ADS8508IBDWR

          ADS8508 SLAS433 – SEPTEMBER 2005 12-BIT 250-KSPS SERIAL CMOS SAMPLING ANALOG-TO-DIGITAL CONVERTER FEATURES APPLICATIONS • • • • • • • • • • • • • • • • • • • • 250-kHz Sampling Rate 4-V, 5-V, 10-V, ±3.33-V, ±5-V, and ±10-V Input Ranges 73-dB SINAD With 45-kHz Input ±0.45 LSB Max INL ±0.45 LSB Max DNL, 12-Bits No Missing Codes ±1 LSB Bipolar Zero Errors ±0.4 PPM/°C Bipolar Zero Error Drift Six Specified Input Ranges SPI Compatible Serial Output with Daisy-Chain (TAG) Feature 5-V Supply Pin-Compatible With ADS7808 (Low Speed) and 16-Bit ADS8509/7809 Uses Internal or External Reference 70-mW Typ Power Dissipation at 250 KSPS 20-Pin SO Package Simple DSP Interface Industrial Process Control Data Acquisition Systems Digital Signal Processing Medical Equipment Instrumentation DESCRIPTION The ADS8508 is a complete 12-bit sampling analog-to-digital (A/D) converter using state-of-the-art CMOS structures. It contains a complete 12-bit, capacitor-based, successive approximation register (SAR) A/D converter with sample-and-hold, reference, clock, and a serial data interface. Data can be output using the internal clock or can be synchronized to an external data clock. The ADS8508 also provides an output synchronization pulse for ease of use with standard DSP processors. The ADS8508 is specified at a 250-kHz sampling rate over the full temperature range. Precision resistors provide various input ranges including ±10 V and 0 V to 5 V, while the innovative design allows operation from a single +5-V supply with power dissipation under 100 mW. The ADS8508 is available in a 20-pin SO package, fully specified for operation over the industrial -40°C to 85°C temperature range. Successive Approximation Register Clock CDAC 9.8 kΩ R1IN BUSY 4.9 kΩ R2IN 2.5 kΩ R3IN EXT/INT 10 kΩ Comparator CAP Buffer Internal +2.5 V Ref Serial Data Out & Control DATACLK DATA R/C SB/BTC CS PWRD 4 kΩ REF 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 the Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters. Copyright © 2005, Texas Instruments Incorporated ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 PACKAGE/ORDERING INFORMATION (1) PRODUCT MINIMUM RELATIVE ACCURACY (LSB) NO MISSING CODE MINIMUM SINAD (dB) SPECIFICATION TEMPERATURE RANGE PACKAGE LEAD PACKAGE DESIGNATOR ADS8508IB ±0.45 12 72 -40°C to 85°C SO-20 DW (1) ORDERING NUMBER ADS8508IBDW ADS8508IBDWR TRANSPORT MEDIA, QTY Tube, 25 Tape and Reel, 2000 For the most current package and ordering information, see the Package Option Addendum at the end of this document, or see the TI website at www.ti.com. ABSOLUTE MAXIMUM RATINGS over operating free-air temperature range (unless otherwise noted) (1) ADS8508 Analog inputs R1IN ±25 V R2IN ±25 V R3IN ±25 V CAP +VANA + 0.3 V to AGND2 - 0.3 V REF Indefinite short to AGND2, momentary short to VANA DGND, AGND2 Ground voltage differences ±0.3 V VANA 6V VDIG to VANA 0.3 V VDIG 6V Digital inputs -0.3 V to +VDIG + 0.3 V Maximum junction temperature 165°C Storage temperature range –65°C to 150°C Internal power dissipation 700 mW Lead temperature (soldering, 10s) (1) 260°C All voltage values are with respect to network ground terminal. ELECTRICAL CHARACTERISTICS At TA = -40°C to 85°C, fs = 250 kHz, VDIG = VANA = 5 V, using internal reference and fixed resistors (See Figure 28 and Figure 29) (unless otherwise specified) PARAMETER TEST CONDITIONS ADS8508IB MIN TYP Resolution MAX 12 UNIT Bits ANALOG INPUT Voltage ranges (1) Impedance (1) Capacitance 50 pF THROUGHPUT SPEED Conversion cycle Acquire and convert Throughput rate 4 250 µs kHz DC ACCURACY INL Integral linearity error -0.45 0.45 LSB (2) DNL Differential linearity error -0.45 0.45 LSB No missing codes Transition (1) (2) (3) 2 noise (3) ±10 V, 0 V to 5 V, etc. (see Table 3) LSB means least significant bit. For the ±10-V input range, one LSB is 4.88 mV. Typical rms noise at worst case transitions and temperatures. 