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ADS5203
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SBAS258A - JUNE 2002 - REVISED JULY 2002
Dual 10 Bit 40MSPS Low Power ANALOG TO DIGITAL CONVERTER
FEATURES D 3.3V Single-Supply Operation D Dual Simultaneous Sample-and-Hold Inputs D Differential or Single-Ended Analog Inputs D Single or Dual Parallel Bus Output D 60dB SNR at fIN = 10.5MHz D 73dB SFDR at fIN = 10.5MHz D Low Power: 240mW D 300MHz Analog Input Bandwidth D 3.3V TTL/CMOS-Compatible Digital I/O D Internal or External Reference D Adjustable Reference Input Range D Power-Down (Standby) Mode D TQFP-48 Package APPLICATIONS D Digital Communications (Baseband Sampling) D Video Processing D Portable Instrumentation D Ultrasound DESCRIPTION
The ADS5203 is a dual 10-bit, 40MSPS Analog-to-Digital Converter (ADC). It simultaneously converts each analog input signal into a 10-bit, binary coded digital word up to a maximum sampling rate of 40MSPS per channel. All digital inputs and outputs are 3.3V TTL/CMOS compatible. An innovative dual-pipeline architecture implemented in a CMOS process and the 3.3V supply results in very low power dissipation. In order to provide maximum flexibility, both top and bottom voltage references can be set from user-supplied voltages. Alternatively, if no external references are available, the on-chip internal references can be used. Both ADCs share a common reference to improve offset and gain matching. If external reference voltage levels are available, the internal references can be powered down independently from the rest of the chip, resulting in even greater power savings. The ADS5203 is characterized for operation from -40C to +85C and is available in a TQFP-48 package.
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters.
Copyright 2002, Texas Instruments Incorporated
ADS5203
www.ti.com SBAS258A - JUNE 2002 - REVISED JULY 2002
ORDERING INFORMATION
PRODUCT ADS5203 PACKAGE-LEAD TQFP-48 PACKAGE DESIGNATOR(1) PFB SPECIFIED TEMPERATURE RANGE -40C to +85C PACKAGE MARKING AZ5203 ORDERING NUMBER ADS5203IPFB TRANSPORT MEDIA, QUANTITY Tray, 250 Tape and Reel, 1000
ADS5203 TQFP-48 PFB -40C to +85C AZ5203 ADS5203IPFBR (1) For the most current specifications and package information, refer to our web site at www.ti.com.
ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature range unless otherwise noted(1). Supply Voltage: AVDD to AGND, DVDD to DGND . . . . . . . . . . -0.5V to 3.6V Supply Voltage: AVDD to DVDD, AGND to DGND . . . . . . . . . . -0.5V to 0.5V Digital Input Voltage Range to DGND . . . . . . . . . . . . -0.5V to DVDD + 0.5V Analog Input Voltage Range to AGND . . . . . . . . . . . . . -0.5V to AVDD + 0.5V
Digital Output Voltage Applied from Ext. Source to DGND . . . . . . . -0.5V to DVDD + 0.5V Reference Voltage Input Range to AGND: VREFT, VREFB . . . . . . -0.5V to AVDD + 0.5V
Operating Free-Air Temperature Range, TA (ADS5203I) . . . -40C to +85C Storage Temperature Range, TSTG . . . . . . . . . . . . . . . . . . -65C to +150C
Soldering Temperature 1.6mm (1/16 inch) from case for 10 seconds . . . . . . . . . . . . 300C
(1) Stresses above these ratings may cause permanent damage. Exposure to absolute maximum conditions for extended periods may degrade device reliability. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those specified is not implied.
