ADS7822B

® ADS 782 2 OPS7822 A65 8 AD ADS7822 For most current data sheet and other product information, visit www.burr-brown.com 12-Bit High Speed 2.7V microPower Sampling ANALOG-TO-DIGITAL CONVERTER FEATURES q 75kHz SAMPLING RATE q MICRO POWER: 0.54mW at 75kHz 0.06mW at 7.5kHz q POWER DOWN: 3µA max q MINI-DIP-8, SOIC-8, AND MSOP-8 q DIFFERENTIAL INPUT q SERIAL INTERFACE DESCRIPTION The ADS7822 is a 12-bit sampling analog-to-digital converter (A/D) with guaranteed specifications over a 2.7V to 3.6V supply range. It requires very little power even when operating at the full 75kHz rate. At lower conversion rates, the high speed of the device enables it to spend most of its time in the power down mode— the power dissipation is less than 60µW at 7.5kHz. The ADS7822 also features operation from 2.0V to 5V, a synchronous serial interface, and a differential input. The reference voltage can be set to any level within the range of 50mV to VCC. Ultra low power and small size make the ADS7822 ideal for battery operated systems. It is also a perfect fit for remote data acquisition modules, simultaneous multi-channel systems, and isolated data acquisition. The ADS7822 is available in a plastic mini-DIP-8, an SOIC-8, or an MSOP-8 package. APPLICATIONS q BATTERY OPERATED SYSTEMS q REMOTE DATA ACQUISITION q ISOLATED DATA ACQUISITION q SIMULTANEOUS SAMPLING, MULTI-CHANNEL SYSTEMS SAR Control VREF DOUT +In CDAC –In S/H Amp Comparator Serial Interface DCLOCK CS/SHDN 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 © 1996 Burr-Brown Corporation PDS-1358C Printed in U.S.A., May, 2000 SPECIFICATIONS At –40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 75kHz, fCLK = 16 • fSAMPLE, unless otherwise specified. ADS7822 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 Gain Error Noise Power Supply Rejection SAMPLING DYNAMICS Conversion Time Acquisition Time Throughput Rate DYNAMIC CHARACTERISTICS Total Harmonic Distortion SINAD Spurious Free Dynamic Range REFERENCE INPUT Voltage Range Resistance Current Drain VIN = 2.5Vp-p at 1kHz VIN = 2.5Vp-p at 1kHz VIN = 2.5Vp-p at 1kHz 0.05 CS = GND, fSAMPLE = 0Hz CS = V CC At Code 710h fSAMPLE = 7.5kHz CS = V CC 5 5 8 0.8 0.001 CMOS IIH = +5µA IIL = +5µA IOH = –250µA IOL = 250µA 2.0 –0.3 2.1 Straight Binary 2.7 2.0 3.6 200 20 180 3.6 2.7 5.25 325 T T T T T T 5.5 0.8 0.4 T T T T T T T T T T T T T T CONDITIONS MIN TYP MAX MIN T T T T T T 12 ±0.5 ±0.5 ±2 ±2 ±3 ±3 ±0.5 ±0.5 ±1 ±1 T T T ±0.25 ±0.25 ±0.75 ±0.75 ±1 ±1 ADS7822B TYP MAX T T T MIN T T T T T T ADS7822C TYP MAX T T T UNITS +In – (–In) +In –In 0 –0.2 –0.2 25 ±1 12 11 VREF VCC +0.2 +1.0 V V V pF µA Bits Bits LSB(1) LSB LSB LSB µVrms dB Clk Cycles Clk Cycles kHz dB dB dB 33 82 12 1.5 75 –82 71 86 VCC T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T 40 3 T T T T V GΩ GΩ µA µA µA DIGITAL INPUT/OUTPUT Logic Family Logic Levels: VIH VIL VOH VOL Data Format V V V V POWER SUPPLY REQUIREMENTS Specified Performance VCC See Notes 2 and 3 See Note 3 Quiescent Current fSAMPLE = 7.5kHz(4,5) fSAMPLE = 7.5kHz(5) Power Down TEMPERATURE RANGE Specified Performance T Specifications same as ADS7822. CS = V CC V V V µA µA µA µA °C 3 –40 +85 T T T T T T Notes: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, one LSB is 0.61mV. (2) The maximum clock rate of the ADS7822 is less than 1.2MHz in this power supply range. (3) See the Typical Performance Curves for more information. (4) fCLK = 1.2MHz, CS = VCC for 145 clock cycles out of every 160. (5) See the Power Dissipation section for more information regarding lower sample rates. 