ADS8341 ADS8 341 SBAS136D – SEPTEMBER 2000 – APRIL 2003 16-Bit, 4-Channel Serial Output Sampling ANALOG-TO-DIGITAL CONVERTER FEATURES DESCRIPTION ● PIN FOR PIN WITH ADS7841 The ADS8341 is a 4-channel, 16-bit sampling Analog-toDigital (A/D) converter with a synchronous serial interface. Typical power dissipation is 8mW at a 100kHz throughput rate and a +5V supply. The reference voltage (VREF) can be varied between 500mV and VCC, providing a corresponding input voltage range of 0V to VREF. The device includes a shutdown mode that reduces power dissipation to under 15µW. The ADS8341 is tested down to 2.7V operation. ● SINGLE SUPPLY: 2.7V to 5V ● 4-CHANNEL SINGLE-ENDED OR 2-CHANNEL DIFFERENTIAL INPUT ● UP TO 100kHz CONVERSION RATE ● 86dB SINAD ● SERIAL INTERFACE ● SSOP-16 PACKAGE Low power, high speed, and an onboard multiplexer make the ADS8341 ideal for battery-operated systems such as personal digital assistants, portable multi-channel data loggers, and measurement equipment. The serial interface also provides low-cost isolation for remote data acquisition. The ADS8341 is available in an SSOP-16 package and is ensured over the –40°C to +85°C temperature range. APPLICATIONS ● ● ● ● ● DATA ACQUISITION TEST AND MEASUREMENT INDUSTRIAL PROCESS CONTROL PERSONAL DIGITAL ASSISTANTS BATTERY-POWERED SYSTEMS SAR DCLK CS CH0 CH1 CH2 Comparator Four Channel Multiplexer Serial Interface and Control CDAC CH3 SHDN DIN DOUT BUSY COM VREF 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. Copyright © 2000-2003, Texas Instruments Incorporated 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. www.ti.com ABSOLUTE MAXIMUM RATINGS(1) PIN CONFIGURATIONS +VCC to GND ........................................................................ –0.3V to +6V Analog Inputs to GND ............................................ –0.3V to +VCC + 0.3V Digital Inputs to GND ........................................................... –0.3V to +6V Power Dissipation .......................................................................... 250mW Maximum Junction Temperature ................................................... +150°C Operating Temperature Range ........................................ –40°C to +85°C Storage Temperature Range ......................................... –65°C to +150°C Lead Temperature (soldering, 10s) ............................................... +300°C Top View NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. Exposure to absolute maximum conditions for extended periods may affect device reliability. ELECTROSTATIC DISCHARGE SENSITIVITY 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. SSOP +VCC 1 16 DCLK CH0 2 15 CS CH1 3 14 DIN CH2 4 13 BUSY 12 DOUT ADS8341 CH3 5 COM 6 11 GND SHDN 7 10 GND VREF 8 9 +VCC 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. PIN DESCRIPTIONS PIN NAME DESCRIPTION 1 2 3 4 5 6 +VCC CH0 CH1 CH2 CH3 COM 7 8 9 10 11 12 13 14 15 16 SHDN VREF +VCC GND GND DOUT BUSY DIN CS DCLK Power Supply, 2.7V to 5V Analog Input Channel 0 Analog Input Channel 1 Analog Input Channel 2 Analog Input Channel 3 Ground Reference for Analog Inputs. Sets zero code voltage in single-ended mode. Connect this pin to ground or ground reference point. Shutdown. When LOW, the device enters a very low power shutdown mode. Voltage Reference Input. See Electrical Characteristics Table for ranges. Power Supply, 2.7V to 5V Ground. Connect to Analog Ground Ground. Connect to Analog Ground. Serial Data Output. Data is shifted on the falling edge of DCLK. This output is high impedance when CS is HIGH. Busy Output. This output is high impedance when CS is HIGH. Serial Data Input. If CS is LOW, data is latched on rising edge of DCLK. Chip Select Input. Controls conversion timing and enables the serial input/output register. External Clock Input. This clock runs the SAR conversion process and synchronizes serial data I/O. Maximum input clock frequency equals 2.4MHz to achieve 100kHz sampling rate. PACKAGE/ORDERING INFORMATION PRODUCT ADS8341E " ADS8341EB " MAXIMUM INTEGRAL LINEARITY ERROR (LSB) NO MISSING CODES ERROR (LSB) SPECIFICATION TEMPERATURE RANGE PACKAGE PACKAGE DESIGNATOR(1) ORDERING NUMBER TRANSPORT MEDIA 8 " 6 " 14 " 15 " –40°C to +85°C " –40°C to +85°C " SSOP-16 " SSOP-16 " DBQ " DBQ " ADS8341E ADS8341E/2K5 ADS8341EB ADS8341EB/2K5 Rails Tape and Reel Rails Tape and Reel NOTE: (1) For the most current specifications and package information, refer to our web site www.ti.com. 2 ADS8341 SBAS136D ELECTRICAL CHARACTERISTICS: +5V At TA = –40°C to +85°C, +VCC = +5V, VREF = +5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. ADS8341E, P PARAMETER CONDITIONS MIN TYP MAX RESOLUTION ANALOG INPUT Full-Scale Input Span