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
RECOMMENDED OPERATING CONDITIONS
over operating free-air temperature range, TA, unless otherwise noted. PARAMETER Power Supply Su ly Supply Voltage Analog and Reference Inputs Reference Input Voltage (top) Reference Input Voltage (bottom) Reference Voltage Differential Reference Input Resistance Reference Input Current Analog Input Voltage, Differential Analog Input Voltage, Single-Ended(1) Analog Input Capacitance Clock Input(2) Analog Outputs CML Voltage CML Output Resistance Digital Inputs High-Level Input Voltage Low-Level Input Voltage Input Capacitance Clock Period Pulse Duration Clock Period tc (80MHz) tw(CLKH), tw(CLKL) (80MHz) tc (40MHz) 12.5 Clock HIGH or LOW 5.25 25 11.25 VIH VIL 2.4 DGND 5 DVDD 0.8 V V pF ns ns ns ns AVDD/2 2.3 V k VREFT VREFB VREFT - VREFB RREF IREF VIN VIN CI fCLK = 1MHz to 80MHz fCLK = 1MHz to 80MHz fCLK = 1MHz to 80MHz fCLK = 80MHz fCLK = 80MHz -1 CML - 1.0 8 0 AVDD 1.9 0.95 0.95 2.0 1.0 1.0 1650 0.62 1 CML + 1.0 2.15 1.1 1.1 V V V mA V V pF V AVDD DVDD DRVDD 3.0 3.3 3.6 V CONDITIONS MIN NOM MAX UNIT
Pulse Duration tw(CLKH) , tw(CLKL) (40MHz) Clock HIGH or LOW (1) Applies only when the signal reference input connects to CML. (2) Clock pin is referenced to AVDD/AVSS. 2
ADS5203
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ELECTRICAL CHARACTERISTICS
over recommended operating conditions with fCLK = 80MHz and use of internal voltage references, unless otherwise noted. PARAMETER Power Supply IDD O erating Su ly Current Operating Supply AVDD DVDD DRVDD PD PD(STBY) tPD tWU IIH IIL 56 AVDD = DVDD = DRVDD = 3.3V, 3 3V CL = 10pF, fIN = 3.5MHz, -1dBFS 10pF 3 5MHz PWDN_REF = `L' PWDN_REF = `H' STDBY = `H', CLK Held HIGH or LOW External Reference -1 -1 1.7 15 240 220 95 550 40 1 1 62 2.2 26 290 240 150 mW W ms s A A mA TEST CONDITIONS MIN TYP MAX UNIT
Power Dissipation Standby Power Wake Up Time Digital Inputs High-Level Input Current on Digital Inputs incl. CLK Low-Level Input Current on Digital Inputs incl. CLK Digital Outputs High-Level Output Voltage Low-Level Output Voltage Output Capacitance High-Impedance State Output Current to High-Level High-Impedance State Output Current to Low-Level Data Output Rise-and-Fall Time Rise and Fall Reference Outputs Reference Top Voltage Reference Bottom Voltage Differential Reference Votage DC Accuracy Integral Nonlinearity, End Point Nonlinearity Differential Nonlinearity Missing Codes Zero Error(3) Full-Scale Error Gain Error
Power-Up Time for All References from Standby
AVDD = DVDD = DRVDD = 3 6V 3.6V
VOH VOL CO IOZH IOZL
AVDD = DVDD = DRVDD = 3.0V at IOH = 50A, Digital Outputs Forced HIGH AVDD = DVDD = DRVDD = 3.0V at IOL = 50A, Digital Outputs Forced LOW
2.8
2.96 0.04 5 0.2
V V pF +1 +1 A A ns ns 2.1 1.05 1.05 V V V
AVDD = DVDD = DRVDD = 3 6V 3.6V CLOAD = 10pF, Single-Bus Mode CLOAD = 10pF, Dual-Bus Mode
-1 -1 3 5 1.9 0.95 0.95 2 1 1.0
VREFTO VREFBO REFT - REFB
Absolute Min/Max Values Valid and Tested for AVDD = 3.3V
INL DNL
Internal References(1) Internal References(2)
TA = -40C to +85C 40C TA = -40C to +85C
-1.5 15 -0.9
0.4 0 4 0.5
+1 5 +1.5 +1
LSB LSB
No Missing Codes Assured 0.12 AVDD = DVDD = DRVDD = 3 3V 3.3V External References (3) 0.28 0.24 1.5 1.5 1.5 %FS %FS %FS