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. ® ADS7822 2 ABSOLUTE MAXIMUM RATINGS(1) VCC ....................................................................................................... +6V Analog Input .............................................................. –0.3V to (VCC + 0.3V) Logic Input ............................................................................... –0.3V to 6V Case Temperature ......................................................................... +100°C Junction Temperature .................................................................... +150°C Storage Temperature ..................................................................... +125°C External Reference Voltage .............................................................. +5.5V NOTE: (1) Stresses above these ratings may permanently damage the device. ELECTROSTATIC DISCHARGE SENSITIVITY Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. BurrBrown Corporation recommends that all integrated circuits be handled and stored using appropriate ESD protection methods. 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 published specifications. PIN CONFIGURATION VREF +In –In GND 1 2 ADS7822 3 4 PDIP-8, SOIC-8, MSOP-8 8 7 6 5 +VCC DCLOCK DOUT CS/SHDN PIN ASSIGNMENTS PIN 1 2 3 4 5 6 7 8 NAME VREF +In –In GND CS/SHDN D OUT DCLOCK +VCC DESCRIPTION Reference Input. Non Inverting Input. Inverting Input. Connect to ground or to remote ground sense point. Ground. Chip Select when LOW, Shutdown Mode when HIGH. The serial output data word is comprised of 12 bits of data. In operation the data is valid on the falling edge of DCLOCK. The second clock pulse after the falling edge of CS enables the serial output. After one null bit the data is valid for the next 12 edges. Data Clock synchronizes the serial data transfer and determines conversion speed. Power Supply. PACKAGE/ORDERING INFORMATION MAXIMUM INTEGRAL LINEARITY ERROR (LSB) ±2 MAXIMUM DIFFERENTIAL LINEARITY ERROR (LSB) ±2 PRODUCT ADS7822E ADS7822E ADS7822EB ADS7822EB ADS7822EC ADS7822EC ADS7822P ADS7822PB ADS7822PC ADS7822U ADS7822U ADS7822UB ADS7822UB ADS7822UC ADS7822UC PACKAGE MSOP-8 PACKAGE DRAWING NUMBER(1) 337 SPECIFICATION TEMPERATURE RANGE –40°C to +85°C PACKAGE MARKING(2) A22 ORDERING NUMBER(3) TRANSPORT MEDIA Tape and Reel " ±1 " ±1 " MSOP-8 " 337 " –40°C to +85°C " ±0.75 " ±0.75 " MSOP-8 " 337 " –40°C to +85°C " ±2 ±1 ±0.75 ±2 ±1 ±0.75 " ±2 ±1 ±0.75 ±2 ±1 ±0.75 " Plastic DIP-8 Plastic DIP-8 Plastic DIP-8 SOIC-8 " 006 006 006 182 –40°C –40°C –40°C –40°C " to to to to +85°C +85°C +85°C +85°C " " " " " " " SOIC-8 " 182 –40°C to +85°C –40°C to +85°C " " " " SOIC-8 " 182 " " ADS7822E/250 ADS7822E/2K5 A22 ADS7822EB/250 " ADS7822EB/2K5 A22 ADS7822EC/250 " ADS7822EC/2K5 ADS7822P ADS7822P ADS7822PB ADS7822PB ADS7822PC ADS7822PC ADS7822U ADS7822U " ADS7822U/2K5 ADS7822UB ADS7822UB " ADS7822UB/2K5 ADS7822UC ADS7822UC " ADS7822UC/2K5 " " Tape and Reel " Tape and Reel " Rails Rails Rails Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel NOTE: (1) For detail drawing and dimension table, please see end of data sheet or Package Drawing File on Web. (2) Performance Grade information is marked on the reel. (3) Models with a slash(/) are available only in Tape and reel in quantities indicated (e.g. /250 indicates 250 units per reel, /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of ”ADS7822E/2K5“ will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to the www.burr-brown.com web site under Applications and Tape and Reel Orientation and Dimensions. ® 3 ADS7822 TYPICAL PERFORMANCE CURVES At TA = +25°C, VCC = +2.7V, VREF = +2.5V, fSAMPLE = 75kHz, fCLK = 16 • fSAMPLE, unless otherwise specified. INTEGRAL LINEARITY ERROR vs CODE 1.00 Differential Linearity Error (LSB) 1.00 0.75 0.50 0.25 0.00 –0.25 –0.50 –0.75 –1.00 0 2048 Code 4095 0 DIFFERENTIAL LINEARITY ERROR vs CODE Integral Linearity Error (LSB) 0.75 0.50 0.25 0.00 –0.25 –0.50 –0.75 –1.00 2048 Code 4095 SUPPLY CURRENT vs TEMPERATURE 350 300 POWER DOWN SUPPLY CURRENT vs TEMPERATURE 120 100 Supply Current (nA) 80 60 40 20 0 Supply Current (µA) 250 200 150 100 50 –50 –25 0 25 50 75 100 Temperature (°C) –50 –25 0 25 50 75 100 Temperature (°C) QUIESCENT CURRENT vs VCC 400 350 Quiescent Current (µA) MAXIMUM SAMPLE RATE vs VCC 1000 300 250 200 150 100 1 2 3 VCC (V) 4 