Absolute Input Range Positive Input - Negative Input Positive Input Negative Input 0 –0.2 –0.2 POWER SUPPLY REQUIREMENTS +VCC Quiescent Current 1.0 20 3 +4.75V < VCC < 5.25V ±8 ±2 4.0 ±0.05 4.0 ✻ ✻ ✻ ✻ = = = = 5Vp-p 5Vp-p 5Vp-p 5Vp-p at at at at 0.024 0 10kHz 10kHz 10kHz 50kHz 2.4 2.4 0.5 ✻ ✻ +VCC 5 40 2.5 0.001 3.0 –0.3 3.5 100 3 1.5 300 7.5 –40 Clk Cycles Clk Cycles kHz ns ns ps MHz MHz MHz ✻ ✻ V GΩ µA µA µA ✻ 5.5 +0.8 4.75 Bits LSB mV LSB(1) % LSB µVrms LSB(1) dB dB dB dB ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ ✻ 0.4 Power Dissipation ±6 ±1 ✻ ±0.024 ✻ ✻ ✻ ✻ V V V V ✻ Straight Binary Specified Performance V V V pF µA ✻ ✻ ✻ ✻ CMOS | IIH | ≤ +5µA | IIL | ≤ +5µA IOH = –250µA IOL = 250µA ✻ ✻ ✻ ✻ ✻ ✻ ✻ –90 86 92 100 DCLK Static BITS ✻ 100 VIN VIN VIN VIN ✻ ✻ 500 30 100 2.4 SHDN = VDD UNITS ✻ 16 4.5 fSAMPLE = 12.5kHz Power-Down Mode(3), CS = +VCC TEMPERATURE RANGE Specified Performance ✻ ✻ ✻ MAX 15 1.2 fSAMPLE = 12.5kHz DCLK Static DIGITAL INPUT/OUTPUT Logic Family Logic Levels VIH VIL VOH VOL Data Format TYP ✻ ✻ 14 Data Transfer Only REFERENCE INPUT Range Resistance Input Current VREF +VCC +0.2 +1.25 25 ±1 SAMPLING DYNAMICS Conversion Time Acquisition Time Throughput Rate Multiplexer Settling Time Aperture Delay Aperture Jitter Internal Clock Frequency External Clock Frequency DYNAMIC CHARACTERISTICS Total Harmonic Distortion(2) Signal-to-(Noise + Distortion) Spurious-Free Dynamic Range Channel-to-Channel Isolation MIN 16 Capacitance Leakage Current SYSTEM PERFORMANCE No Missing Codes Integral Linearity Error Offset Error Offset Error Match Gain Error Gain Error Match Noise Power-Supply Rejection ADS8341EB, PB 5.25 2.0 ✻ ✻ ✻ ✻ ✻ V mA µA µA mW ✻ °C ✻ 3 10 +85 ✻ ✻ Same specifications as ADS8341E. NOTES: (1) LSB means Least Significant Bit. With VREF equal to +5.0V, one LSB is 76µV. (2) First five harmonics of the test frequency. (3) Auto power-down mode (PD1 = PD0 = 0) active or SHDN = GND. ADS8341 SBAS136D 3 ELECTRICAL CHARACTERISTICS: +2.7V At TA = –40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. ADS8341E, P PARAMETER CONDITIONS MIN TYP MAX RESOLUTION ANALOG INPUT Full-Scale Input Span Absolute Input Range MIN TYP 16 Positive Input - Negative Input Positive Input Negative Input 0 –0.2 –0.2 VREF +VCC +0.2 +0.2 Capacitance Leakage Current SYSTEM PERFORMANCE No Missing Codes Integral Linearity Error Offset Error Offset Error Match Gain Error Gain Error Match Noise Power-Supply Rejection ADS8341EB, PB ✻ ✻ ✻ 14 ✻ BITS ✻ ✻ ✻ V V V pF µA 15 1.2 1.0 20 3 +2.7 < VCC < +3.3V ±12 ±1 4.0 ±0.05 4.0 ✻ ✻ ✻ ✻ ±8 ±0.5 ✻ ±0.0024 ✻ ✻ 16 ✻ 4.5 ✻ 100 ✻ ✻ ✻ ✻ 500 30 100 2.4 SHDN = VDD When Used with Internal Clock Data Transfer Only UNITS ✻ ✻ 25 ±1 SAMPLING DYNAMICS Conversion Time Acquisition Time Throughput Rate Multiplexer Settling Time Aperture Delay Aperture Jitter Internal Clock Frequency External Clock Frequency MAX 0.024 0.024 0 2.4 2.0 2.4 ✻ ✻ ✻ ✻ ✻ ✻ Bits LSB mV LSB % of FSR LSB µVrms LSB(1) Clk Cycles Clk Cycles kHz ns ns ps MHz MHz MHz MHz DYNAMIC CHARACTERISTICS Total Harmonic Distortion(2) Signal-to-(Noise + Distortion) Spurious-Free Dynamic Range Channel-to-Channel Isolation REFERENCE INPUT Range Resistance Input Current VIN VIN VIN VIN = = = = 2.5Vp-p 2.5Vp-p 2.5Vp-p 2.5Vp-p at at at at 10kHz 10kHz 10kHz 50kHz 0.5 +VCC DCLK Static 5 13 2.5 0.001 fSAMPLE = 12.5kHz DCLK Static DIGITAL INPUT/OUTPUT Logic Family Logic Levels VIH VIL VOH VOL Data Format POWER SUPPLY REQUIREMENTS +VCC Quiescent Current ✻ 40 3 | IIH | ≤ +5µA | IIL | ≤ +5µA IOH = –250µA IOL = 250µA +VCC • 0.7 –0.3 +VCC • 0.8 5.5 +0.8 ✻ ✻ ✻ 1.2 220 Power Dissipation 3.2 –40 ✻ ✻ ✻ ✻ 0.4 2.7 ✻ V GΩ µA µA µA ✻ V V V V ✻ Straight Binary Specified Performance dB dB dB dB ✻ ✻ ✻ ✻ ✻ CMOS fSAMPLE = 12.5kHz Power-Down Mode(3), CS = +VCC TEMPERATURE RANGE Specified Performance ✻ ✻ ✻ ✻ –90 86 92 100 3.6 1.85 ✻ ✻ 3 5 +85 ✻ ✻ ✻ ✻ ✻ V mA µA µA mW ✻ °C ✻ Same specifications as ADS8341E. NOTES: (1) LSB means Least Significant Bit. With VREF equal to +5.0V, one LSB is 76µV. (2) First five harmonics of the test frequency. (3) Auto power-down mode (PD1 = PD0 = 0) active or SHDN = GND. 4 ADS8341 SBAS136D TYPICAL CHARACTERISTICS: +5V At TA = +25°C, +VCC = +5V, VREF = +5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. FREQUENCY SPECTRUM (4096 Point FFT; fIN = 9.985kHz, –0.2dB) 0 0 –20 –20 –40 –40 Amplitude (dB) Amplitude (dB) FREQUENCY SPECTRUM (4096 Point FFT; fIN = 1.001kHz, –0.2dB) –60 –80 –100 –60 –80 –100 –120 –120 –140 –140 –160 –160 0 10 20 30 40 50 0 10 20 30 40 Frequency (kHz) Frequency (kHz) SIGNAL-TO-NOISE RATIO AND SIGNAL-TO-(NOISE+DISTORTION) vs INPUT FREQUENCY SPURIOUS-FREE DYNAMIC RANGE AND TOTAL HARMONIC DISTORTION vs INPUT FREQUENCY 100 50 100 –100 