(1) Integral nonlinearity refers to the deviation of each individual code from a line drawn from zero to full-scale. The point used as zero occurs 1/2LSB before the first code transition. The full-scale point is defined as a level 1/2LSB beyond the last code transition. The deviation is measured from the center of each particular code to the best-fit line between these two endpoints. (2) An ideal ADC exhibits code transitions that are exactly 1LSB apart. DNL is the deviation from this ideal value. Therefore, this measure indicates how uniform the transfer function step sizes are. The ideal step size is defined here as the step size for the device under test, (i.e., (last transition level - first transition level)/(2n - 2)). Using this definition for DNL separates the effects of gain and offset error. A minimum DNL better than -1LSB ensures no missing codes. (3) Zero error is defined as the difference in analog input voltage--between the ideal voltage and the actual voltage--that will switch the ADC output from code 0 to code 1. The ideal voltage level is determined by adding the voltage corresponding to 1/2LSB to the bottom reference level. The voltage corresponding to 1LSB is found from the difference of top and bottom references divided by the number of ADC output levels (1024). Full-scale error is defined as the difference in analog input voltage--between the ideal voltage and the actual voltage--that will switch the ADC output from code 1022 to code 1023. The ideal voltage level is determined by subtracting the voltage corresponding to 1.5LSB from the top reference level. The voltage corresponding to 1LSB is found from the difference of top and bottom references divided by the number of ADC output levels (1024).
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ADS5203
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DYNAMIC PERFORMANCE(1)
TA = TMIN to TMAX, AVDD = DVDD = DRVDD = 3.3V, fIN = -1dBFS, Internal Reference, fCLK = 80MHz, fS = 40MSPS, and Differential Input Range = 2Vp-p, unless otherwise noted. PARAMETER Effective Number of Bits ENOB TEST CONDITIONS fIN = 3.5MHz fIN = 10.5MHz fIN = 20MHz fIN = 3.5MHz fIN = 10.5MHz fIN = 20MHz Signal-to-Noise Ratio g SNR fIN = 3.5MHz fIN = 10.5MHz fIN = 20MHz fIN = 3.5MHz fIN = 10.5MHz fIN = 20MHz Spurious-Free Dynamic Range y g SFDR fIN = 3.5MHz fIN = 10.5MHz fIN = 20MHz See Note (2) IMD f1 = 9.5MHz, f2 = 9.9MHz 57 MIN 9.3 TYP 9.7 9.7 9.6 -71 -71 -68 60.5 60.5 60 60 60 60 75 69 73 70.5 300 -68 -75 0.016 1.75 -66 MAX UNIT Bits Bits Bits dB dB dB dB dB dB dB dB dB dB dB dB MHz dBc dBc % FS
Total Harmonic Distortion
THD
Signal-to-Noise Ratio + Distortion g
SINAD
Analog Input Bandwidth 2-Tone Intermodulation Distortion A/B Channel Crosstalk A/B Channel Offset Mismatch
A/B Channel Full-Scale Error Mismatch 0.016 1.0 % FS (1) These specifications refer to a 25 series resistor and 15pF differential capacitor between A/B+ and A/B- inputs; any source impedance will bring the bandwidth down. (2) Analog input bandwidth is defined as the frequency at which the sampled input signal is 3dB down on unity gain and is limited by the input switch impedance.
