susceptible) circuits as far away from each other as possible. In order to best manage the return currents, pure digital circuits that generate high switching currents should be closest to the power supply entry. This will keep the highest frequency return current paths short and prevent them from traveling over the sensitive MxFE and analog portions of the ground plane. Also, these circuits should be generously bypassed at each device that will further reduce the high frequency ground currents. The MxFE should be placed adjacent to the digital circuits such that the ground return currents from the digital sections will not flow in the ground plane under the MxFE. The analog circuits should be placed furthest from the power supply.
In general, if the component placing guidelines discussed earlier can be implemented, it is best to have at least one continuous ground plane for the entire board. All ground connections should be made as short as possible. This will result in the lowest impedance return paths and the quietest ground connections. If the components cannot be placed in a manner that would keep the high frequency ground currents from traversing under the MxFE and analog components, it may be necessary to put current steering channels into the ground plane to route the high frequency currents around these sensitive areas. These current steering channels should be made only when and where necessary.
Signal Routing
The digital RX and TX signal paths should be kept as short as possible. Also, the impedance of these traces should have a controlled impedance of about 50 . This will prevent poor signal integrity and the high currents that can occur during undershoot or overshoot caused by ringing. If the signal traces cannot be kept shorter than about 1.5 inches, then series termination resistors (33 to 47 ) should be placed close to all signal sources. It is a good idea to series terminate all clock signals at their source regardless of trace length.
The receive RX+/RX- signals are the most sensitive signals on the entire board. Careful routing of these signals is essential for good The AD9975 has several pins that are used to decouple sensitive receive path performance. The RX+/RX- signals form a differential internal nodes. These pins are REFIO, REFB, and REFT. The decoupair and should be routed together as a pair. By keeping the traces pling capacitors connected to these points should have low ESR adjacent to each other, noise coupled onto the signals will appear and ESL. These capacitors should be placed as close to the MxFE as common mode and will be largely rejected by the MxFE receive as possible and be connected directly to the analog ground plane. input. Keeping the driving point impedance of the receive signal low The resistor connected to the FS ADJ pin should also be placed and placing any low-pass filtering of the signals close to the MxFE close to the device and connected directly to the analog ground plane. will further reduce the possibility of noise corrupting these signals. REV. 0 -19-
AD9975
OUTLINE DIMENSIONS 48-Lead Plastic Quad Flatpack [LQFP] 1.4 mm Thick (ST-48)
Dimensions shown in millimeters
PIN 1 INDICATOR 9.00 BSC
48 1 37 36
0.75 0.60 0.45
1.45 1.40 1.35
0.20 0.09
SEATING PLANE
TOP VIEW
(PINS DOWN)
7.00 BSC
0.15 0.05
SEATING PLANE
7 3.5 0 0.08 MAX COPLANARITY
VIEW A
12 13 24 25
VIEW A
ROTATED 90 CCW
0.50 BSC
0.27 0.22 0.17
COMPLIANT TO JEDEC STANDARDS MS-026BBC
-20-
REV. 0
PRINTED IN U.S.A.
C03061-0-8/02(0)
1.60 MAX

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