SNR SINAD 80 70 –90 THD(1) 80 –80 70 –70 (1) First Nine Harmonics of the Input Frequency 60 60 10 100 –60 1 10 100 Frequency (kHz) Frequency (kHz) EFFECTIVE NUMBER OF BITS vs INPUT FREQUENCY CHANGE IN SIGNAL-TO-(NOISE+DISTORTION) vs TEMPERATURE 15.0 0.2 14.5 0.0 Delta from 25°C (dB) Effective Number of Bits 1 14.0 13.5 13.0 12.5 –0.2 –0.4 –0.6 –0.8 12.0 –1.0 11.5 –1.2 11.0 fIN = 9.985kHz, –0.2dB –1.4 1 10 Frequency (kHz) ADS8341 SBAS136D 100 –50 –25 0 25 50 75 100 Temperature (°C) 5 THD (dB) 90 SFDR (dB) SNR and SINAD (dB) SFDR 90 TYPICAL CHARACTERISTICS: +5V (Cont.) At TA = +25°C, +VCC = +5V, VREF = +5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. DIFFERENTIAL LINEARITY ERROR vs CODE 4 2 3 DLE (LSBS) ILE (LSBS) INTEGRAL LINEARITY ERROR vs CODE 3 1 0 2 1 –1 0 –2 –1 –3 0000h 4000h 8000h C000h –2 0000h FFFFh 4000h 8000h C000h FFFFh Output Code Output Code CHANGE IN OFFSET vs TEMPERATURE CHANGE IN GAIN vs TEMPERATURE 1.0 0.40 Change in Gain (LSB) Change in Offset (LSB) 0.30 0.50 0.00 –0.50 –1.00 0.20 0.10 0.00 –0.10 –0.20 –0.30 –1.50 –40 –20 0 20 40 60 80 –40 100 –20 0 20 40 60 80 100 80 100 Temperature (°C) Temperature (°C) WORST CASE CHANNEL-TO-CHANNEL OFFSET MATCH vs TEMPERATURE WORST CASE CHANNEL-TO-CHANNEL GAIN MATCH vs TEMPERATURE 0.35 2.50 0.30 Gain Match (LSB) Offset Match (LSB) 2.00 1.50 1.00 0.25 0.20 0.15 0.10 0.50 0.05 0.00 0.00 –40 –20 0 20 40 Temperature (°C) 6 60 80 100 –40 –20 0 20 40 60 Temperature (°C) ADS8341 SBAS136D TYPICAL CHARACTERISTICS: +5V (Cont.) At TA = +25°C, +VCC = +5V, VREF = +5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. IQ vs TEMPERATURE 1.45 IQ (mA) 1.40 1.35 1.20 1.25 –40 –20 0 20 40 60 80 100 Temperature (°C) ADS8341 SBAS136D 7 TYPICAL CHARACTERISTICS: +2.7V At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. FREQUENCY SPECTRUM (4096 Point FFT; fIN = 9.985kHz, –0.2dB) 0 0 –20 –20 –40 –40 Amplitude (dB) Amplitude (dB) FREQUENCY SPECTRUM (4096 Point FFT; fIN = 1.001kHz, –0.2dB) –60 –80 –100 –60 –80 –100 –120 –120 –140 –140 –160 –160 0 10 20 30 40 50 0 10 20 30 40 50 Frequency (kHz) Frequency (kHz) SIGNAL-TO-NOISE RATIO AND SIGNAL-TO-(NOISE+DISTORTION) vs INPUT FREQUENCY SPURIOUS-FREE DYNAMIC RANGE AND TOTAL HARMONIC DISTORTION vs INPUT FREQUENCY 100 90 SNR 90 SFDR 80 SFDR (dB) SNR and SINAD (dB) 85 SINAD 75 80 THD(1) 70 70 60 65 (1) First nine harmonics of the input frequency. 50 60 1 10 100 1 EFFECTIVE NUMBER OF BITS vs INPUT FREQUENCY CHANGE IN SIGNAL-TO-(NOISE+DISTORTION) vs TEMPERATURE 2.0 fIN = 9.985kHz, –0.2dB 1.5 Delta from 25°C (dB) 14 Effective Number of Bits 100 Frequency (kHz) 15 13 12 11 10 1.0 0.5 0.0 –0.5 –1.0 9 –1.5 –2.0 8 1 10 Frequency (kHz) 8 10 Frequency (kHz) 100 –50 –25 0 25 50 75 100 Temperature (°C) ADS8341 SBAS136D TYPICAL CHARACTERISTICS: +2.7V (Cont.) At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. DIFFERENTIAL LINEARITY ERROR vs CODE 4 2 3 DLE (LSBS) ILE (LSBS) INTEGRAL LINEARITY ERROR vs CODE 3 1 0 2 1 –1 0 –2 –1 –3 0000h 4000h 8000h C000h –2 0000h FFFFh 4000h Output Code CHANGE IN OFFSET vs TEMPERATURE FFFFh CHANGE IN GAIN vs TEMPERATURE 0.200 0.20 0.100 Change in Gain (LSB) Change in Offset (LSB) C000h Output Code 0.30 0.10 0.00 –0.10 –0.20 0.000 –0.100 –0.200 –0.300 –0.30 –40 –20 0 20 40 60 80 –40 100 –20 0 20 40 60 80 100 80 100 Temperature (°C) Temperature (°C) WORST CASE CHANNEL-TO-CHANNEL OFFSET MATCH vs TEMPERATURE WORST CASE CHANNEL-TO-CHANNEL GAIN MATCH vs TEMPERATURE 0.400 0.200 0.300 0.150 Gain Match (LSB) Offset Match (LSB) 8000h 0.200 0.100 0.000 0.100 0.050 0.000 –40 –20 0 20 40 Temperature (°C) ADS8341 SBAS136D 60 80 100 –40 –20 0 20 40 60 Temperature (°C) 9 TYPICAL CHARACTERISTICS: +2.7V (Cont.) At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 100kHz, and fCLK = 24 • fSAMPLE = 2.4MHz, unless otherwise noted. IQ vs TEMPERATURE SUPPLY CURRENT vs +VSS 1.15 1.6 fSAMPLE = 100kHz VREF vs +VSS 1.10 1.4 IQ (mA) Supply Current (mA) 1.5 1.3 1.05 1.2 1.00 1.1 0.95 1.0 2.5 3.0 3.5 4.0 +VSS (V) 10 4.5 5.0 –40 –20 0 20 40 60 80 100 Temperature (°C) ADS8341 SBAS136D THEORY OF OPERATION The input current on the analog inputs depends on the conversion rate of the device. During the sample period, the source must charge the internal sampling capacitor (typically 25pF). After the capacitor has been fully charged, there is no further input current. The rate of charge transfer from the analog source to the converter is a function of conversion rate. The ADS8341 is a classic Successive Approximation Register (SAR) A/D converter. The architecture is based on capacitive redistribution which inherently includes a sampleand-hold function. The converter is fabricated on a 0.6µm CMOS process. The basic operation of the ADS8341 is shown