5 Sample Rate (kHz) 100 10 1 1 2 3 VCC (V) 4 5 ® ADS7822 4 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VCC = +2.7V, VREF = +2.5V, fSAMPLE = 75kHz, fCLK = 16 • fSAMPLE, unless otherwise specified. CHANGE IN OFFSET vs REFERENCE VOLTAGE 1.2 1.0 Change in Offset (LSB) 0.6 CHANGE IN OFFSET vs TEMPERATURE VCC = 5V 0.6 0.4 0.2 0.0 –0.2 –0.4 –0.6 –0.8 1 2 3 Reference Voltage (V) 4 5 Delta from 25°C (LSB) 0.8 0.4 0.2 0 –0.2 –0.4 –0.6 –50 –25 0 25 50 Temperature (°C) 75 100 CHANGE IN GAIN vs REFERENCE VOLTAGE 2.5 2.0 Change in Gain (LSB) CHANGE IN GAIN vs TEMPERATURE 0.15 VCC = 5V 0.1 1.5 1.0 0.5 0.0 –0.5 –1.0 –1.5 1 2 3 Reference Voltage (V) 4 5 Delta from 25°C (LSB) 0.05 0 –0.05 –0.1 –0.15 –50 –25 0 25 50 75 100 Temperature (°C) EFFECTIVE NUMBER OF BITS vs REFERENCE VOLTAGE 12 PEAK-TO-PEAK NOISE vs REFERENCE VOLTAGE 10 9 Peak-to-Peak Noise (LSB) Effective Number of Bits (rms) 11.75 11.5 11.25 11 10.75 10.5 10.25 10 0.1 VCC = 5V VCC = 5V 8 7 6 5 4 3 2 1 0 1 Reference Voltage (V) 10 0.1 1 Reference Voltage (V) 10 ® 5 ADS7822 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VCC = +2.7V, VREF = +2.5V, fSAMPLE = 75kHz, fCLK = 16 • fSAMPLE, unless otherwise specified. SPURIOUS FREE DYNAMIC RANGE and SIGNAL-TO-NOISE RATIO vs FREQUENCY 100 Spurious Free Dynamic Range and Signal-to-Noise Ratio (dB) TOTAL HARMONIC DISTORTION vs FREQUENCY 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 80 70 60 50 40 30 20 10 0 1 10 Frequency (kHz) 100 Signal-to-Noise Ratio Total Harmonic Distortion (dB) 90 Spurious Free Dynamic Range 1 10 Frequency (kHz) 100 SIGNAL-TO-(NOISE + DISTORTION) vs FREQUENCY 100 SIGNAL-TO-(NOISE + DISTORTION) vs INPUT LEVEL Signal-to-(Noise Ratio Plus Distortion) (dB) 1 10 Frequency (kHz) 100 80 70 60 50 40 30 20 10 0 –40 –35 –30 –25 –20 –15 Input Level (dB) –10 –5 0 Signal-to-(Noise + Distortion) (dB) 90 80 70 60 50 40 30 20 10 0 REFERENCE CURRENT vs SAMPLE RATE 14 12 REFERENCE CURRENT vs TEMPERATURE (Code = 710h) 14 12 Reference Current (µA) Reference Current (µA) 10 8 6 4 2 0 0 15 30 45 60 75 Sample Rate (kHz) 10 8 6 4 2 –50 –25 0 25 50 75 100 Temperature (°C) ® ADS7822 6 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VCC = +2.7V, VREF = +2.5V, fSAMPLE = 75kHz, fCLK = 16 • fSAMPLE, unless otherwise specified. POWER SUPPLY REJECTION vs RIPPLE FREQUENCY 0 –10 CHANGE IN INTEGRAL LINEARITY AND DIFFERENTIAL LINEARITY vs REFERENCE VOLTAGE 0.20 Delta from +2.5V Reference (LSB) VCC = 5V 0.15 0.10 0.05 0.00 –0.05 –0.10 Change in Differential Linearity (LSB) 1 2 3 Reference Voltage (V) 4 5 Change in Integral Linearity (LSB) Power Supply Rejection (dB) –20 –30 –40 –50 –60 –70 –80 –90 1 10 100 1000 Ripple Frequency (kHz) 10000 THEORY OF OPERATION The ADS7822 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 0.6µ CMOS process. The architecture and process allow the ADS7822 to acquire and convert an analog signal at up to 75,000 conversions per second while consuming very little power. The ADS7822 requires an external reference, an external clock, and a single power source (VCC). The external reference can be any voltage between 50mV and VCC. The value of the reference voltage directly sets the range of the analog input. The reference input current depends on the conversion rate of the ADS7822. The external clock can vary between 10kHz (625Hz throughput) and 1.2MHz (75kHz throughput). The duty cycle of the clock is essentially unimportant as long as the minimum high and low times are at least 400ns (VCC = 2.7V or greater). The minimum clock frequency is set by the leakage on the capacitors internal to the ADS7822. The analog input is provided to two input pins: +In and –In. When a conversion is initiated, the differential input on these pins is sampled on the internal capacitor array. While a conversion is in progress, both inputs are disconnected from any internal function. The digital result of the conversion is clocked out by the DCLOCK input and is provided serially, most significant bit first, on the DOUT pin. The digital data that is provided on the DOUT pin is for the conversion currently in progress—there is no pipeline delay. It is possible to continue to clock the ADS7822 after the conversion is complete and to obtain the serial data least significant bit first. See the digital timing section for more information. 