12 Bits 0.1 LSB ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 ELECTRICAL CHARACTERISTICS (continued) At TA = -40°C to 85°C, fs = 250 kHz, VDIG = VANA = 5 V, using internal reference and fixed resistors (See Figure 28 and Figure 29) (unless otherwise specified) PARAMETER TEST CONDITIONS ±10 V range Full-scale error (4) (5) All other ranges Full-scale error drift MIN All other ranges Full-scale error drift Ext. Ref. with 0.1% external fixed resistors 0.5 0.5 ±7 0.5 -0.5 0.5 ±2 -1 -5 5 4 V and 5 V range -3 3 Power supply sensitivity (VDIG = VANA = VD) 1-µF Capacitor to CAP +4.75 V < VD < +5.25 V %FS mV ppm/°C 10 V range Recovery to rated accuracy after power down %FS ppm/°C 1 ±0.4 Unipolar zero error drift UNIT ppm/°C -0.5 Bipolar zero error drift Unipolar zero error (4) MAX -0.5 Ext. Ref. Bipolar zero error (4) TYP -0.5 Int. Ref. ±10 V range Full-scale error (4) (5) Int. Ref. with 0.1% external fixed resistors ADS8508IB mV ±2 ppm/°C 1 ms -0.5 0.5 LSB AC ACCURACY SFDR Spurious-free dynamic range fI = 45 kHz THD Total harmonic distortion fI = 45 kHz SINAD SNR Signal-to-(noise+distortion) Signal-to-noise ratio Full-power fI = 45 kHz 86 -95 72 –60-dB Input fI = 45 kHz 72 bandwidth (7) dB (6) 95 -86 dB 73 dB 32 dB 73 dB 500 kHz SAMPLING DYNAMICS Aperture delay Transient response 5 FS Step ns 2 Overvoltage recovery (8) 150 µs ns REFERENCE Internal reference voltage No load 2.48 Internal reference source current (must use external buffer) 2.52 1 Internal reference drift 2.3 Ext. 2.5-V Ref. 2.5 V µA 8 External reference voltage range for specified linearity External reference current drain 2.5 ppm/°C 2.7 V 100 µA DIGITAL INPUTS Logic levels VIL Low-level input voltage -0.3 0.8 V VIH High-level input voltage 2.0 VDIG +0.3 V V (4) (5) (6) (7) (8) As measured with circuit shown in Figure 28 and Figure 29. Adjustable to zero with external potentiometer. Factory calibrated with 0.1%, 0.25-W resistors. For bipolar input ranges, full-scale error is the worst case of -full-scale or +full-scale uncalibrated deviation from ideal first and last code transitions, divided by the transition voltage (not divided by the full-scale range) and includes the effect of offset error. For unipolar input ranges, full-scale error is the deviation of the last code transition divided by the transition voltage. It also includes the effect of offset error. All specifications in dB are referred to a full-scale ±10-V input. Full-power bandwidth is defined as the full-scale input frequency at which signal-to-(noise + distortion) degrades to 60 dB. Recovers to specified performance after 2 x FS input overvoltage. 3 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 ELECTRICAL CHARACTERISTICS (continued) At TA = -40°C to 85°C, fs = 250 kHz, VDIG = VANA = 5 V, using internal reference and fixed resistors (See Figure 28 and Figure 29) (unless otherwise specified) PARAMETER TEST CONDITIONS ADS8508IB MIN TYP MAX UNIT IIL Low-level input current VIL = 0 V ±10 µA IIH High-level input current VIH = 5 V ±10 µA DIGITAL OUTPUTS Data format (Serial 16-bits) Data coding (Binary 2's complement or straight binary) Pipeline delay (Conversion results only available after completed conversion.) Data clock (Selectable for internal or external data clock) Internal clock (output only when transmitting data) EXT/INT Low External clock (can run continually but not recommended for optimum performance) EXT/INT High VOL Low-level output voltage ISINK = 1.6 mA VOH High-level output voltage ISOURCE = 500 µA Leakage current Output capacitance 9 0.1 MHz 26 MHz 0.4 V Hi-Z state, VOUT = 0 V to VDIG ±5 µA Hi-Z state 15 pF V 4 V POWER SUPPLIES VDIG Digital input voltage VANA Analog input voltage IDIG Digital input current IANA Analog input current Must be ≤ VANA 4.75 5 5.25 4.75 5 5.25 V 4 mA 10 mA POWER DISSIPATION PWRD Low fS = 250 kHz 70 PWRD High 100 50 mW µW TEMPERATURE RANGE Specified performance -40 85 °C Derated performance (9) -55 125 °C Storage -65 150 °C THERMAL RESISTANCE (ΘJA) SO (9) 4 75 °C/W The internal reference may not be started correctly beyond the industrial temperature range (-40°C to 85°C), therefore use of an external reference is recommended. ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 TIMING REQUIREMENTS, TA = –40°C to 85°C PARAMETER MIN TYP MAX 6 20 ns 2.2 µs tw1 Pulse duration, convert td1 Delay time, BUSY from R/C low tw2 Pulse duration, BUSY low td2 Delay time, BUSY, after end of conversion 5 td3 Delay time, aperture 5 tconv Conversion time tacq Acquisition time tconv + tacq 40 UNIT ns ns ns 2.2 1.8 µs µs Cycle time 4 µs td4 Delay time, R/C Low to internal DATACLK output 270 ns tc1 Cycle time, internal DATACLK 110 ns td5 Delay time, data valid to internal DATACLK high 15 35 ns td6 Delay time, data valid after internal DATACLK low 20 35 ns tc2 Cycle time, external DATACLK 35 ns tw3 Pulse duration, external DATACLK high 15 ns tw4 Pulse duration, external DATACLK low 15 tsu1 Setup time, R/C rise/fall to external DATACLK high 15 tsu2 Setup time, R/C transition to CS transition 10 td7 Delay time, SYNC, after external DATACLK high 3 35 ns td8 Delay