in Figure 1. The device requires an external reference and an external clock. It operates from a single supply of 2.7V to 5.25V. The external reference can be any voltage between 500mV and +VCC. The value of the reference voltage directly sets the input range of the converter. The average reference input current depends on the conversion rate of the ADS8341. A2 A1 A0 CH0 0 0 1 +IN 1 0 1 0 1 0 1 1 0 CH1 CH2 CH3 COM –IN +IN –IN +IN –IN +IN –IN TABLE I. Single-Ended Channel Selection (SGL/DIF HIGH). The analog input to the converter is differential and is provided via a four-channel multiplexer. The input can be provided in reference to a voltage on the COM pin (which is generally ground) or differentially by using two of the four input channels (CH0 - CH3). The particular configuration is selectable via the digital interface. A2 A1 A0 CH0 CH1 0 0 1 +IN –IN CH2 CH3 1 0 1 –IN +IN 0 1 0 +IN –IN 1 1 0 –IN +IN COM TABLE II. Differential Channel Control (SGL/DIF LOW). ANALOG INPUT A2-A0 (Shown 001B) Figure 2 shows a block diagram of the input multiplexer on the ADS8341. The differential input of the converter is derived from one of the four inputs in reference to the COM pin or two of the four inputs. Table I and Table II show the relationship between the A2, A1, A0, and SGL/DIF control bits and the configuration of the analog multiplexer. The control bits are provided serially via the DIN pin, see the Digital Interface section of this data sheet for more details. CH0 CH1 CH2 +IN CH3 Converter –IN When the converter enters the hold mode, the voltage difference between the +IN and –IN inputs, as shown in Figure 2, is captured on the internal capacitor array. The voltage on the –IN input is limited between –0.2V and 1.25V, allowing the input to reject small signals that are common to both the +IN and –IN input. The +IN input has a range of –0.2V to +VCC + 0.2V. COM SGL/DIF (Shown HIGH) FIGURE 2. Simplified Diagram of the Analog Input. +2.7V to +5V ADS8341 1µF + to 10µF 0.1µF Single-ended or differential analog inputs External VREF 0.1µF + 1 +VCC DCLK 16 2 CH0 CS 15 3 CH1 DIN 14 4 CH2 BUSY 13 5 CH3 DOUT 12 6 COM GND 11 7 SHDN GND 10 8 VREF +VCC Serial/Conversion Clock Chip Select Serial Data In Serial Data Out 9 1µF FIGURE 1. Basic Operation of the ADS8341. ADS8341 SBAS136D 11 rate and reference voltage. As the current from the reference is drawn on each bit decision, clocking the converter more quickly during a given conversion period will not reduce overall current drain from the reference. REFERENCE INPUT The external reference sets the analog input range. The ADS8341 will operate with a reference in the range of 500mV to +VCC. Keep in mind that the analog input is the difference between the +IN input and the –IN input, see Figure 2. For example, in the single-ended mode, a 1.25V reference, with the COM pin grounded, the selected input channel (CH0 - CH3) will properly digitize a signal in the range of 0V to 1.25V. If the COM pin is connected to 0.5V, the input range on the selected channel is 0.5V to 1.75V. DIGITAL INTERFACE Figure 3 shows the typical operation of the ADS8341’s digital interface. This diagram assumes that the source of the digital signals is a microcontroller or digital signal processor with a basic serial interface (note that the digital inputs are over-voltage tolerant up to 5.5V, regardless of +VCC). Each communication between the processor and the converter consists of eight clock cycles. One complete conversion can be accomplished with three serial communications, for a total of 24 clock cycles on the DCLK input. There are several critical items concerning the reference input and its wide voltage range. As the reference voltage is reduced, the analog voltage weight of each digital output code is also reduced. This is often referred to as the LSB (least significant bit) size and is equal to the reference voltage divided by 65,536. Any offset or gain error inherent in the A/D converter will appear to increase, in terms of LSB size, as the reference voltage is reduced. For example, if the offset of a given converter is 2LSBs with a 2.5V reference, then it will typically be 10LSBs with a 0.5V reference. In each case, the actual offset of the device is the same, 76µV. The first eight cycles are used to provide the control byte via the DIN pin. When the converter has enough information about the following conversion to set the input multiplexer appropriately, it enters the acquisition (sample) mode. After three more clock cycles, the control byte is complete and the converter enters the conversion mode. At this point, the input sample-and-hold goes into the hold mode. The next 16 clock cycles accomplish the