7 ANALOG INPUT The +In and –In input pins allow for a differential input signal. Unlike some converters of this type, the –In input is not re-sampled later in the conversion cycle. When the converter goes into the hold mode, the voltage difference between +In and –In is captured on the internal capacitor array. The range of the –In input is limited to –0.2V to +1V. Because of this, the differential input can be used to reject only small signals that are common to both inputs. Thus, the –In input is best used to sense a remote signal ground that may move slightly with respect to the local ground potential. The input current on the analog inputs depends on a number of factors: sample rate, input voltage, source impedance, and power down mode. Essentially, the current into the ADS7822 charges the internal capacitor array during the sample period. After this capacitance has been fully charged, there is no further input current. The source of the analog input voltage must be able to charge the input capacitance (25pF) to a 12-bit settling level within 1.5 clock cycles. When the converter goes into the hold mode or while it is in the power down mode, the input impedance is greater than 1GΩ. Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, the –In input should not drop below GND – 200mV or exceed GND + 1V. The +In input should always remain within the range of GND – 200mV to VCC + 200mV. Outside of these ranges, the converter’s linearity may not meet specifications. ® ADS7822 REFERENCE INPUT The external reference sets the analog input range. The ADS7822 will operate with a reference in the range of 50mV to VCC. There are several important implications of this. As the reference voltage is reduced, the analog voltage weight of each digital output code is reduced. This is often referred to as the LSB (least significant bit) size and is equal to the reference voltage divided by 4096. This means that 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. The noise inherent in the converter will also appear to increase with lower LSB size. With a 2.5V reference, the internal noise of the converter typically contributes only 0.32 LSB peak-to-peak of potential error to the output code. When the external reference is 50mV, the potential error contribution from the internal noise will be 50 times larger—16 LSBs. The errors due to the internal noise are gaussian in nature and can be reduced by averaging consecutive conversion results. For more information regarding noise, consult the typical performance curves “Effective Number of Bits vs Reference Voltage” and “Peak-to-Peak Noise vs Reference Voltage.” Note that the effective number of bits (ENOB) figure is calculated based on the converter’s signal-to-(noise + distortion) ratio with a 1kHz, 0dB input signal. SINAD is related to ENOB as follows SINAD = 6.02 • ENOB + 1.76 With lower reference voltages, extra care should be taken to provide a clean layout including adequate bypassing, a clean 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 external sources of error such as nearby digital signals and electromagnetic interference. DIGITAL INTERFACE SIGNAL LEVELS The digital inputs of the ADS7822 can accommodate logic levels up to 6V regardless of the value of VCC. Thus, the ADS7822 can be powered at 3V and still accept inputs from logic powered at 5V. The CMOS digital output (DOUT) will swing 0V to VCC. If VCC is 3V and this output is connected to a 5V CMOS logic input, then that IC may require more supply current than normal and may have a slightly longer propagation delay. SERIAL INTERFACE The