ADS7844

® ADS 784 4 ADS7844 ADS 7 844 12-Bit, 8-Channel Serial Output Sampling ANALOG-TO-DIGITAL CONVERTER FEATURES q SINGLE SUPPLY: 2.7V to 5V q 8-CHANNEL SINGLE-ENDED OR 4-CHANNEL DIFFERENTIAL INPUT q UP TO 200kHz CONVERSION RATE q ±1 LSB MAX INL AND DNL q GUARANTEED NO MISSING CODES q 72dB SINAD q SERIAL INTERFACE q 20-LEAD QSOP AND 20-LEAD SSOP PACKAGES q ALTERNATE SOURCE FOR MAX147 DESCRIPTION The ADS7844 is an 8-channel, 12-bit sampling analog-to-digital converter (ADC) with a synchronous serial interface. Typical power dissipation is 3mW at a 200kHz throughput rate and a +5V supply. The reference voltage (VREF) can be varied between 100mV and VCC, providing a corresponding input voltage range of 0V to VREF. The device includes a shutdown mode which reduces power dissipation to under 1µW. The ADS7844 is guaranteed down to 2.7V operation. Low power, high speed, and on-board multiplexer make the ADS7844 ideal for battery operated systems such as personal digital assistants, portable multichannel data loggers, and measurement equipment. The serial interface also provides low-cost isolation for remote data acquisition. The ADS7844 is available in a 20-lead QSOP package and the MAX147 equivalent 20-lead SSOP package and is guaranteed over the –40°C to +85°C temperature range. APPLICATIONS q DATA ACQUISITION q TEST AND MEASUREMENT q INDUSTRIAL PROCESS CONTROL q PERSONAL DIGITAL ASSISTANTS q BATTERY-POWERED SYSTEMS CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM VREF CDAC Eight Channel Multiplexer CS Comparator Serial Interface and Control SHDN DIN DOUT BUSY SAR DCLK International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © 1998 Burr-Brown Corporation PDS-1463D Printed in U.S.A. July, 1999 SPECIFICATION: +5V At TA = –40°C to +85°C, +VCC = +5V, VREF = +5V, f SAMPLE = 200kHz, and fCLK = 16 • fSAMPLE = 3.2MHz, unless otherwise noted. ADS7844E, N PARAMETER ANALOG INPUT Full-Scale Input Span Absolute Input Range Capacitance Leakage Current SYSTEM PERFORMANCE Resolution No Missing Codes Integral Linearity Error Differential Linearity Error Offset Error Offset Error Match Gain Error Gain Error Match Noise Power Supply Rejection SAMPLING DYNAMICS Conversion Time Acquisition Time Throughput Rate Multiplexer Settling Time Aperture Delay Aperture Jitter 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 = = = = 5Vp-p 5Vp-p 5Vp-p 5Vp-p at at at at 10kHz 10kHz 10kHz 50kHz 0.1 DCLK Static f SAMPLE = 12.5kHz DCLK Static DIGITAL INPUT/OUTPUT Logic Family Logic Levels VIH VIL VOH VOL Data Format POWER SUPPLY REQUIREMENTS +VCC Quiescent Current 5 45 2.5 0.001 CMOS | IIH | ≤ +5µA | IIL | ≤ +5µA IOH = –250µA IOL = 250µA 3.0 –0.3 3.5 Straight Binary Specified Performance f SAMPLE = 12.5kHz Power-Down Mode(3), CS = +VCC Power Dissipation TEMPERATURE RANGE Specified Performance T Same specifications as ADS7844E, ADS7844N. NOTE: (1) LSB means Least Significant Bit. With VREF equal to +5.0V, one LSB is 1.22mV. (2) First five harmonics of the test frequency. (3) Auto power-down mode (PD1 = PD0 = 0) active or SHDN = GND. –40 4.75 550 300 5.25 900 3 4.5 +85 T T T T T T 5.5 +0.8 0.4 T T T V µA µA µA mW °C T T T CONDITIONS MIN TYP MAX MIN T T T T T T T ±0.8 0.15 0.1 30 70 ±2 ±3 1.0 ±4 1.0 ±0.5 T T T T ±1 ±1 T T ±3 T ADS7844EB, NB TYP MAX T T T UNITS Positive Input - Negative Input Positive Input Negative Input 0 –0.2 –0.2 25 ±1 12 12 VREF +VCC +0.2 +1.25 V V V pF µA Bits Bits LSB(1) LSB LSB LSB LSB LSB µVrms dB Clk Cycles Clk Cycles kHz ns ns ps dB dB dB dB 12 3 200 500 30 100 –76 71 76 120 +VCC 100 3 T T T T T T T T T –78 72 78 T T T T T T T V GΩ µA µA µA T T T V V V V The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. ® ADS7844 