actual analog-to-digital conversion. Likewise, the noise or uncertainty of the digitized output will increase with lower LSB size. With a reference voltage of 500mV, the LSB size is 7.6µV. This level is below the internal noise of the device. As a result, the digital output code will not be stable and vary around a mean value by a number of LSBs. The distribution of output codes will be gaussian and the noise can be reduced by simply averaging consecutive conversion results or applying a digital filter. Control Byte Also shown in Figure 3 is the placement and order of the control bits within the control byte. Tables III and IV give detailed information about these bits. The first bit, the ‘S’ bit, must always be HIGH and indicates the start of the control byte. The ADS8341 will ignore inputs on the DIN pin until the start bit is detected. The next three bits (A2 A0) select the active input channel or channels of the input multiplexer (see Tables I and II and Figure 2). With a lower reference voltage, care should be taken to provide a clean layout including adequate bypassing, a clean (low-noise, low-ripple) power supply, a low-noise reference, and a low-noise input signal. Because the LSB size is lower, the converter will also be more sensitive to nearby digital signals and electromagnetic interference. The voltage into the VREF input is not buffered and directly drives the Capacitor Digital-to-Analog Converter (CDAC) portion of the ADS8341. Typically, the input current is 13µA with a 2.5V reference. This value will vary by microamps depending on the result of the conversion. The reference current diminishes directly with both conversion Bit 7 (MSB) Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 (LSB) S A2 A1 A0 — SGL/DIF PD1 PD0 TABLE III. Order of the Control Bits in the Control Byte. CS tACQ DCLK 1 8 Idle DIN S A2 8 1 Acquire A1 A0 1 8 1 8 Conversion Idle SGL/ PD1 PD0 DIF S (START) A2 Acquire A1 A0 1 Conversion SGL/ PD1 PD0 DIF (START) BUSY DOUT 15 (MSB) 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 (LSB) Zero Filled... 15 14 (MSB) FIGURE 3. Conversion Timing, 24-Clocks per Conversion, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port. 12 ADS8341 SBAS136D BIT NAME 7 DESCRIPTION PD1 PD0 0 0 Power-down between conversions. When each conversion is finished, the converter enters a low power mode. At the start of the next conversion, the device instantly powers up to full power. There is no need for additional delays to assure full operation and the very first conversion is valid. Single-Ended/Differential Select Bit. Along with bits A2 - A0, this bit controls the setting of the multiplexer input, see Tables I and II. 1 0 Selects Internal Clock Mode 0 1 Reserved for Future Use Power-Down Mode Select Bits. See Table V for details. 1 1 No power-down between conversions, device always powered. Selects external clock mode. S Start Bit. Control byte starts with first HIGH bit on DIN. 6-4 A2 - A0 Channel Select Bits. Along with the SGL/DIF bit, these bits control the setting of the multiplexer input, see Tables I and II. 2 SGL/DIF 1-0 PD1 - PD0 Description TABLE IV. Descriptions of the Control Bits within the Control Byte. TABLE V. Power-Down Selection. The SGL/DIF bit controls the multiplexer input mode: either single-ended (HIGH) or differential (LOW). In single-ended mode, the selected input channel is referenced to the COM pin. In differential mode, the two selected inputs provide a differential input. See Tables I and II and Figure 2 for more information. The last two bits (PD1 - PD0) select the powerdown mode, as shown in Table V. If both inputs are HIGH, the device is always powered up. If both inputs are LOW, the device enters a power-down mode between conversions. When a new conversion is initiated, the device will resume normal operation instantly—no delay is needed to allow the device to power up and the very first conversion will be valid. PD0 = 1 for external clock mode. After enabling the required clock mode, only then should the ADS8341 be set to power-down between conversions (i.e., PD1 = PD0 = 0). The ADS8341 maintains the clock mode it was in prior to entering the power-down modes. External Clock Mode Clock Modes The ADS8341 can be used with an external serial clock or an internal clock to perform the successive-approximation conversion. In both clock modes, the external clock shifts data in and out of the device. Internal clock mode is selected when PD1 is HIGH and PD0 is LOW. If the user decides to switch from one clock mode to the other, an extra conversion cycle will be required before the ADS8341 can switch to the new mode. The extra cycle is required because the PD0 and PD1 control