ADS7822 communicates with microprocessors and other digital systems via a synchronous 3-wire serial interface as shown in Figure 1 and Table I. The DCLOCK signal synchronizes the data transfer with each bit being transmitted on the falling edge of DCLOCK. Most receiving systems will capture the bitstream on the rising edge of DCLOCK. However, if the minimum hold time for DOUT is acceptable, the system can use the falling edge of DCLOCK to capture each bit. tCYC CS/SHDN tSUCS DCLOCK tCSD DOUT Hi-Z Null Bit Null Bit Power Down Hi-Z B11 B10 B9 (MSB) B8 B7 B6 B5 B4 B3 B2 B1 B0(1) B11 B10 B9 B8 tSMPL tCONV tDATA Note: (1) After completing the data transfer, if further clocks are applied with CS LOW, the A/D will output LSB-First data then followed with zeroes indefinitely. tCYC CS/SHDN tSUCS DCLOCK tCSD DOUT Hi-Z Null Bit Hi-Z B11 B10 B9 (MSB) B8 B7 B6 B5 B4 B3 B2 B1 B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 (1) Power Down tSMPL tCONV tDATA Note: (1) After completing the data transfer, if further clocks are applied with CS LOW, the A/D will output zeroes indefinitely. tDATA: During this time, the bias current and the comparator power down and the reference input becomes a high impedance node, leaving the CLK running to clock out LSB-First data or zeroes. FIGURE 1. ADS7822 Basic Timing Diagrams. ® ADS7822 8 SYMBOL tSMPL tCONV tCYC tCSD tSUCS thDO tdDO tdis ten tf tr DESCRIPTION Analog Input Sample Time Conversion Time Throughput Rate CS Falling to DCLOCK LOW CS Falling to DCLOCK Rising DCLOCK Falling to Current DOUT Not Valid DCLOCK Falling to Next DOUT Valid CS Rising to DOUT Tri-State DCLOCK Falling to DOUT Enabled DOUT Fall Time D OUT Rise Time MIN 1.5 TYP MAX 2.0 UNITS Clk Cycles Clk Cycles 12 75 0 30 15 130 40 75 90 110 200 80 175 200 200 kHz ns ns ns ns ns ns ns ns periods, DOUT will output the conversion result, most significant bit first. After the least significant bit (B0) has been output, subsequent clocks will repeat the output data but in a least significant bit first format. After the most significant bit (B11) has been repeated, DOUT will tri-state. Subsequent clocks will have no effect on the converter. A new conversion is initiated only when CS has been taken HIGH and returned LOW. DATA FORMAT The output data from the ADS7822 is in straight binary format as shown in Table II. This table represents the ideal output code for the given input voltage and does not include the effects of offset, gain error, or noise. DESCRIPTION Full Scale Range Least Significant Bit (LSB) Full Scale Midscale Midscale – 1 LSB Zero ANALOG VALUE VREF VREF/4096 VREF –1 LSB VREF/2 VREF/2 – 1 LSB 0V DIGITAL OUTPUT STRAIGHT BINARY BINARY CODE 1111 1111 1111 1000 0000 0000 0111 1111 1111 0000 0000 0000 HEX CODE FFF 800 7FF 000 TABLE I. Timing Specifications (VCC = 2.7V and above, –40°C to +85°C. A falling CS signal initiates the conversion and data transfer. The first 1.5 to 2.0 clock periods of the conversion cycle are used to sample the input signal. After the second falling DCLOCK edge, DOUT is enabled and will output a LOW value for one clock period. For the next 12 DCLOCK TABLE II. Ideal Input Voltages and Output Codes. 1.4V 3kΩ DOUT 100pF CLOAD Test Point DOUT tr tf VOH VOL Voltage Waveforms for DOUT Rise and Fall Times, tr, tf Load Circuit for tdDO, tr, and tf Test Point DCLOCK VIL tdDO DOUT thDO Voltage Waveforms for DOUT Delay Times, tdDO VOH VOL DOUT 3kΩ 100pF CLOAD Load Circuit for tdis and ten VCC tdis Waveform 2, ten tdis Waveform 1 CS/SHDN VIH CS/SHDN DCLOCK 1 2 DOUT Waveform 1(1) tdis DOUT Waveform 2(2) Voltage Waveforms for tdis 90% 10% DOUT ten Voltage Waveforms for ten VOL B11 NOTES: (1) Waveform 1 is for an output with internal conditions such that the output is HIGH unless disabled by the output control. (2) Waveform 2 is for an output with internal conditions such that the output is LOW unless disabled by the output control. FIGURE 2. Timing Diagrams and Test Circuits