time, data valid 2 20 ns td9 Delay time, CS to rising edge td10 tsu3 td11 Delay time, final external DATACLK to BUSY falling edge tsu3 Setup time, TAG valid 0 ns th1 Hold time, TAG valid 2 ns ns tC2 + 5 ns ns 10 ns Delay time, previous data available after CS, R/C low 2 µs Setup time, BUSY transition to first external DATACLK 5 ns 1 µs DW PACKAGE (TOP VIEW) R1IN 1 AGND1 2 20 VDIG 19 VANA R2IN 3 R3IN 4 18 PWRD CAP 5 16 CS REF 6 15 R/C AGND2 7 14 TAG SB/BTC 8 13 DATA EXT/INT 9 12 DATACLK DGND 10 17 BUSY 11 SYNC 5 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Terminal Functions TERMINAL NAME DESCRIPTION NO. I/O AGND1 2 – Analog ground. Used internally as ground reference point. Minimal current flow. AGND2 7 – Analog ground BUSY 17 O Busy output. Falls when a conversion is started, and remains low until the conversion is completed and the data is latched into the output shift register. CAP 5 – Reference buffer capacitor. 2.2-µF Tantalum to ground. CS 16 – Chip select. Internally ORed with R/C DATA 13 O Serial data output. Data is synchronized to DATACLK, with the format determined by the level of SB/BTC. In the external clock mode, after 16 bits of data, the ADS8508 outputs the level input on TAG as long as CS is low and R/C is high (see Figure 8 and Figure 9). If EXT/INT is low, data is valid on both the rising and falling edges of DATACLK, and between conversions DATA stays at the level of the TAG input when the conversion was started. DATACLK 12 I/O Either an input or an output depending on the EXT/INT level. Output data is synchronized to this clock. If EXT/INT is low, DATACLK transmits 16 pulses after each conversion, and then remains low between conversions. DGND 10 – Digital ground EXT/INT 9 – Selects external or internal clock for transmitting data. If high, data is output synchronized to the clock input on DATACLK. If low, a convert command initiates the transmission of the data from the previous conversion, along with 16-clock pulses output on DATACLK. NC – – No connect PWRD 18 I Power down input. If high, conversions are inhibited and power consumption is significantly reduced. Results from the previous conversion are maintained in the output shift register. R/C 15 I Read/convert input. With CS low, a falling edge on R/C puts the internal sample-and-hold into the hold state and starts a conversion. When EXT/INT is low, this also initiates the transmission of the data results from the previous conversion. If EXT/INT is high, a rising edge on R/C with CS low, or a falling edge on CS with R/C high, transmits a pulse on SYNC and initiates the transmission of data from the previous conversion. REF 6 I/O R1IN 1 I Analog input. See Table 3 for input range connections. R2IN 3 I Analog input. See Table 3 for input range connections. R3IN 4 I Analog input. See Table 3 for input range connections. SB/BTC 8 O Select straight binary or binary 2's complement data output format. If high, data is output in a straight binary format. If low, data is output in a binary 2's complement format. SYNC 11 O Sync output. This pin is used to supply a data synchronization pulse when the EXT level is high and at least one external clock pulse has occured when not in the read mode. See the external clock modes desciptions. TAG 14 I Tag input for use in the external clock mode. If EXT is high, digital data input from TAG is output on DATA with a delay that is dependent on the external clock mode. See Figure 8 and Figure 9. VANA 19 I Analog supply input. Nominally +5 V. Connect directly to pin 20, and decouple to ground with 0.1-µF ceramic and 10-µF tantalum capacitors. VDIG 20 I Digital supply input. Nominally +5 V. Connect directly to pin 19. Must be ≤ VANA. 6 Reference input/output. Outputs internal 2.5-V reference. Can also be driven by external system reference. In both cases, bypass to ground with a 2.2-µF tantalum capacitor. ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 PARAMETER MEASUREMENT INFORMATION CS R/C R/C CS tsu1 tsu1 tsu1 External DATACLK tsu1 External DATACLK CS Set Low, Discontinuous Ext DATACLK R/C Set Low, Discontinuous Ext DATACLK BUSY CS tsu2 tsu2 tsu3 1 External DATACLK R/C 2 CS Set Low, Discontinuous Ext DATACLK Figure 1. Critical Timing tw1 tw1 R/C td1 td1 tw2 tw2 BUSY td2 td3 STATUS Nth Conversion Error Correction tconv td4 td2 td3 td11 (N+1)th Accquisition td11 Error (N+1)th Conversion Correction tconv tacq tc1 (N+2)th Accquisition tacq td4 Internal 1 DATACLK TAG = 0 12 1 2 12 td6 td5 DATA 2 D11 D0 (N−1)th Conversion Data CS, EXT/INT, and TAG are tied low TAG = 0 D11 D0 TAG = 0 Nth Conversion Data 8 starts READ Figure 2. Basic Conversion Timing - Internal DATACLK (Read Previous Data During Conversion) 7 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 PARAMETER MEASUREMENT INFORMATION (continued) tw1 tw1 R/C td1 td1 tw2 tw2 BUSY td2 td3 STATUS Error Correction Nth Conversion td2 td3 td11 td11 (N+1)th Accquisition (N+1)th Conversion tacq tconv (N+2)th Accquisition tacq tconv tsu3 tsu1 Error Correction tsu3 tsu1 External 1 DATACLK 12 No more data to shift out DATA TAG = 0 2 1 TAG = 0 1 12 Nth Data EXT/INT tied high, CS and TAG are tied low 12 No more data to shift out TAG = 0 2 1 TAG = 0 12 (N+1)th Data TAG = 0 tw1 + tsu1 starts READ Figure 3. Basic Conversion Timing - External DATACLK tw1 R/C td1 tsu1 tw2 td1 BUSY td2 td3 STATUS td3 td11 Nth Conversion Error Correction (N+1) th Accquisition tsu3 tconv tacq tc2 External DATACLK tsu1 tw4 tw3 0 1 2 3 4 5 7 8 9 10 11 12 SYNC = 0 td8 T00 EXT/INT tied high, CS tied low D10 D09 D08 D07 D06 D04 D03 D02 D01 D00 Null T00 Txx T02 T03 T04 T05 T06 T08 T09 T10 T11 T12 Null T17 Tyy th1 tsu3 TAG td8 Nth Conversion Data D11 DATA T01 tw1 + tsu1 starts READ Figure 4. Read After Conversion (Discontinuous External DATACLK) 8 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 PARAMETER MEASUREMENT INFORMATION (continued) tw1 R/C td1 tw2 BUSY td10 td3 td2 Error Correction Nth Conversion STATUS tsu3 tconv tc2 tsu1 External tw3 td11 tw4 1 0 DATACLK 2 3 4 5 8 9 10 11 12 SYNC = 0 td8 Nth Conversion Data D10 D11 DATA D09 D08 D07 D06 D04 D03 D02 td8 D01 D00 Rising DATACLK change DATA, tw1 + tsu1 Starts READ TAG is not recommended for this mode. There is not enough time to do so without violating td11. EXT/INT tied high, CS and TAG tied low Figure 5. Read During Conversion (Discontinuous External DATACLK) tw1 R/C td1 tsu1 td1 tsu1 tw2 BUSY td2 td3 Error Nth Conversion Correction STATUS td3 td11 (N+1)th Accquisition tconv tacq tc2 tsu3 tsu1 External DATACLK 0 2 1 tsu1 tw4 tw3 3 4 5 6 7 9 10 11 12 13 14 tc2 td7 SYNC =0 td8 Nth Conversion Data D11 DATA T00 EXT/INT tied high, CS tied low D09 D08 D07 D06 D04 D03 D02 D01 D00 Null T02 T03 T04 T05 T06 T12 T13 T14 T15 T16 T17 T00 Txx th1 tsu3 TAG td8 D10 T01 Tyy tw1 + tsu1 starts READ Figure 6. Read After Conversion With SYNC (Discontinuous External DATACLK) 9 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 PARAMETER MEASUREMENT INFORMATION (continued) tw1 R/C td1 tw2 BUSY td3 td10 td2 Error Correction Nth Conversion STATUS tsu3 tconv tsu1 tsu1 External tw3 tsu1 0 DATACLK tc2 tw4 1 2 3 td11 4 5 6 7 10 11 12 13 14 td7 tc2 SYNC = 0 td8 EXT/INT tied high, CS and TAG tied low td8 Nth Conversion Data D11 DATA D10 D09 D08 D07 D06 D04 D03 D02 D01 D00 tw1 + tsu1 Starts READ TAG is not recommended for this mode. There is not enough time to do so without violating td11. Figure 7. Read During Conversion With SYNC (Discontinuous External DATACLK) 10 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Tag 14 Tag 13 Tag 12 Tag 11 Tag 1 TAG Tag 2 Bit 11 (MSB) DATA Tag 0 t c2 t su2 t su1 0 SYNC BUSY R/C CS External DATACLK t w1 t su2 t d1 t w3 t d7 1 t c2 t w4 2 t d8 3 Bit 10 4 Bit 1 13 14 Bit 0 (LSB) Tag 0 t d9 Tag 1 Tag 15 PARAMETER MEASUREMENT INFORMATION (continued) Figure 8. Conversion and Read Timing with Continuous External DATACLK (EXT/INT Tied High) Read After Conversions (Not Recommended) 11 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Tag 14 Tag 13 Tag 1 TAG Tag 12 Bit 15 (MSB) DATA SYNC t d1 BUSY R/C CS External DATACLK t su2 t w3 t c2 t w1 t w4 t su1 t su1 Tag 0 t c2 t d8 t d10 Bit 0 (LSB) Tag 0 t d8 Tag 1 Tag 15 PARAMETER MEASUREMENT INFORMATION (continued) Figure 9. Conversion and Read Timing with Continuous External DATACLK (EXT/INT Tied High) Read Previous Conversion Result During Conversion (Not Recommended) 12 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 TYPICAL CHARACTERISTICS TOTAL HARMONIC DISTORTION vs FREE-AIR TEMPERATURE 110 90 80 70 −20 0 20 40 60 −95 −90 −85 −80 −70 −40 80 70 60 50 −75 TA − Free-Air-Temperature −
C fi = 45 kHz −20 0 20 40 60 40 −40 80 −20 0 20 40 60 80 TA − Free-Air-Temperature −
C TA − Free-Air-Temperature −
C Figure 10. Figure 11. Figure 12. SIGNAL-TO-NOISE AND DISTORTION vs FREE-AIR TEMPERATURE SIGNAL-TO-NOISE RATIO vs INPUT FREQUENCY SIGNAL-TO-NOISE AND DISTORTION vs INPUT FREQUENCY 70 60 50 SINAD − Signal-to-Noise and Distortion − dB 80 80 SNR − Signal-to-Noise − dB SINAD − Signal-to-Noise and Distortion − dB 80 SNR − Signal-to-Noise − dB 100 −40 75 70 65 60 55 fi = 45 kHz 40 −40 50 −20 0 20 40 60 80 1 10 100 1000 80 75 70 65 60 55 50 1 fi − Input Frequency − kHz TA − Free-Air-Temperature −