bits need to be written to the ADS8341 prior to the change in clock modes. When power is first applied to the ADS8341, the user must set the desired clock mode. It can be set by writing PD1 = 1 and PD0 = 0 for internal clock mode or PD1 = 1 and In external clock mode, the external clock not only shifts data in and out of the ADS8341, it also controls the A/D conversion steps. BUSY will go HIGH for one clock period after the last bit of the control byte is shifted in. Successiveapproximation bit decisions are made and appear at DOUT on each of the next 16 DCLK falling edges (see Figure 3). Figure 4 shows the BUSY timing in external clock mode. Since one clock cycle of the serial clock is consumed with BUSY going high (while the MSB decision is being made), 16 additional clocks must be given to clock out all 16 bits of data; thus, one conversion takes a minimum of 25 clock cycles to fully read the data. Since most microprocessors communicate in 8-bit transfers, this means that an additional transfer must be made to capture the LSB. There are two ways of handling this requirement. One is shown in Figure 3, where the beginning of the next control byte appears at the same time the LSB is being clocked out of the ADS8341. This method allows for maximum throughput and 24 clock cycles per conversion. CS tCSS tCL tCH tBD tBD tD0 tCSH DCLK tDS DIN tDH PD0 tBDV tBTR BUSY tDV DOUT tTR 15 14 FIGURE 4. Detailed Timing Diagram. ADS8341 SBAS136D 13 The other method is shown in Figure 5, which uses 32 clock cycles per conversion; the last seven clock cycles simply shift out zeros on the DOUT line. BUSY and DOUT go into a high-impedance state when CS goes high; after the next CS falling edge, BUSY will go LOW. SYMBOL DESCRIPTION MIN tACQ Acquisition Time 1.5 TYP MAX UNITS tDS DIN Valid Prior to DCLK Rising 100 ns tDH DIN Hold After DCLK HIGH 10 ns µs tDO DCLK Falling to DOUT Valid 200 ns Internal Clock Mode tDV CS Falling to DOUT Enabled 200 ns tTR CS Rising to DOUT Disabled 200 ns In internal clock mode, the ADS8341 generates its own conversion clock internally. This relieves the microprocessor from having to generate the SAR conversion clock and allows the conversion result to be read back at the processor’s convenience, at any clock rate from 0MHz to 2.0MHz. BUSY goes LOW at the start of conversion and then returns HIGH when the conversion is complete. During the conversion, BUSY will remain LOW for a maximum of 8µs. Also, during the conversion, DCLK should remain LOW to achieve the best noise performance. The conversion result is stored in an internal register; the data may be clocked out of this register any time after the conversion is complete. tCSS CS Falling to First DCLK Rising 100 ns tCSH CS Rising to DCLK Ignored 0 ns tCH DCLK HIGH 200 ns tCL DCLK LOW 200 tBD DCLK Falling to BUSY Rising 200 ns tBDV CS Falling to BUSY Enabled 200 ns tBTR CS Rising to BUSY Disabled 200 ns ns TABLE VI. Timing Specifications (+VCC = +2.7V to 3.6V, TA = –40°C to +85°C, CLOAD = 50pF). SYMBOL DESCRIPTION MIN tACQ Acquisition Time 900 tDS DIN Valid Prior to DCLK Rising 50 ns tDH DIN Hold After DCLK HIGH 10 ns If CS is LOW when BUSY goes LOW following a conversion, the next falling edge of the external serial clock will write out the MSB on the DOUT line. The remaining bits (D14-D0) will be clocked out on each successive clock cycle following the MSB. If CS is HIGH when BUSY goes LOW then the DOUT line will remain in tri-state until CS goes LOW, as shown in Figure 6. CS does not need to remain LOW once a conversion has started. Note that BUSY is not tri-stated when CS goes HIGH in internal clock mode. Data can be shifted in and out of the ADS8341 at clock rates exceeding 2.4MHz, provided that the minimum acquisition time tACQ, is kept above 1.7µs. TYP MAX UNITS ns tDO DCLK Falling to DOUT Valid 100 ns tDV CS Falling to DOUT Enabled 70 ns 70 ns tTR CS Rising to DOUT Disabled tCSS CS Falling to First DCLK Rising 50 ns tCSH CS Rising to DCLK Ignored 0 ns tCH DCLK HIGH 150 ns tCL DCLK LOW 150 tBD DCLK Falling to BUSY Rising 100 ns tBDV CS Falling to BUSY Enabled 70 ns tBTR CS Rising to BUSY Disabled 70 ns ns TABLE VII. Timing Specifications (+VCC = +4.75V to +5.25V, TA = –40°C to +85°C, CLOAD = 50pF). Digital Timing Figure 4 and Tables VI and VII provide detailed timing for the digital interface of the ADS8341. CS tACQ DCLK 1 8 Idle DIN S A2 1 8 Acquire A1 A0 1 1 8 8 Conversion Idle SGL/ DIF PD1 PD0 (START) BUSY DOUT 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Zero Filled... 0 (MSB) (LSB) FIGURE 5. External Clock Mode 32 Clocks Per Conversion. CS tACQ DCLK 1 8 Idle DIN S A2 Acquire A1 A0 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Conversion SGL/ PD1 PD0 DIF (START) BUSY DOUT 15 (MSB) 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Zero Filled... (LSB) FIGURE 6. Internal Clock Mode Timing. 