2 SPECIFICATION: +2.7V At TA = –40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. ADS7844E, N PARAMETER ANALOG INPUT Full-Scale Input Span Absolute Input Range Capacitance Leakage Current SYSTEM PERFORMANCE Resolution No Missing Codes Integral Linearity Error Differential Linearity Error Offset Error Offset Error Match Gain Error Gain Error Match Noise Power Supply Rejection SAMPLING DYNAMICS Conversion Time Acquisition Time Throughput Rate Multiplexer Settling Time Aperture Delay Aperture Jitter 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 0.1 DCLK Static fSAMPLE = 12.5kHz DCLK Static DIGITAL INPUT/OUTPUT Logic Family Logic Levels VIH VIL VOH VOL Data Format POWER SUPPLY REQUIREMENTS +VCC Quiescent Current 5 13 2.5 0.001 CMOS | IIH | ≤ +5µA | IIL | ≤ +5µA IOH = –250µA IOL = 250µA +VCC • 0.7 –0.3 +VCC • 0.8 Straight Binary Specified Performance fSAMPLE = 12.5kHz Power-Down Mode(3), CS = +VCC Power Dissipation TEMPERATURE RANGE Specified Performance T Same specifications as ADS7844E, ADS7844N. NOTE: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, one LSB is 610mV. (2) First five harmonics of the test frequency. (3) Auto power-down mode (PD1 = PD0 = 0) active or SHDN = GND. –40 2.7 280 220 3.6 650 3 1.8 +85 T T T T 5.5 +0.8 0.4 T T T T T T V µA µA µA mW °C T T T CONDITIONS MIN TYP MAX MIN T T T T T T T ±0.8 0.15 0.1 30 70 ±2 ±3 1.0 ±4 1.0 ±0.5 T T T T ±1 ±1 T T ±3 T ADS7844EB, NB TYP MAX T T T UNITS Positive Input - Negative Input Positive Input Negative Input 0 –0.2 –0.2 25 ±1 12 12 VREF +VCC +0.2 +0.2 V V V pF µA Bits Bits LSB(1) LSB LSB LSB LSB LSB µVrms dB Clk Cycles Clk Cycles kHz ns ns ps dB dB dB dB 12 3 125 500 30 100 –75 71 78 100 +VCC 40 3 T T T T T T T T T –77 72 80 T T T T T T T V GΩ µA µA µA T T T V V V V ® 3 ADS7844 PIN CONFIGURATION Top View PIN DESCRIPTIONS PIN 1 2 3 4 5 6 7 8 9 NAME CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM DESCRIPTION Analog Input Channel 0. Analog Input Channel 1. Analog Input Channel 2. Analog Input Channel 3. Analog Input Channel 4. Analog Input Channel 5. Analog Input Channel 6. Analog Input Channel 7. 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 Specification Table for ranges. Power Supply, 2.7V to 5V. Ground Ground Serial Data Output. Data is shifted on the falling edge of DCLK. This output is high impedance when CS is high. Busy Output. Busy goes low when the DIN control bits are being read and also when the device is converting. The 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. Active LOW. Data will not be clocked into DIN unless CS is low. When CS is high DOUT is high impedance. External Clock Input. The clock speed determines the conversion rate by the equation fCLK = 16 • fSAMPLE. Power Supply CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM 1 2 3 4 5 ADS7844 6 7 8 9 20 19 18 17 16 15 14 13 12 11 +VCC DCLK CS DIN BUSY DOUT GND GND +VCC VREF 10 11 12 13 14 15 SHDN VREF +VCC GND GND DOUT SHDN 10 16 BUSY ABSOLUTE MAXIMUM RATINGS(1) +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 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. 17 18 DIN CS 19 20 CLK +VCC ELECTROSTATIC DISCHARGE SENSITIVITY This integrated circuit can be damaged by ESD. Burr-Brown 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. PACKAGE/ORDERING INFORMATION MINIMUM RELATIVE ACCURACY (LSB) ±2 " " " ±1 " " " MAXIMUM GAIN ERROR (LSB) ±4 " " " ±3 " " " SPECIFICATION TEMPERATURE RANGE –40°C to +85°C " " " –40°C to +85°C " " " PACKAGE DRAWING NUMBER(1) 349 " 334 " 349 " 334 " PRODUCT ADS7844E " ADS7844N " ADS7844EB " ADS7844NB " PACKAGE 20-Lead QSOP " 20-Lead SSOP " 20-Lead QSOP " 20-Lead SSOP " ORDERING NUMBER(2) ADS7844E