C 10 100 1000 fi − Input Frequency − kHz Figure 13. Figure 14. Figure 15. SPURIOUS FREE DYNAMIC RANGE vs INPUT FREQUENCY TOTAL HARMONIC DISTORTION vs INPUT FREQUENCY BIPOLAR ZERO SCALE ERROR vs FREE-AIR TEMPERATURE 90 80 70 60 50 −100 5 −90 4 Bipolar Zero Scale Error − mV 100 THD − Total Harmonic Distortion − dB SFDR − Spurious Free Dynamic Range − dB SIGNAL-TO-NOISE RATIO vs FREE-AIR TEMPERATURE −100 fi = 45 kHz THD − Total Harmonic Distortion − dB SFDR − Spurious Free Dynamic Range − dB SPURIOUS FREE DYNAMIC RANGE vs FREE-AIR TEMPERATURE −80 −70 −60 −50 −40 −30 −20 −10 10 100 fi − Input Frequency − kHz Figure 16. 1000 2 1 0 −1 −2 −3 −4 0 1 External Reference, ±10-V Range 3 1 10 100 fi − Input Frequency − kHz Figure 17. 1000 −5 −40 −25 −10 5 20 35 50 65 80 TA − Free-Air Temperature −
C Figure 18. 13 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 TYPICAL CHARACTERISTICS (continued) INTERNAL REFERENCE VOLTAGE vs FREE-AIR TEMPERATURE 0.20 0.10 Internal Reference Voltage − V Full Scale Error − %FSR 0.15 External Reference, ±10 V Range for 5 Representative Parts 0.05 0 −0.05 −0.10 −0.15 −0.20 −40 −25 −10 5 20 35 50 65 80 TA − Free-Air Temperature −
C SUPPLY CURRENT vs FREE-AIR TEMPERATURE 2.510 20 2.508 19 2.506 18 2.504 17 Supply Current − mA FULL SCALE ERROR vs FREE-AIR TEMPERATURE 2.502 2.500 2.498 2.496 2.494 16 15 14 13 12 2.492 11 2.490 −55 −35 −15 5 10 −40 −25 −10 5 25 45 65 85 105 Figure 19. 20 Figure 20. Figure 21. INL 0.5 fs = 250 KSPS 0.4 0.3 INL − LSBs 0.2 0.1 0 −0.1 −0.2 −0.3 −0.4 −0.5 0 512 1024 1536 2048 2560 3072 3584 4096 3072 3584 4096 Code (Binary 2’s Complement in Decimal) Figure 22. DNL 0.5 fs = 250 KSPS 0.4 0.3 DNL − LSBs 0.2 0.1 0 −0.1 −0.2 −0.3 −0.4 −0.5 0 512 1024 1536 2048 2560 Code (Binary 2’s Complement in Decimal) Figure 23. 14 35 50 65 TA − Free-Air Temperature −
C TA − Free-Air Temperature −
C 80 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 TYPICAL CHARACTERISTICS (continued) FFT (1 kHz Input) 20 8192 Points fs = 250 KSPS fi = 1 kHz, 0dB SINAD = 73.47 dB THD = −94.03 dB 0 Amplitude − dB −20 −40 −60 −80 −100 −120 −140 −160 0 25 50 75 100 125 75 100 125 f − Frequency − Hz Figure 24. FFT (45 kHz Input) 20 8192 Points fs = 250 KSPS fi = 45 kHz, 0dB SINAD = 73.62 dB THD = −94.03 dB 0 Amplitude − dB −20 −40 −60 −80 −100 −120 −140 −160 0 25 50 f − Frequency − Hz Figure 25. 15 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 BASIC OPERATION Two signals control conversion in the ADS8508: CS and R/C. These two signals are internally ORed together. To start a conversion the chip must be selected, CS low, and the conversion signal must be active, R/C low. Either signal can be brought low first. Conversion starts on the falling edge of the second signal. BUSY goes low when conversion starts and returns high after the data from that conversion is shifted into the internal storage register. Sampling begins when BUSY goes high. To reduce the number of control pins CS can be tied low permanently. The R/C pin now controls conversion and data reading exclusively. In the external clock mode this means that the ADS8508 will clock out data whenever R/C is brought high and the external clock is active. In the internal clock mode data is clocked out every convert cycle regardless of the states of CS and R/C. The ADS8508 provides a TAG input for cascading multiple converters together. READING DATA The conversion result is available as soon as BUSY returns to high therefore, data always represents the conversion previously completed even when it is read during a conversion. The ADS8508 outputs serial data in either straight binary or binary two’s compliment format. The SB/BTC pin controls the format. Data is shifted out MSB first. The first conversion immediately following a power-up will not produce a valid conversion result. Data can be clocked out with either the internally generated clock or with an external clock. The EXT/INT pin controls this function. If external clock is used the TAG input can be used to daisy-chain multiple ADS8508 data pins together. INTERNAL DATACLK In the internal clock mode data for the previous conversion is clocked out during each conversion period. The internal data clock is synchronized to the internal conversion clock so that is does not interfere with the conversion process. The DATACLK pin becomes an output when EXT/INT