14 ADS8341 SBAS136D Data Format The ADS8341 output data is in straight binary format, as shown in Figure 7. This figure shows the ideal output code for the given input voltage and does not include the effects of offset, gain, or noise. If DCLK is active and CS is LOW while the ADS8341 is in auto power-down mode, the device will continue to dissipate some power in the digital logic. The power can be reduced to a minimum by keeping CS HIGH. The differences in supply current for these two cases are shown in Figure 9. FS = Full-Scale Voltage = VREF 1LSB = VREF/65,536 1000 1LSB 11...111 fCLK = 24 • fSAMPLE Supply Current (µA) Output Code 11...110 11...101 00...010 00...001 100 fCLK = 2.4MHz 10 TA = 25°C +VCC = +2.7V VREF = +2.5V PD1 = PD0 = 0 00...000 0V FS – 1LSB 1 Input Voltage(1) (V) 1k 10k Voltage at converter input, after multiplexer: +IN – (–IN). (See Figure 2.) POWER DISSIPATION There are three power modes for the ADS8341: full power (PD1 - PD0 = 11B), auto power-down (PD1 - PD0 = 00B), and shutdown (SHDN LOW). The affects of these modes varies depending on how the ADS8341 is being operated. For example, at full conversion rate and 24-clocks per conversion, there is very little difference between full power mode and auto power-down, a shutdown (SHDN LOW) will not lower power dissipation. When operating at full-speed and 24-clocks per conversion (as shown in Figure 3), the ADS8341 spends most of its time acquiring or converting. There is little time for auto powerdown, assuming that this mode is active. Thus, the difference between full power mode and auto power-down is negligible. If the conversion rate is decreased by simply slowing the frequency of the DCLK input, the two modes remain approximately equal. However, if the DCLK frequency is kept at the maximum rate during a conversion, but conversion are simply done less often, then the difference between the two modes is dramatic. Figure 8 shows the difference between reducing the DCLK frequency (“scaling” DCLK to match the conversion rate) or maintaining DCLK at the highest frequency and reducing the number of conversion per second. In the later case, the converter spends an increasing percentage of its time in power-down mode (assuming the auto power-down mode is active). ADS8341 SBAS136D 1M FIGURE 8. Supply Current versus Directly Scaling the Frequency of DCLK with Sample Rate or Keeping DCLK at the Maximum Possible Frequency. 14 TA = 25°C +VCC = +2.7V VREF = +2.5V fCLK = 24 • fSAMPLE PD1 = PD0 = 0 12 Supply Current (µA) FIGURE 7. Ideal Input Voltages and Output Codes. 100k fSAMPLE (Hz) NOTE(1): 10 8 6 CS LOW (GND) 4 2 CS HIGH (+VCC) 0 0.09 0.00 1k 10k 100k 1M fSAMPLE (Hz) FIGURE 9. Supply Current versus State of CS. Operating the ADS8341 in auto power-down mode will result in the lowest power dissipation, and there is no conversion time “penalty” on power-up. The very first conversion will be valid. SHDN can be used to force an immediate power-down. 15 NOISE The noise floor of the ADS8341 itself is extremely low, as can be seen from Figures 10 thru 13, and is much lower than competing A/D converters. The ADS8341 was tested at both 5V and 2.7V and in both the internal and external clock modes. A low-level DC input was applied to the analog input pins and the converter was put through 5,000 conversions. The digital output of the A/D converter will vary in output code due to the internal noise of the ADS8341. This is true for all 16-bit SAR-type A/D converters. Using a histogram to plot the output codes, the distribution should appear bell-shaped with the peak of the bell curve representing the nominal code for the input value. The ±1σ, ±2σ, and ±3σ distributions will represent the 68.3%, 95.5%, and 99.7%, respectively, of all codes. The transition noise can be calculated by dividing the number of codes measured by 6 and this will yield the ±3σ distribution or 99.7% of all codes. Statistically, up to 3 codes could fall outside the distribution when executing 1000 conversions. The ADS8341, with < 3 output codes for the ±3σ distribution, will yield a < ±0.5 LSB transition noise at 5V operation. Remember, to achieve this low noise performance, the peak-to-peak noise of the input signal and reference must be < 50µV. 3619 683 638 31 7FFD 29 7FFE 7FFF 8000 8001 Code FIGURE 12. Histogram of 5,000 Conversions of a DC Input at the Code Transition, 2.7V operation external clock mode. 3572 4606 790 586 30 7FFD 22 7FFE 7FFF 8000 8001 Code FIGURE 13. Histogram of 5,000 Conversions of a DC Input at the Code Center, 