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ADS5203
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PIN CONFIGURATION
Terminal Functions
TERMINAL
NAME DRVDD DRVSS DA 9..0
NO. 1,13 12, 24 14-23
I/O I I O
Supply Voltage for Output Drivers Digital Ground for Output Drivers
DESCRIPTION
DB 9..0 OE COUT COUT SELB DVSS CLK DVDD AVDD MODE AVSS B- B+ REFT
2-11 48 26 25 44 43 47 45 27,37,41 46 28,36,40 35 34 31
O I O O I I I I I I I I I I/O
Data Outputs for Bus A. D9 is MSB. This is the primary bus. Data from both input channels can be output on this bus or data from the A channel only. Pins SELB and MODE select the output mode. The data outputs are in tri-state during power-down (refer to Timing Options table). Data Outputs for Bus B. D9 is MSB. This is the second bus. Data is output from the B-channel when dual bus output mode is selected. The data outputs are in tri-state during power-down and single-bus modes (refer to Timing Options table). Output Enable. A LOW on this terminal will enable the data output bus, COUT and COUT. Latch Clock for the Data Outputs. COUT is in tri-state during power-down. Inverted Latch Clock or multiplexer control for the Data Outputs. COUT is in tri-state during power-down. Selects either single-bus data output or dual-bus data output. A LOW selects dual-bus data output. Digital Ground Clock Input. The input is sampled on each rising edge of CLK when using a 40MHz input and alternate rising edges when using an 80MHz input. The clock pin is referenced to AVDD and AVSS to reduce noise coupling from digital logic. Digital Supply Voltage Analog Supply Voltage Selects the COUT and COUT output mode. Analog Ground Negative Input for the Analog B Channel Positive Input for the Analog B Channel Reference Voltage Top. The voltage at this terminal defines the top reference voltage for the ADC. Sufficient filtering should be applied to this input: the use of 0.1F capacitor between REFT and AVSS is recommended. Additionally a 0.1F capacitor should be connected between REFT and REFB. Reference Voltage Bottom. The voltage at this terminal defines the bottom reference voltage for the ADC. Sufficient filtering should be applied to this input: the use of 0.1F capacitor between REFB and AVSS is recommended. Additionally a 0.1F capacitor should be connected between REFT and REFB. Common-Mode Level. This voltage is equal to (AVDD - AVSS)/2. An external capacitor of 0.1F should be connected between this terminal and AVSS when CML is used as a bias voltage. No capacitor is required if CML is not used. Power-Down for Internal Reference Voltages. A HIGH on this terminal disables the internal reference circuit. Standby Input. A HIGH on this terminal will power down the device. Negative Input for the Analog A Channel Positive Input for Analog A Channel This pin must be connected to DVDD. It should not be left floating.
REFB
30
I/O
CML PDWN_REF STBY A- A+ TP
32 33 42 39 38 29
O I I I I
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ADS5203
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TIMING REQUIREMENTS
PARAMETER Input Clock Rate Conversion Rate Clock Duty Cycle (40MHz) Clock Duty Cycle (80MHz) Output Delay Time Mux Setup Time Mux Hold Time Output Setup Time Output Hold Time Pipeline Delay (latency, channels A and B) Pipeline Delay (latency, channels A and B) Pipeline Delay (latency, channel A) Pipeline Delay (latency, channel B) Pipeline Delay (latency, channel A) Pipeline Delay (latency, channel B) Aperture Delay Time Aperture Jitter Disable Time, OE Rising to Hi-Z Enable Time, OE Falling to Valid Data td(o) ts(m) th(m) ts(o) th(o) td(pipe) td(pipe) td(pipe) td(pipe) td(pipe) td(pipe) td(a) tJ(a) tdis ten CL = 10pF CL = 10pF MODE = 0, SELB = 0 MODE = 1, SELB = 0 MODE = 0, SELB = 1 MODE = 0, SELB = 1 MODE = 1, SELB = 1 MODE = 1, SELB = 1 CL = 10pF 9 1.7 9 1.5 fCLK TEST CONDITIONS MIN 1 1 45 42 50 50 9 10.4 2.1 10.4 2.2 8 4 8 9 8 9 3 1.5 5 5 8 8 TYP MAX 80 40 55 58 14 UNIT MHz MSPS % % ns ns ns ns ns CLK Cycles CLK Cycles CLK Cycles CLK Cycles CLK Cycles CLK Cycles ns ps, rms ns ns
TIMING OPTIONS
OPERATING MODE 80MHz Input Clock, Dual-Bus Output, COUT = 40MHz 40MHz Input Clock, Dual-Bus Output, COUT = 40MHz 80MHz Input Clock, Single-Bus Output, COUT = 40MHz 80MHz Input Clock, Single-Bus Output, COUT = 80MHz MODE 0 1 0 1 SELB 0 0 1 1 TIMING DIAGRAM FIGURE 1 2 3 4
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ADS5203
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TIMING DIAGRAMS
Sample A1 and B1 Analog_A Analog_B 0 CLK(1) CLK40INT(2) td(pipe) ADCOUTA[9:0](3) td(pipe) ADCOUTB[9:0](3) DA[9:0] td(o) DB[9:0] DAB[19:0] COUT COUT NOTES: (1) In this option CLK = 80MHz. (2) CLK40INT refers to 40MHz Internal Clock, per channel. (3) Internal signal only. B1 B2 B3 A&B 3 B4 A&B 4 B5 A&B 5 DAB[19:0] is used to illustrate the placement of the busses DA and DB A&B 1 A&B 2 ts(o) th(o) B1 td(o) A1 B2 A2 B3 A3 B4 A4 B5 A5 A1 A2 A3 A4 A5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Figure 1. Dual Bus Output--Option 1.