for the Parameters in Table I. ® 9 ADS7822 POWER DISSIPATION Supply Current (µA) 1000 TA = 25°C fCLK = 1.2MHz 100 VCC = 5.0V VREF = 5.0V 10 VCC = 2.7V VREF = 2.5V The architecture of the converter, the semiconductor fabrication process, and a careful design allow the ADS7822 to convert at up to a 75kHz rate while requiring very little power. Still, for the absolute lowest power dissipation, there are several things to keep in mind. The power dissipation of the ADS7822 scales directly with conversion rate. So, the first step to achieving the lowest power dissipation is to find the lowest conversion rate that will satisfy the requirements of the system. In addition, the ADS7822 is in power down mode under two conditions: when the conversion is complete and whenever CS is HIGH (see Figure 1). Ideally, each conversion should occur as quickly as possible, preferably, at a 1.2MHz clock rate. This way, the converter spends the longest possible time in the power down mode. This is very important as the converter not only uses power on each DCLOCK transition (as is typical for digital CMOS components) but also uses some current for the analog circuitry, such as the comparator. The analog section dissipates power continuously, until the power down mode is entered. Figure 3 shows the current consumption of the ADS7822 versus sample rate. For this graph, the converter is clocked at 1.2MHz regardless of the sample rate—CS is HIGH for the remaining sample period. Figure 4 also show current consumption versus sample rate. However, in this case, the DCLOCK period is 1/16th of the sample period—CS is HIGH for one DCLOCK cycle out of every 16. There is an important distinction between the power down mode that is entered after a conversion is complete and the full power down mode which is enabled when CS is HIGH. While both shutdown the analog section, the digital section is completely shutdown only when CS is HIGH. Thus, if CS is left LOW at the end of a conversion and the converter is continually clocked, the power consumption will not be as low as when CS is HIGH. See Figure 5 for more information. Power dissipation can also be reduced by lowering the power supply voltage and the reference voltage. The ADS7822 will operate over a VCC range of 2.0V to 5.25V. However, at voltages below 2.7V, the converter will not run at a 75kHz sample rate. See the typical performance curves for more information regarding power supply voltage and maximum sample rate. SHORT CYCLING Another way of saving power is to utilize the CS signal to short cycle the conversion. Because the ADS7822 places the latest data bit on the DOUT line as it is generated, the converter can easily be short cycled. This term means that the conversion can be terminated at any time. For example, if only 8 bits of the conversion result are needed, then the conversion can be terminated (by pulling CS HIGH) after the 8th bit has been clocked out. 1 0.1 1 10 100 Sample Rate (kHz) FIGURE 3. Maintaining fCLK at the Highest Possible Rate Allows Supply Current to Drop Linearly with Sample Rate. 1000 Supply Current (µA) 100 10 TA = 25°C VCC = 2.7V VREF = 2.5V fCLK = 16 • fSAMPLE 0.1 1 10 100 1 Sample Rate (kHz) FIGURE 4. Scaling fCLK Reduces Supply Current Only Slightly with Sample Rate. 10.0 8.0 TA = 25°C VCC = 2.7V VREF = 2.5V fCLK = 16 • fSAMPLE Supply Current (µA) 6.0 4.0 2.0 0.0 0.050 0.00 0.1 CS LOW (GND) CS HIGH (VCC) 1 10 100 Sample Rate (kHz) FIGURE 5. Shutdown Current with CS HIGH is 50nA Typically, Regardless of the Clock. Shutdown Current with CS LOW Varies with Sample Rate. ® ADS7822 10 This technique can be used to lower the power dissipation (or to increase the conversion rate) in those applications where an analog signal is being monitored until some condition becomes true. For example, if the signal is outside a predetermined range, the full 12-bit conversion result may not be needed. If so, the conversion can be terminated after the first