2.7V operation internal clock mode. 0 194 7FFC 7FFE 7FFF 200 0 8000 8001 Code FIGURE 10. Histogram of 5,000 Conversions of a DC Input at the Code Transition, 5V operation external clock mode. 4614 0 203 7FFC 7FFE 7FFF 183 0 8000 8001 AVERAGING The noise of the A/D converter can be compensated by averaging the digital codes. By averaging conversion results, transition noise will be reduced by a factor of 1/√n, where n is the number of averages. For example, averaging 4 conversion results will reduce the transition noise by 1/2 to ±0.25 LSBs. Averaging should only be used for input signals with frequencies near DC. For AC signals, a digital filter can be used to low-pass filter and decimate the output codes. This works in a similar manner to averaging; for every decimation by 2, the signalto-noise ratio will improve 3dB. Code FIGURE 11. Histogram of 5,000 Conversions of a DC Input at the Code Center, 5V operation internal clock mode. 16 ADS8341 SBAS136D LAYOUT For optimum performance, care should be taken with the physical layout of the ADS8341 circuitry. This is particularly true if the reference voltage is low and/or the conversion rate is high. The basic SAR architecture is sensitive to glitches or sudden changes on the power supply, reference, ground connections, and digital inputs that occur just prior to latching the output of the analog comparator. Thus, during any single conversion for an n-bit SAR converter, there are n “windows” in which large external transient voltages can easily affect the conversion result. Such glitches might originate from switching power supplies, nearby digital logic, and high power devices. The degree of error in the digital output depends on the reference voltage, layout, and the exact timing of the external event. The error can change if the external event changes in time with respect to the DCLK input. With this in mind, power to the ADS8341 should be clean and well bypassed. A 0.1µF ceramic bypass capacitor should be placed as close to the device as possible. In addition, a 1µF to 10µF capacitor and a 5Ω or 10Ω series resistor may be used to low-pass filter a noisy supply. ADS8341 SBAS136D The reference should be similarly bypassed with a 0.1µF capacitor. Again, a series resistor and large capacitor can be used to low-pass filter the reference voltage. If the reference voltage originates from an op amp, make sure that it can drive the bypass capacitor without oscillation (the series resistor can help in this case). The ADS8341 draws very little current from the reference on average, but it does place larger demands on the reference circuitry over short periods of time (on each rising edge of DCLK during a conversion). The ADS8341 architecture offers no inherent rejection of noise or voltage variation in regards to the reference input. This is of particular concern when the reference input is tied to the power supply. Any noise and ripple from the supply will appear directly in the digital results. While high frequency noise can be filtered out as discussed in the previous paragraph, voltage variation due to line frequency (50Hz or 60Hz) can be difficult to remove. The GND pin should be connected to a clean ground point. In many cases, this will be the “analog” ground. Avoid connections that are too near the grounding point of a microcontroller or digital signal processor. If needed, run a ground trace directly from the converter to the power supply entry point. The ideal layout will include an analog ground plane dedicated to the converter and associated analog circuitry. 17 PACKAGE DRAWINGS DBQ (R-PDSO-G**) PLASTIC SMALL-OUTLINE 24 PINS SHOWN 0.012 (0,30) 0.008 (0,20) 0.025 (0,64) 24 0.005 (0,13) M 13 0.244 (6,20) 0.228 (5,80) 0.008 (0,20) NOM 0.157 (3,99) 0.150 (3,81) 1 Gage Plane 12 A 0.010 (0,25) 0°– 8° 0.069 (1,75) MAX 0.035 (0,89) 0.016 (0,40) Seating Plane 0.010 (0,25) 0.004 (0,10) 0.004 (0,10) PINS ** 16 20 24 28 A MAX 0.197 (5,00) 0.344 (8,74) 0.344 (8,74) 0.394 (10,01) A MIN 0.188 (4,78) 0.337 (8,56) 0.337 (8,56) 0.386 (9,80) DIM 4073301/E 10/00 NOTES: A. B. C. D. 18 All linear dimensions are in inches (millimeters). This drawing is subject to change without notice. Body dimensions do not include mold flash or protrusion not to exceed 0.006 (0,15). Falls within JEDEC MO-137 ADS8341 SBAS136D PACKAGE OPTION ADDENDUM www.ti.com 3-Oct-2003 PACKAGING INFORMATION ORDERABLE DEVICE STATUS(1) PACKAGE TYPE PACKAGE DRAWING PINS PACKAGE QTY ADS8341E ACTIVE SSOP DBQ 16 100 ADS8341E/2K5 ACTIVE SSOP DBQ 16 2500 ADS8341EB ACTIVE SSOP DBQ 16 100 ADS8341EB/2K5 ACTIVE SSOP DBQ 16 2500 (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. 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