Sample A1 and B1 Analog_A Analog_B 1 CLK(1) td(pipe) ADCOUTA[9:0](2) td(pipe) ADCOUTB[9:0](2) DA[9:0] td(o) DB[9:0] DAB[19:0] B1 B2 B3 A&B 3 B4 A&B 4 B5 A&B 5 B1 td(o) A1 B2 A2 B3 A3 B4 A4 B5 A5 A1 A2 A3 A4 A5 2 3 4 5 6 7 8 9 10
DAB[19:0] is used to illustrate the combined busses DA and DB A&B 1 A&B 2 ts(o) th(o)
COUT COUT NOTE: (1) In this option CLK = 40MHz, per channel. (2) Internal signal only
Figure 2. Dual Bus Output--Option 2.
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ADS5203
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TIMING DIAGRAMS (Cont.)
Sample A1 and B1 Analog_A Analog_B 0 CLK(1) CLK40INT(2) td(pipe) ADCOUTA[9:0](3) td(pipe) ADCOUTB[9:0](3) td(o) DA[9:0] A1 B1 A2 B2 A3 B3 th(o) ts(o) COUT th(o) ts(o) COUT NOTES: (1) In this option CLK = 80MHz. (2) CLK40INT refers to 40MHz Internal Clock, per channel. (3) Internal signal only. A4 B4 A5 B5 B1 td(o) B2 B3 B4 B5 A1 A2 A3 A4 A5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Figure 3. Single Bus Output--Option 1.
Sample A1 and B1 Analog_A Analog_B 0 CLK(1) CLK40INT(2) td(pipe) ADCOUTA[9:0](3) td(pipe) ADCOUTB[9:0](3) td(o) DA[9:0] A1 B1 th(o) ts(o) COUT ts(m) th(m) COUT NOTES: (1) In this option CLK = 80MHz. (2) CLK40INT refers to 40MHz Internal Clock, per channel. (3) Internal signal only. A2 B2 A3 B3 A4 B4 A5 B5 B1 td(o) B2 B3 B4 B5 A1 A2 A3 A4 A5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Figure 4. Single Bus Output--Option 2.
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ADS5203
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TYPICAL CHARACTERISTICS
At TA = 25C, AVDD = DVDD = DRVDD = 3.3V, fIN = -0.5dBFS, Internal Reference, fCLK = 80MHz, fS = 40MSPS, Differential Input Range = 2Vp-p, 25 series resistor, and 15pF differential capacitor at A/B+ and A/B- inputs, unless otherwise noted.
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ADS5203
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Typical Characteristics (Cont.)
At TA = 25C, AVDD = DVDD = DRVDD = 3.3V, fIN = -0.5dBFS, Internal Reference, fCLK = 80MHz, fS = 40MSPS, Differential Input Range = 2Vp-p, 25 series resistor, and 15pF differential capacitor at A/B+ and A/B- inputs, unless otherwise noted.
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ADS5203
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PRINCIPLE OF OPERATION
The ADS5203 implements a dual high-speed, 10-bit, 40MSPS converter in a cost-effective CMOS process. The differential inputs on each channel are sampled simultaneously. Signal inputs are differential and the clock signal is single-ended. The clock signal is either 80MHz or 40MHz, depending on the device configuration set by the user. Powered from 3.3V, the dual-pipeline design architecture ensures low-power operation and 10-bit resolution. The digital inputs are 3.3V TTL/CMOS compatible. Internal voltage references are included for both bottom and top voltages. Alternatively, the user may apply externally generated reference voltages. In doing so, the input range can be modified to suit the application. The ADC is a 5-stage pipelined ADC with 4 stages of fully-differential switched capacitor sub-ADC/MDAC pairs and a single sub-ADC in stage 5. All stages deliver 2 bits of the final conversion result. A digital error correction is used to compensate for modest comparator offsets in the sub-ADCs.