is low. 12 clock pulses are generated at the beginning of each conversion after timing t8 is satisfied, i.e. you can only read previous conversion result during conversion. DATACLK returns to low when it is inactive. The 12 bits of serial data are shifted out the DATA pin synchronous to this clock with each bit available on a rising and then a falling edge. DATA pin returns to the state of TAG pin input sensed at the start of transmission. EXTERNAL DATACLK The external clock mode offers several ways to retrieve conversion results. However, since the external clock cannot be synchronized to the internal conversion clock care must be taken to avoid corrupting the data. When EXT/INT is set high, the R/C and CS signals control the read state. When the read state is initiated the result from the previously completed conversion is shifted out the DATA pin synchronous to the external clock that is connected to the DATACLK pin. Each bit is available on a falling and then a rising edge. The maximum external clock speed of 28.5 MHz allows data shifted out quickly either at the beginning of conversion or the beginning of sampling. There are several modes of operation available when using an external clock. It is recommended that the external clock run only while reading data. This is the discontinuous clock mode. Since the external clock is not synchronized to the internal clock that controls conversion slight changes in the external clock can cause conflicts that can corrupt the conversion process. Specifications with a continuously running external clock cannot be guaranteed. It is especially important that the external clock does not run during the second half of the conversion cycle (approximately the time period specified by td11, see timing table). In the discontinuous clock mode data can be read during conversion or during sampling, with or without a SYNC pulse. Data read during a conversion must meet the td11 timing specification. Data read during sampling must be complete before starting a conversion. 16 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Whether reading during sampling or during conversion a SYNC pulse is generated whenever at least one rising edge of the external clock occurs while the part is not in the read state. In the discontinuous external clock with SYNC mode a SYNC pulse follows the first rising edge after the read command. The data is shifted out after the SYNC pulse. The first rising clock edge after the read command generates a SYNC pulse. The SYNC pulse can be detected on the next falling edge and then the next rising edge. Successively, each bit can be read first on the falling edge and then on the next rising edge. Thus 13 clock pulses after the read command are required to read on the falling edge, and 14 clock pulses are necessary to read on the rising edge. Table 2. DATACLK Pulses DESCRIPTION DATACLK PULSES REQUIRED WITH SYNC WITHOUT SYNC Read on falling edge of DATACLK 13 12 Read on rising edge of DATACLK 14 13 If the clock is entirely inactive when not in the read state no SYNC pulse is generated. In this case the first rising clock edge shifts out the MSB. The MSB can be read on the first falling edge or on the next rising edge. In this discontinuous external clock mode with no SYNC, 12 clocks are necessary to read the data on the falling edge and 13 clocks for reading on the rising edge. Data always represents the conversion already completed. TAG FEATURE The TAG feature allows the data from multiple ADS8508 converters to be read on a single serial line. The converters are cascaded together using the DATA pins as outputs and the TAG pins as inputs as illustrated in Figure 26. The DATA pin of the last converter drives the processor's serial data input. Data is then shifted through each converter, synchronous to the externally supplied data clock, onto the serial data line. The internal clock cannot be used for this configuration. The preferred timing uses the discontinuous, external, data clock during the sampling period. Data must be read during the sampling period because there is not sufficient time to read data from multiple converters during a conversion period without violating the td11 constraint (see the EXTERNAL DATACLOCK section). The sampling period must be sufficiently long to allow all data words to be read before starting a new conversion. Note, in Figure 26, that a NULL bit separates the data word from each converter. The state of the DATA pin at the end of a READ cycle reflects the state