ADS7844E/2K5 ADS7844N ADS7844N/1K ADS7844EB ADS7844EB/2K5 ADS7844NB ADS7844NB/1K TRANSPORT MEDIA Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “ADS7844/2K5” will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B of Burr-Brown IC Data Book. ® ADS7844 4 TYPICAL PERFORMANCE CURVES:+5V At TA = +25°C, +VCC = +5V, VREF = +5V, fSAMPLE = 200kHz, and fCLK = 16 • fSAMPLE = 3.2MHz, unless otherwise noted. FREQUENCY SPECTRUM (4096 Point FFT; fIN = 1,123Hz, –0.2dB) 0 –20 Amplitude (dB) FREQUENCY SPECTRUM (4096 Point FFT; fIN = 10.3kHz, –0.2dB) 0 –20 –40 –60 –80 –100 –120 Amplitude (dB) –40 –60 –80 –100 –120 0 25 50 Frequency (kHz) 75 100 0 25 50 Frequency (kHz) 75 100 SIGNAL-TO-NOISE RATIO AND SIGNAL-TO(NOISE+DISTORTION) vs INPUT FREQUENCY 74 SNR 73 SNR and SINAD (dB) 80 85 SPURIOUS FREE DYNAMIC RANGE AND TOTAL HARMONIC DISTORTION vs INPUT FREQUENCY –85 SFDR –80 SFDR (dB) 72 SINAD 71 70 75 THD –75 70 –70 69 68 1 10 Input Frequency (kHz) 100 65 1 10 Input Frequency (kHz) –65 100 EFFECTIVE NUMBER OF BITS vs INPUT FREQUENCY 12.0 CHANGE IN SIGNAL-TO-(NOISE+DISTORTION) vs TEMPERATURE 0.6 0.4 Delta from +25°C (dB) Effective Number of Bits 11.8 0.2 0.0 –0.2 –0.4 fIN = 10kHz, –0.2dB –0.6 11.6 11.4 11.2 11.0 1 10 Input Frequency (kHz) 100 –40 –20 0 20 40 60 80 100 Temperature (°C) ® 5 ADS7844 THD (dB) TYPICAL PERFORMANCE CURVES:+2.7V At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. FREQUENCY SPECTRUM (4096 Point FFT; fIN = 1,129Hz, –0.2dB) 0 –20 Amplitude (dB) FREQUENCY SPECTRUM (4096 Point FFT; fIN = 10.6kHz, –0.2dB) 0 –20 –40 –60 –80 –100 –120 Amplitude (dB) –40 –60 –80 –100 –120 0 15.6 31.3 Frequency (kHz) 46.9 62.5 0 15.6 31.3 Frequency (kHz) 46.9 62.5 SIGNAL-TO-NOISE RATIO AND SIGNAL-TO(NOISE+DISTORTION) vs INPUT FREQUENCY 78 SNR 74 SPURIOUS FREE DYNAMIC RANGE AND TOTAL HARMONIC DISTORTION vs INPUT FREQUENCY 90 85 SFDR 80 –80 –75 –70 THD 65 60 –65 –60 –55 –50 1 10 Input Frequency (kHz) 100 75 70 –90 –85 SNR and SINAD (dB) 70 SFDR (dB) 66 SINAD 62 58 54 1 10 Input Frequency (kHz) 100 55 50 EFFECTIVE NUMBER OF BITS vs INPUT FREQUENCY 12.0 11.5 Effective Number of Bits CHANGE IN SIGNAL-TO-(NOISE+DISTORTION) vs TEMPERATURE 0.4 fIN = 10kHz, –0.2dB 0.2 Delta from +25°C (dB) 11.0 10.5 10.0 9.5 9.0 1 10 Input Frequency (kHz) 100 0.0 –0.2 –0.4 –0.6 –0.8 –40 –20 0 20 40 60 80 100 Temperature (˚C) ® ADS7844 6 THD (dB) TYPICAL PERFORMANCE CURVES:+2.7V (CONT) At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. SUPPLY CURRENT vs TEMPERATURE 400 350 Supply Current (µA) 140 120 POWER DOWN SUPPLY CURRENT vs TEMPERATURE 300 250 200 150 100 –40 –20 0 20 40 60 80 100 Temperature (˚C) Supply Current (nA) 100 80 60 40 20 –40 –20 0 20 40 60 80 100 Temperature (˚C) INTEGRAL LINEARITY ERROR vs CODE 1.00 0.75 0.50 ILE (LSB) 1.00 0.75 0.50 DIFFERENTIAL LINEARITY ERROR vs CODE 0.25 0.00 –0.25 –0.50 –0.75 –1.00 000H DLE (LSB) 800H Output Code FFFH 0.25 0.00 –0.25 –0.50 –0.75 –1.00 000H 800H Output Code FFFH CHANGE IN GAIN vs TEMPERATURE 0.15 0.10 CHANGE IN OFFSET vs TEMPERATURE 0.6 0.4 Delta from +25˚C (LSB) 0.05 0.00 –0.05 –0.10 –0.15 –40 –20 0 20 40 60 80 100 Temperature (˚C) Delta from +25˚C (LSB) 0.2 0.0 –0.2 –0.4 –0.6 –40 –20 0 20 40 60 80 100 Temperature (˚C) ® 7 ADS7844 TYPICAL PERFORMANCE CURVES (CONT) At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. REFERENCE CURRENT vs SAMPLE RATE 14 12 Reference Current (µA) REFERENCE CURRENT vs TEMPERATURE 18 16 Reference Current (µA) 10 8 6 4 2 0 0 25 50 75 100 125 Sample Rate (kHz) 14 12 10 8 6 –40 –20 0 20 40 60 80 100 Temperature (˚C) SUPPLY CURRENT vs +VCC 320 300 MAXIMUM SAMPLE RATE vs +VCC 1M Supply Current (µA) Sample Rate (Hz) 280 260 240 220 200 180 2 fSAMPLE = 