The analog input signal is sampled on capacitors CSP and CSN while the internal device clock is low. The sampled voltage is transferred to capacitors CHP and CHN and held on these while the internal device clock is high. The SHA can sample both single-ended and differential input signals. The load presented to the AIN pin consists of the switched input sampling capacitor CS (approximately 2pF) and its various stray capacitances. A simplified equivalent circuit for the switched capacitor input is shown in Figure 6. The switched capacitor circuit is modeled as a resistor RIN. fCLK is the clock frequency, which is 40MHz at full speed, and CS is the sampling capacitor. Using 25 series resistors and a differential 15pF capacitor at the A/B+ and A/B- inputs is recommended to reduce noise.
NOTE: AIN can be any variation of A or B inputs.
SAMPLE-AND-HOLD AMPLIFIER
Figure 5 shows the internal SHA architecture. The circuit is balanced and fully differential for good supply noise rejection. The sampling circuit has been kept as simple as possible to obtain good performance for high-frequency input signals.
fCLK = 40MHz
VCM
VCM = 0.5 S (V(A/B+) + V(A/B-))
Figure 6. Equivalent Circuit for the Switched Capacitor Input.
ANALOG INPUT, DIFFERENTIAL CONNECTION
The analog input of the ADS5203 is a differential architecture that can be configured in various ways depending on the signal source and the required level of performance. A fully differential connection will deliver the best performance from the converter. The analog inputs must not go below AVSS or above AVDD. The inputs can be biased with any common-mode voltage provided that the minimum and maximum input voltages stay within the range AVSS to AVDD. It is recommended to bias the inputs with a common-mode voltage around AVDD/2. This can be accomplished easily with the output voltage source CML, which is equal to AVDD/2. CML is made available to the user to help simplify circuit design. This output voltage source is not designed to be a reference or to be loaded but makes an excellent DC bias source and stays well within the analog input common-mode voltage range over temperature.
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Figure 5. SHA Architecture.
ADS5203
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Table 1 lists the digital outputs for the corresponding analog input voltages. Table 1. Output Format for Differential Configuration
DIFFERENTIAL INPUT VIN = (A+/B+) - (A-/B-), REFT - REFB = 1V ANALOG INPUT VOLTAGE DIGITAL OUTPUT CODE VIN = +1V 3FFH VIN = 0 200H VIN = -1V 000H
DC-COUPLED DIFFERENTIAL ANALOG INPUT CIRCUIT
Driving the analog input differentially can be achieved in various ways. Figure 7 gives an example where a single-ended signal is converted into a differential signal by using a fully differential amplifier such as the THS4141. The input voltage applied to VOCM of the THS4141 shifts the output signal into the desired common-mode level. VOCM can be connected to CML of the ADS5203, the common-mode level is shifted to AVDD/2.
Figure 8.
AC-Coupled Differential Transformer.
Input
with
ANALOG INPUT, SINGLE-ENDED CONFIGURATION
For a single-ended configuration, the input signal is applied to only one of the two inputs. The signal applied to the analog input must not go below AVSS or above AVDD. The inputs can be biased with any common-mode voltage provided that the minimum and maximum input voltage stays within the range AVSS to AVDD. It is recommended to bias the inputs with a common-mode voltage around AVDD/2. This can be accomplished easily with the output voltage source CML, which is equal to AVDD/2. An example for this is shown in Figure 9.
Figure 7. Single-Ended to Differential Conversion Using the THS4141. Figure 9. AC-Coupled, Single-Ended Configuration.
AC-COUPLED DIFFERENTIAL ANALOG INPUT CIRCUIT
Driving the analog input differentially can be achieved by using a transformer-coupling, as illustrated in Figure 8. The center tap of the transformer is connected to the voltage source CML, which sets the common-mode voltage to AVDD/2. No buffer is required at the output of CML since the circuit is balanced and no current is drawn from CML.