of the TAG pin at the start of the cycle. This is true in all READ modes, including the internal clock mode. For example, when a single converter is used in the internal clock mode the state of the TAG pin determines the state of the DATA pin after all 12 bits have shifted out. When multiple converters are cascaded together this state forms the NULL bit that separates the words. Thus, with the TAG pin of the first converter grounded as shown in Figure 26 the NULL bit becomes a zero between each data word. 17 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Processor ADS8508A DATA CS R/C DATACLK TAG SCLK ADS 8508B TAG(A) DATA CS R/C DATACLK TAG TAG(B) GPIO GPIO SDI Null D A00 Q D Q D Null D A11 Q D Q D B00 DATA (A) A12 Q D Q D B11 Q B12 DATA (B) Q DATACLK R/C (both A & B) BUSY (both A & B) SYNC (both A & B) External DATACLK 1 2 3 4 12 13 DATA ( A ) A11 A10 A09 A01 A00 DATA ( B ) B11 B10 B09 B01 B00 EXT/INT tied high, CS of both converter A and B, TAG input of converter A are tied low. 14 15 16 17 Null TAG(A) = 0 A Nth Conversion Data Null A11 A10 A09 B 30 A01 31 32 A00 Null A TAG(A) = 0 . Figure 26. Timing of TAG Feature With Single Conversion (Using External DATACLK) ANALOG INPUTS The ADS8508 has six analog input ranges as shown in Table 3. The offset and gain specifications are factory calibrated with 0.1%, ¼-W, external resistors as shown in Figure 28 and Figure 29. The external resistors can be omitted if larger gain and offset errors are acceptable or if using software calibration. The hardware trim circuitry shown in Figure 28 and Figure 29 can reduce the errors to zero. The analog input pins R1IN, R2IN, and R3IN have ±25-V overvoltage protection. The input signal must be referenced to AGND1. This will minimized the ground loop problem typical to analog designs. The analog input should be driven by a low impedance source. A typical driving circuit using OPA627 or OPA132 is shown in Figure 27. The ADS8508 can operate with its internal 2.5-V reference or an external reference. An external reference connected to pin 6 (REF) bypasses the internal reference. The external reference must drive the 4-kΩ resistor that separates pin 6 from the internal reference (see the illustration on page 1). The load will vary with the difference between the internal and external reference voltages. The external reference voltage can vary from 2.3 V to 2.7 V. The internal reference will be approximately 2.5 V. The reference, whether internal or external, is buffered internally with a buffer with its output on pin 5 (CAP). The ADS8508 is factory tested with 2.2-µF capacitors connected to pins 5 and 6 (CAP and REF). Each capacitor should be placed as close as possible to its pin. The capacitor on pin 6 band limits the internal reference noise. A smaller capacitor can be used but it may degrade SNR and SINAD The capacitor on pin 5 stabilizes the reference buffer and provides switching charge to the CDAC during conversion. Capacitors smaller than 1 µF can cause the buffer to become unstable may not hold sufficient charge for the CDAC. The parts are tested to specifications with 2.2 µF so larger capacitors are not necessary. The ESR (equivalent series resistance) of these compensation capacitors is also critical. Keep the total ESR under 3 Ω. See the TYPICAL CHARACTERISTICS section for how the performance is affected by ESR. Neither the internal reference nor the buffer should be used to drive an external load. Such loading can degrade performance. Any load on the internal reference causes a voltage drop across the 4-kΩ resistor and will affect gain. The internal buffer is capable of driving ±2-mA loads but any load can cause perturbations of the reference at the CDAC, degrading performance. It should be pointed out that, unlike other competitor’s parts with similar input structure, the ADS8508 does not require a second high speed amplifier used as buffer to isolate the CAP pin from the signal dependent current in the R3IN pin but can tolerate it if one do exist. 18 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 The external reference voltage can vary from 2.3 V to 2.7 V. The reference voltage determines the size of the least significant bit (LSB). The larger reference voltages produce a larger LSB, which can improve SNR. Smaller reference voltages can degrade SNR. +15V 2.2 F 22 pF ADS8508 200
100 nF GND R1IN 1 k
Pin 7 1 k
Pin 2 Vin 22 pF 10 pF Pin 1 AGND 100
Pin5 − OPA 627 Pin 6 + Pin3 R2IN GND 33.2 k