n-bits, where n might be as low as 3 or 4. This results in lower power dissipation in both the converter and the rest of the system, as they spend more time in the power down mode. can drive the bypass capacitor without oscillation (the series resistor can help in this case). Keep in mind that while the ADS7822 draws very little current from the reference on average, there are still instantaneous current demands placed on the external reference circuitry. Also, keep in mind that the ADS7822 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 described in the previous paragraph, voltage variation due to the line frequency (50Hz or 60Hz), can be difficult to remove. The GND pin on the ADS7822 should be placed on a clean ground point. In many cases, this will be the “analog” ground. Avoid connecting the GND pin too close to the grounding point for a microprocessor, microcontroller, or digital signal processor. If needed, run a ground trace directly from the converter to the power supply connection point. The ideal layout will include an analog ground plane for the converter and associated analog circuitry. LAYOUT For optimum performance, care should be taken with the physical layout of the ADS7822 circuitry. This will be particularly true if the reference voltage is low and/or the conversion rate is high. At a 75kHz conversion rate, the ADS7822 makes a bit decision every 830ns. That is, for each subsequent bit decision, the digital output must be updated with the results of the last bit decision, the capacitor array appropriately switched and charged, and the input to the comparator settled to a 12-bit level all within one clock cycle. The basic SAR architecture is sensitive to spikes on the power supply, reference, and ground connections that occur just prior to latching the comparator output. 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 spikes might originate from switching power supplies, digital logic, and high power devices, to name a few. This particular source of error can be very difficult to track down if the glitch is almost synchronous to the converter’s DCLOCK signal—as the phase difference between the two changes with time and temperature, causing sporadic misoperation. With this in mind, power to the ADS7822 should be clean and well bypassed. A 0.1µF ceramic bypass capacitor should be placed as close to the ADS7822 package as possible. In addition, a 1 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, be careful that the op amp APPLICATION CIRCUITS Figures 6 and 7 show some typical application circuits for the ADS7822. Figure 6 uses an ADS7822 and a multiplexer to provide for a flexible data acquisition circuit. A resistor string provides for various voltages at the multiplexer input. The selected voltage is buffered and driven into VREF. As shown in Figure 6, the input range of the ADS7822 is programmable to 100mV, 200mV, 300mV, or 400mV. The 100mV range would be useful for sensors such as the thermocouple shown. Figure 7 shows a basic data acquisition system. The ADS7822 input range is 0V to VCC, as the reference input is connected directly to the power supply. The 5Ω resistor and 1µF to 10µF capacitor filter the microcontroller “noise” on the supply, as well as any high-frequency noise from the supply itself. The exact values should be picked such that the filter provides adequate rejection of the noise. ® 11 ADS7822 +3V +3V +3V R8 26kΩ R7 5Ω OPA237 0.4V R9 1kΩ 0.3V C1 10µF MUX R10 1kΩ 0.2V R11 1kΩ 0.1V R12 1kΩ D1 R1 150kΩ R3 500kΩ R2 59kΩ R6 1MΩ C3 0.1µF VREF C2 0.1µF U2 DCLOCK ADS7822 DOUT CS/SHDN U1 R5 500Ω C5 0.1µF µP 3-Wire Interface U4 A0 A1 TC1 Thermocouple TC2 TC3 C4 10µF R4 1kΩ U3 ISO Thermal Block FIGURE 6. Thermocouple Application Using a MUX to Scale the Input Range of the ADS7822. +2.7V to +3.6V 5Ω + 1µF to 10µF ADS7822 VREF 0.1µF +In –In GND CS DOUT DCLOCK VCC + 1µF to 10µF Microcontroller FIGURE 7. Basic Data Acquisition System. ® ADS7822 12