The signal amplitude to achieve full scale is 2Vp-p. The signal, which is applied at A/B+ is centered at the bias voltage. The input A/B- is also centered at the bias voltage. The CML output is connected via a 4.7k resistor to bias the input signal. There is a direct DC-coupling from CML to A/B- while this input is AC-decoupled through the 10F and 0.1F capacitors. The decoupling minimizes the coupling of A/B+ into the A/B- path.
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ADS5203
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Table 2 lists the digital outputs for the corresponding analog input voltages. Table 2. Output Format for Single-Ended Configuration.
SINGLE-ENDED INPUT, REFT - REFB = 1V ANALOG INPUT VOLTAGE DIGITAL OUTPUT CODE V(A/B+) = VCML + 1V 3FFH V(A/B+) = VCML 200H V(A/B+) = VCML - 1V 000H
DIGITAL INPUTS
Digital inputs are CLK, STDBY, PWDN_REF, OE, MODE, and SELB. These inputs don't have a pull-down resistor to ground, therefore, they should not be left floating. The CLK signal at high frequencies should be considered as an `analog' input. CLK should be referenced to AVDD and AVSS to reduce noise coupling from the digital logic. Overshoot/undershoot should be minimized by proper termination of the signal close to the ADS5203. An important cause of performance degradation for a high-speed ADC is clock jitter. Clock jitter causes uncertainty in the sampling instant of the ADC, in addition to the inherent uncertainty on the sampling instant caused by the part itself, as specified by its aperture jitter. There is a theoretical relationship between the frequency f and resolution (2N) of a signal that needs to be sampled on one hand, and on the other hand the maximum amount of aperture error dtmax that is tolerable. It is given by the following relation: dtmax = 1/[ f 2(N+1)] As an example, for a 10-bit converter with a 20MHz input, the jitter needs to be kept less than 7.8ps in order not to have changes in the LSB of the ADC output due to the total aperture error.
REFERENCE TERMINALS
The ADS5203's input range is determined by the voltages on its REFB and REFT pins. The ADS5203 has an internal voltage reference generator that sets the ADC reference voltages REFB = 1V and REFT = 2V. The internal ADC references must be decoupled to the PCB AVSS plane. The recommended decoupling scheme is shown in Figure 10. The internal reference voltages commonmode voltage is 1.5V.
DIGITAL OUTPUTS
The output of ADS5203 is an unsigned binary code. Capacitive loading on the output should be kept as low as possible (a maximum loading of 10pF is recommended) to ensure best performance. Higher output loading causes higher dynamic output currents and can, therefore, increase noise coupling into the part's analog front end. To drive higher loads, the use of an output buffer is recommended. When clocking output data from ADS5203, it is important to observe its timing relation to COUT. Please refer to the timing section for detailed information on the pipeline latency in the different modes. For safest system timing, COUT and COUT should be used to latch the output data, (see Figures 1 to 4). In Figure 4, COUT can be used by the receiving device to identify whether the data presently on the bus is from channel A or B.
Figure 10. Recommended External Decoupling for the Internal ADC Reference. External ADC references can also be chosen. The ADS5203 internal references must be disabled by tying PWDN_REF HIGH before applying the external reference sources to the REFT and REFB pins. The external reference voltages common-mode voltage should be 1.5V for best ADC performance.
LAYOUT, DECOUPLING, AND GROUNDING RULES
Proper grounding and layout of the PCB on which the ADS5203 is populated is essential to achieve the stated performance. It is advised to use separate analog and digital ground planes that are spliced underneath the IC. The ADS5203 has digital and analog pins on opposite sides of the package to make this easier. Since there is no connection internally between analog and digital grounds, they have to be joined on the PCB. It is advised to do this at one point in close proximity to the ADS5203.
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Figure 11. External ADC Reference Configuration.