12.5kHz VREF = +VCC 100k 10k VREF = +VCC 1k 2.5 3 3.5 +VCC (V) 4 4.5 5 2 2.5 3 3.5 +VCC (V) 4 4.5 5 ® ADS7844 8 THEORY OF OPERATION The ADS7844 is a classic successive approximation register (SAR) analog-to-digital (A/D) converter. The architecture is based on capacitive redistribution which inherently includes a sample/hold function. The converter is fabricated on a 0.6µs CMOS process. The basic operation of the ADS7844 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 100mV 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 ADS7844. The analog input to the converter is differential and is provided via an eight-channel multiplexer. The input can be provided in reference to a voltage on the COM pin (which is generally ground) or differentially by using four of the eight input channels (CH0 - CH7). The particular configuration is selectable via the digital interface. A2 0 1 0 1 0 1 0 1 A1 0 0 0 0 1 1 1 1 A0 0 0 1 1 0 0 1 1 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM +IN +IN +IN +IN +IN +IN +IN +IN –IN –IN –IN –IN –IN –IN –IN –IN ANALOG INPUT Figure 2 shows a block diagram of the input multiplexer on the ADS7844. The differential input of the converter is derived from one of the eight inputs in reference to the COM pin or four of the eight 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. When the converter enters the hold mode, the voltage difference between the +IN and –IN inputs (see 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 which are common to both the +IN and –IN input. The +IN input has a range of –0.2V to +VCC + 0.2V. 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 (typi- A2 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 A0 0 1 0 1 0 1 0 1 CH0 +IN CH1 –IN CH2 CH3 CH4 CH5 CH6 CH7 +IN –IN +IN –IN +IN –IN –IN +IN –IN +IN –IN +IN –IN +IN TABLE I. Single-Ended Channel Selection (SGL/DIF HIGH). TABLE II. Differential Channel Control (SGL/DIF LOW). +2.7V to +5V ADS7844 1 2 Single-ended or differential analog inputs 3 4 5 6 7 8 9 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM +VCC 20 DCLK 19 CS 18 DIN 17 BUSY 16 DOUT 15 GND 14 GND 13 +VCC 12 VREF 11 0.1µF 1µF to 10µF Serial/Conversion Clock Chip Select Serial Data In Serial Data Out 10 SHDN 1µF to 10µF FIGURE 1. Basic Operation of the ADS7844. ® 9 ADS7844 A2-A0 (shown 00oB)(1) CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 +IN Likewise, the noise or uncertainty of the digitized output will increase with lower LSB size. With a reference voltage of 100mV, the LSB size is 24µV. 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. 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 ADS7844. 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 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. Converter –IN COM NOTE: (1) See Truth Tables, Table 1 & Table 2 for address coding. SGL/DIF (shown HIGH) DIGITAL INTERFACE Figure 3 shows the typical operation of the ADS7844’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. The first eight clock 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/hold goes into the hold mode. The next twelve clock cycles accomplish the actual analogto-digital conversion. A thirteenth clock cycle is needed for the last bit of the conversion result. Three more clock cycles are needed to complete the last byte (DOUT will be LOW). These will be ignored by the converter. 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 ADS7844 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). 