GND R3IN Pin4 CAP 2.2 F GND AGND2 2.2 F GND 100 nF 2.2 F −15 V GND Figure 27. Typical Driving Circuitry (±10 V, No Trim) Table 3. Input Range Connections (see Figure 28 and Figure 29 for complete information) ANALOG INPUT RANGE CONNECT R1IN VIA 200 Ω TO CONNECT R2IN VIA 100 Ω TO CONNECT R3 TO IMPEDANCE ±10 V VIN AGND CAP 11.5 kΩ ±5 V AGND VIN CAP 6.7 kΩ ±3.33 V VIN VIN CAP 5.4 kΩ 0 V to 10 V AGND VIN AGND 6.7 kΩ 0 V to 5 V AGND AGND VIN 5.0 kΩ 0 V to 4 V VIN AGND VIN 5.4 kΩ Table 4. Control Truth Table SPECIFIC FUNCTION CS R/C BUSY EXT/INT DATACLK PWRD SB/BTC OPERATION Initiate conversion and output data using internal clock 1>0 0 1 0 Output 0 x 0 1>0 1 0 Output 0 x Initiates conversion n. Data from conversion n - 1 clocked out on DATA synchronized to 16 clock pulses output on DATACLK. Initiate conversion and output data using external clock 1>0 0 1 1 Input 0 x Initiates conversion n. 0 1>0 1 1 Input 0 x Initiates conversion n. 1>0 1 1 1 Input x x Outputs a pulse on SYNC followed by data from conversion n clocked out synchronized to external DATACLK. 1>0 1 0 1 Input 0 x 0 0>1 0 1 Input 0 x Outputs a pulse on SYNC followed by data from conversion n - 1 clocked out synchronized to external DATACLK. (1) Conversion n in process. Incorrect conversions 0 0 0>1 x x 0 x CS or R/C must be HIGH or a new conversion will be initiated without time for acquisition. Power down x x x x x 0 x Analog circuitry powered. Conversion can proceed.. x x x x x 1 x Analog circuitry disabled. Data from previous conversion maintained in output registers. (1) See Figure XXX for the constraints on previous data valid during a conversion. 19 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Table 4. Control Truth Table (continued) Selecting output format x x x x x x 0 Serial data is output in binary 2s complement format. x x x x x x 1 Serial data is output in straight binary format. Table 5. Output Codes and Ideal Input Voltages DIGITAL OUTPUT DESCRIPTION Full-scale range BINARY 2's COMPLEMENTS (SB/BTC LOW) ANALOG INPUT BINARY CODE HEX CODE BINARY CODE HEX CODE 3.99902 V 0111 1111 1111 7FF 1111 1111 1111 FFF 2V 0000 0000 0000 000 1000 0000 0000 800 4.99756 V 1.99902 V 1111 1111 1111 FFF 0111 1111 1111 7FF 0V 0V 1000 0000 0000 800 0000 0000 0000 000 ±10 ±5 ±3.33 V 0 V to 5 V 0 V to 10 V 0 V to 4 V Least significant bit (LSB) 4.88 mV 2.44 mV 1.63 mV 1.22 mV 2.44 mV 0.98 mV Full scale (FS - 1LSB) 9.99512 V 4.997567 V 3.33171 V 4.99878 V 9.99756 V Midscale 0V 0V 0V 2.5 V 5V One LSB below midscale -4.88 mV -2.44 mV -1.63 mV 2.49878 V -Full scale -10 V -5 V -3.333333 V 0V 20 STRAIGHT BINARY (SB/BTC HIGH) ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Input Range With Trim (Adjust Offset First at 0 V, Then Adjust Gain) Without Trim 200 Ω 200 Ω R1IN R1IN AGND1 AGND1 100 Ω 100 Ω 0 V − 10 V VIN R2IN 33.2 kΩ R3IN R2IN VIN 33.2 kΩ 2.2 µF +5V CAP + 50 kΩ 2.2 µF + R3IN 2.2 µF + +5V CAP 576 kΩ 50 kΩ REF REF + 2.2 µF AGND2 200 Ω AGND2 200 Ω R1IN R1IN AGND1 100 Ω R2IN 33.2 kΩ VIN +5 V CAP 2.2 µF + +5 V 50 kΩ 2.2 µF REF 2.2 µF R2IN 33.2 kΩ R3IN VIN 0V−5V AGND1 100 Ω + + R3IN CAP 576 kΩ 50 kΩ 2.2 µF AGND2 200 Ω R1IN VIN AGND1 100 Ω 100 Ω R2IN R2IN R3IN R3IN 33.2 kΩ +5 V + 33.2 kΩ +5 V CAP 2.2 µF AGND2 R1IN AGND1 2.2 µF REF 200 Ω VIN 0V−4V + + REF 2.2 µF + 576 kΩ 50 kΩ 50 kΩ 2.2 µF AGND2 CAP REF + AGND2 Figure 28. Offset/Gain Circuits for Unipolar Input Ranges 21 ADS8508 www.ti.com SLAS433 – SEPTEMBER 2005 Input Range With Trim (Adjust Offset First at 0 V, Then Adjust Gain) Without Trim 200 Ω VIN 200 Ω R1IN R1IN VIN AGND1 AGND1 100 Ω 100 Ω R2IN ±10 V R2IN +5 V R3IN 33.2 kΩ + 2.2 F + R3IN +5 V 50 kΩ CAP 2.2 F 33.2 kΩ REF 2.2 µF 576 kΩ + 2.2 µF + CAP REF 50 kΩ AGND2 AGND2 200 Ω 200 Ω R1IN R1IN AGND1 AGND1 100 Ω VIN 33.2 kΩ ±5V R3IN CAP 50 kΩ 2.2 µF + 2.2 µF R2IN R3IN + 2.2 µF +5 V +5 V + 100 Ω VIN 33.2 kΩ R2IN CAP 576 kΩ 50 kΩ REF REF + 2.2 µF AGND2 200 Ω AGND2 200 Ω VIN R1IN 100 Ω R1IN VIN 100 Ω AGND1 AGND1 R2IN R2IN R3IN 33.2 kΩ ±3.3 V 33.2 kΩ 2.2 µF +5 V CAP + + REF 2.2 F AGND2 CAP +5 V 50 kΩ 2.2 F 576 kΩ 50 kΩ + 2.2 µF Figure 29. Offset/Gain Circuits for Bipolar Input Ranges 22 R3IN + REF AGND2 PACKAGE OPTION ADDENDUM www.ti.com 10-Dec-2020 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan (2) Lead finish/ Ball material MSL Peak Temp Op Temp (°C) Device Marking (3) (4/5) (6) ADS8508IBDW ACTIVE SOIC DW 20 25 RoHS & Green NIPDAU Level-2-260C-1 YEAR -40 to 85 ADS8508IB ADS8508IBDWR ACTIVE SOIC DW 20 2000 RoHS & Green NIPDAU Level-2-260C-1 YEAR -40 to 85 ADS8508IB (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of