ADS5203
www.ti.com SBAS258A - JUNE 2002 - REVISED JULY 2002
As for power supplies, separate analog and digital supply pins are provided on the part (AVDD/DVDD). The supply to the digital output drivers is kept separate as well (DRVDD). Lowering the voltage on this supply to 3.0V instead of the nominal 3.3V improves performance because of the lower switching noise caused by the output buffers. Due to the high sampling rate and switched-capacitor architecture, the ADS5203 generates transients on the supply and reference lines. Proper decoupling of these lines is, therefore, essential.
4. Analog Input Bandwidth--The analog input bandwidth is defined as the max. frequency of a 1dBFS input sine that can be applied to the device for which an extra 3dB attenuation is observed in the reconstructed output signal. 5. Output Timing--Output timing td(o) is measured from the 1.5V level of the CLK input falling edge to the 10%/90% level of the digital output. The digital output load is not higher than 10pF. Output hold time th(o) is measured from the 1.5V level of the COUT input rising edge to the 10%/90% level of the digital output. The digital output is load is not less than 2pF. Aperture delay td(A) is measured from the 1.5V level of the CLK input to the actual sampling instant. The OE signal is asynchronous. OE timing tdis is measured from the VIH(MIN) level of OE to the high-impedance state of the output data. The digital output load is not higher than 10pF. OE timing ten is measured from the VIL(MAX) level of OE to the instant when the output data reaches VOH(min) or VOL(max) output levels. The digital output load is not higher than 10pF. 6. Pipeline Delay (latency)--The number of clock cycles between conversion initiation on an input sample and the corresponding output data being made available from the ADC pipeline. Once the data pipeline is full, new valid output data is provided on every clock cycle. The first valid data is available on the output pins after the latency time plus the output delay time td(o) through the digital output buffers. Note that a minimum td(o) is not guaranteed because data can transition before or after a CLK edge. It is possible to use CLK for latching data, but at the risk of the prop delay varying over temperature, causing data to transition one CLK cycle earlier or later. The recommended method is to use the latch signals COUT and COUT which are designed to provide reliable setup and hold times with respect to the data out. 7. Wake-Up Time--Wake-up time is from the power-down state to accurate ADC samples being taken, and is specified for external reference sources applied to the device and an 80MHz clock applied at the time of release of STDBY. Cells that need to power up are the bandgap, bias generator, SHAs, and ADCs. 8. Power-Up Time--Power-up time is from the power-down state to accurate ADC samples being taken with an 80MHz clock applied at the time of release of STDBY. Cells that need to power up are the bandgap, internal reference circuit, bias generator, SHAs, and ADCs.
NOTES
1. Integral Nonlinearity (INL)--Integral nonlinearity refers to the deviation of each individual code from a line drawn from zero to full-scale. The point used as zero occurs 1/2LSB before the first code transition. The full-scale point is defined as a level 1/2LSB beyond the last code transition. The deviation is measured from the center of each particular code to the true straight line between these two endpoints. 2. Differential Nonlinearity (DNL)--An ideal ADC exhibits code transitions that are exactly 1LSB apart. DNL is the deviation from this ideal value. Therefore, this measure indicates how uniform the transfer function step sizes are. The ideal step size is defined here as the step size for the device under test (i.e., (last transition level - first transition level)/(2n - 2)). Using this definition for DNL separates the effects of gain and offset error. A minimum DNL better than -1LSB ensures no missing codes. 3. Zero and Full-Scale Error--Zero error is defined as the difference in analog input voltage--between the ideal voltage and the actual voltage--that will switch the ADC output from code 0 to code 1. The ideal voltage level is determined by adding the voltage corresponding to 1/2LSB to the bottom reference level. The voltage corresponding to 1LSB is found from the difference of top and bottom references divided by the number of ADC output levels (1024). Full-scale error is defined as the difference in analog input voltage--between the ideal voltage and the actual voltage--that will switch the ADC output from code 1022 to code 1023. The ideal voltage level is determined by subtracting the voltage corresponding to 1.5LSB from the top reference level. The voltage corresponding to 1LSB is found from the difference of top and bottom references divided by the number of ADC output levels (1024).
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ADS5203
www.ti.com SBAS258A - JUNE 2002 - REVISED JULY 2002
PACKAGE DRAWING
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