10 FIGURE 2. Simplified Diagram of the Analog Input. cally 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. REFERENCE INPUT The external reference sets the analog input range. The ADS7844 will operate with a reference in the range of 100mV to +VCC. Keep in mind that the analog input is the difference between the +IN input and the –IN input as shown in Figure 2. For example, in the single-ended mode, a 1.25V reference, and with the COM pin grounded, the selected input channel (CH0 - CH7) 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. 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 4096. 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 2 LSBs with a 2.5V reference, then it will typically be 10 LSBs with a 0.5V reference. In each case, the actual offset of the device is the same, 1.22mV. ® ADS7844 Bit 7 (MSB) S Bit 6 A2 Bit 5 A1 Bit 4 A0 Bit 3 — Bit 2 SGL/DIF Bit 1 PD1 Bit 0 (LSB) PD0 TABLE III. Order of the Control Bits in the Control Byte. BIT 7 NAME S DESCRIPTION Start Bit. Control byte starts with first HIGH bit on DIN. A new control byte starts with every 15th clock cycle. Channel Select Bits. Along with the SGL/DIF bit, these bits control the setting of the multiplexer input as detailed in Tables I and II. Not Used. Single-Ended/Differential Select Bit. Along with bits A2 - A0, this bit controls the setting of the multiplexer input as detailed in Tables I and II. Power-Down Mode Select Bits. See Table V for details. 6-4 A2 - A0 3 2 — SGL/DIF 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. 16-Clocks per Conversion The control bits for conversion n+1 can be overlapped with conversion ‘n’ to allow for a conversion every 16 clock cycles, as shown in Figure 4. This figure also shows possible serial communication occurring with other serial peripherals between each byte transfer between the processor and the converter. This is possible provided that each conversion completes within 1.6ms of starting. Otherwise, the signal that has been captured on the input sample/hold may droop enough to affect the conversion result. In addition, the ADS7844 is fully powered while other serial communications are taking place. 1-0 PD1 - PD0 TABLE IV. Descriptions of the Control Bits within the Control Byte. CS tACQ DCLK 1 8 1 8 1 8 DIN S A2 A1 Idle A0 SGL/ PD1 PD0 DIF Acquire Conversion Idle (START) BUSY DOUT 11 (MSB) 10 9 8 7 6 5 4 3 2 1 0 (LSB) Zero Filled... FIGURE 3. Conversion Timing, 24-Clocks per Conversion, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port. CS DCLK 1 8 1 8 1 8 1 DIN S CONTROL BITS S CONTROL BITS BUSY DOUT 11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 FIGURE 4. Conversion Timing, 16-Clocks per Conversion, 8-bit Bus Interface. No DCLK delay required with dedicated serial port. ® 11 ADS7844 PD1 0 PD0 0 Description 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. Reserved for future use. Reserved for future use. No power-down between conversions, device always powered. SYMBOL tACQ tDS tDH tDO tDV tTR tCSS tCSH tCH tCL tBD tBDV tBTR DESCRIPTION Acquisition Time DIN Valid Prior to DCLK Rising DIN Hold After DCLK HIGH DCLK Falling to DOUT Valid CS Falling to DOUT Enabled CS Rising to DOUT Disabled CS Falling to First DCLK Rising CS Rising to DCLK Ignored DCLK HIGH DCLK LOW DCLK Falling to BUSY Rising CS Falling to BUSY Enabled CS Rising to BUSY Disabled MIN 1.5 100 10 TYP MAX UNITS µs ns ns 200 200 200 100 0 200 200 200 200 200 ns ns ns ns ns ns ns ns ns ns 0 1 1 1 0 1 TABLE V. Power-Down Selection. Digital Timing Figure 5 and Tables VI and VII provide detailed timing for the digital interface of the ADS7844. 15-Clocks per Conversion Figure 6 provides the fastest way to clock the ADS7844. This method will not work with the serial interface of most microcontrollers and digital signal processors as they are generally not capable of providing 15 clock cycles per serial transfer. However, this method could be used with field programmable gate arrays (FPGAs) or application specific integrated circuits (ASICs). Note that this effectively increases the maximum conversion rate of the converter beyond the values given in the specification tables, which assume 16 clock cycles per conversion. TABLE VI. Timing Specifications (+VCC = +2.7V to 3.6V, TA = –40°C to +85°C, CLOAD = 50pF). SYMBOL tACQ tDS tDH tDO tDV tTR tCSS tCSH tCH tCL tBD tBDV tBTR DESCRIPTION Acquisition Time DIN Valid Prior to DCLK Rising DIN Hold After DCLK HIGH DCLK Falling to DOUT Valid CS Falling to DOUT Enabled CS Rising to DOUT Disabled CS Falling to First DCLK Rising CS Rising to DCLK Ignored DCLK HIGH DCLK LOW DCLK Falling to BUSY Rising CS Falling to BUSY Enabled CS Rising to BUSY Disabled 50 0 150 150 100 70 70 MIN 900 50 10 100 70 70 TYP MAX UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns TABLE VII. Timing Specifications (+VCC = +4.75V to +5.25V, TA = –40°C to +85°C, CLOAD = 50pF). CS tCSS tCH tCL tBD tBD tD0 tCSH DCLK tDS tDH PD0 tBDV tBTR DIN BUSY tDV tTR 11 10 DOUT FIGURE 5. Detailed Timing Diagram. CS DCLK 1 15 A2 A1 A0 SGL/ PD1 PD0 DIF 1 S A2 A1 A0 SGL/ PD1 PD0 DIF 15 S 1 A2 A1 A0 DIN BUSY DOUT S 11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 8 7 6 5 4 3 2 FIGURE 6. Maximum Conversion Rate, 15-Clocks per Conversion. ® ADS7844 12 Data Format The ADS7844 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. FS = Full-Scale Voltage = VREF 1 LSB = VREF/4096 1 LSB 11...111 11...110 Output Code 1000 fCLK = 16 • fSAMPLE Supply Current (µA) 100 10 fCLK = 2MHz TA = 25°C +VCC = +2.7V VREF = +2.5V PD1 = PD0 = 0 1k 10k fSAMPLE (Hz) 100k 1M 1 11...101 00...010 00...001 00...000 FIGURE 8. Supply Current vs Directly Scaling the Frequency of DCLK with Sample Rate or Keeping DCLK at the Maximum Possible Frequency. 0V Input Voltage(1) (V) Note 1: Voltage at converter input, after multiplexer: +IN–(–IN). See Figure 2. FS – 1 LSB 14 12 Supply Current (µA) FIGURE 7. Ideal Input Voltages and Output Codes. POWER DISSIPATION There are three power modes for the ADS7844: 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 ADS7844 is being operated. For example, at full conversion rate and 16 clocks per conversion, there is very little difference between full power mode and auto power-down. Likewise, if the device has entered auto power-down, a shutdown (SHDN LOW) will not lower power dissipation. When operating at full-speed and 16-clocks per conversion (as shown in Figure 4), the ADS7844 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). If DCLK is active and CS is LOW while the ADS7844 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. 13 10 8 6 4 2 0 0.09 0.00 1k TA = 25°C +VCC = +2.7V VREF = +2.5V fCLK = 16 • fSAMPLE PD1 = PD0 = 0 CS LOW (GND) CS HIGH (+VCC) 10k fSAMPLE (Hz) 100k 1M FIGURE 9. Supply Current vs State of CS. Operating the ADS7844 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. LAYOUT For optimum performance, care should be taken with the physical layout of the ADS7844 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 ® ADS7844 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 ADS7844 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 lowpass filter a noisy supply. The reference should be similarly bypassed with a 0.1µF capacitor. Again, a series resistor and large capacitor can be used to lowpass 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 ADS7844 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 ADS7844 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 which 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. ® ADS7844 14