ADS7818E

® ADS 781 8 ADS7818 For most current data sheet and other product information, visit www.burr-brown.com 12-Bit High Speed Low Power Sampling ANALOG-TO-DIGITAL CONVERTER FEATURES q 500kHz THROUGHPUT RATE q 2.5V INTERNAL REFERENCE q LOW POWER: 11mW q SINGLE SUPPLY +5V OPERATION q DIFFERENTIAL INPUT q SERIAL INTERFACE q GUARANTEED NO MISSING CODES q MINI-DIP-8 AND MSOP-8 q UNIPOLAR INPUT RANGE DESCRIPTION The ADS7818 is a 12-bit sampling analog-to-digital converter (A/D) complete with sample/hold, internal 2.5V reference, and synchronous serial interface. Typical power dissipation is 11mW at a 500kHz throughput rate. The device can be placed into a power down mode which reduces dissipation to just 2.5mW. The input range is zero to two times the reference voltage, and the internal reference can be overdriven by an external voltage. Low power, small size, and high-speed make the ADS7818 ideal for battery operated systems such as wireless communication devices, portable multi-channel data loggers, and spectrum analyzers. The serial interface also provides low-cost isolation for remote data acquisition. The ADS7818 is available in a plastic mini-DIP-8 or an MSOP-8 package and is guaranteed over the –40°C to +85°C temperature range. APPLICATIONS q BATTERY OPERATED SYSTEMS q DIGITAL SIGNAL PROCESSING q HIGH SPEED DATA ACQUISITION q WIRELESS COMMUNICATION SYSTEMS CLK SAR CONV +In CDAC –In S/H Amp Comparator Serial Interface DATA Buffer Internal +2.5V Ref VREF 10kΩ ±30% 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 1 PDS-1408B ADS7818 May, 2000 Printed in U.S.A. SPECIFICATIONS At TA = –40°C to +85°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, internal reference, unless otherwise specified. ADS7818P, E PARAMETER ANALOG INPUT Full-Scale Input Span(1) Absolute Input Range Capacitance Leakage Current SYSTEM PERFORMANCE Resolution No Missing Codes Integral Linearity Error Differential Linearity Error Offset Error Gain Error(3) Common-Mode Rejection Noise Power Supply Rejection SAMPLING DYNAMICS Conversion Time Acquisition Time Throughput Rate Aperture Delay Aperture Jitter Step Response DYNAMIC CHARACTERISTICS Signal-to-Noise Ratio Total Harmonic Distortion(4) Signal-to-(Noise+Distortion) Spurious Free Dynamic Range Usable Bandwidth REFERENCE OUTPUT Voltage Source Current(5) Drift Line Regulation REFERENCE INPUT Range Resistance(6) DIGITAL INPUT/OUTPUT Logic Family Logic Levels: VIH VIL VOH VOL Data Format POWER SUPPLY REQUIREMENT +VCC Quiescent Current Power Dissipation TEMPERATURE RANGE Specified Performance T Specifications same as ADS7818P,E. NOTES: (1) Ideal input span, does not include gain or offset error. (2) LSB means Least Significant Bit, with VREF equal to +2.5V, one LSB is 1.22mV. (3) Measured relative to an ideal, full-scale input (+In – (–In)) of 4.999V. Thus, gain error includes the error of the internal voltage reference. (4) Calculated on the first nine harmonics of the input frequency. (5) If the internal reference is required to source current to an external load, the reference voltage will change due to the internal 10kΩ resistor. (6) Can vary ±30%. VIN VIN VIN VIN = 5Vp-p at 100kHz = 5Vp-p at 100kHz = 5Vp-p at 100kHz = 5Vp-p at 100kHz SINAD > 68dB CONDITIONS MIN TYP MAX MIN T T T T T T T ±1 ±0.8 ±2 ± 12 70 50 150 1.2 1.625 0.350 500 5 30 350 72 –78 70 78 350 2.50 20 0.6 2.0 to Internal Reference Voltage 10 CMOS |IIH| ≤ +5µA |IIL| ≤ +5µA IOH = –500µA I OL = 500µA 3.0 –0.3 3.5 Straight Binary Specified Performance fSAMPLE = 500kHz Power Down fSAMPLE = 500kHz Power Down 4.75 2.2 0.5 11 2.5 –40 5.25 T T T T T T VCC +0.3 0.8 0.4 T T T T T T V mA mA mW mW °C 2.55 T T T T T T –82 72 82 T T T T T T T T T ±2 ±5 ±30 ±50 ±0.5 ±0.5 ±1 ±7 T T T T ±1 ±1 T ±15 ±35 ADS7818PB, EB TYP MAX T T T UNITS +In – (–In) +In –In 0 –0.2 –0.2 15 1 12 12 5 VCC +0.2 +0.2 V V V pF µA Bits Bits LSB(2) LSB LSB LSB LSB dB dB µVrms LSB µs µs kHz ns ps ns dB dB dB dB kHz V µA ppm / °C mV V kΩ 25°C –40°C to +85°C DC, 0.2Vp-p 1MHz, 0.2Vp-p Worst Case ∆, +VCC = 5V ±5% –72 70 75 –75 68 72 IOUT = 0 Static Load IOUT = 0 4.75V ≤ VCC ≤ 5.25V 2.475 2.525 50 2.48 2.52 T T T T T T T V V V V 20 T +85 T 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. ® ADS7818 2 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 (VCC + 0.3V) Power Dissipation .......................................................................... 325mW 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. 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 Top View VREF +In –In GND 1 2 ADS7818 3 4 Plastic Mini-DIP-8 8 7 6 5 +VCC CLK DATA CONV VREF +In –In GND 1 2 ADS7818 3 4 MSOP-8 8 7 6 5 +VCC CLK DATA CONV PIN ASSIGNMENTS PIN 1 2 3 4 5 6 NAME VREF +In –In GND CONV DATA DESCRIPTION Reference Output. Decouple to ground with a 0.1µF ceramic capacitor and a 2.2µF tantalum capacitor. Non-Inverting Input. Inverting Input. Connect to ground or to remote ground sense point. Ground. Convert Input. Controls the sample/hold mode, start of conversion, start of serial data transfer, type of serial transfer, and power down mode. See the Digital Interface section for more information. Serial Data Output. The 12-bit conversion result is serially transmitted most significant bit first with each bit valid on the rising edge of CLK. By properly controlling the CONV input, it is possibly to have the data transmitted least significant bit first. See the Digital Interface section for more information. Clock Input. Synchronizes the serial data transfer and determines conversion speed. Power Supply. Decouple to ground with a 0.1µF ceramic capacitor and a 10µF tantalum capacitor. 7 8 CLK +VCC PACKAGE/ORDERING INFORMATION MAXIMUM INTEGRAL LINEARITY ERROR (LSB) ±2 MAXIMUM DIFFERENTIAL LINEARITY ERROR (LSB) N/S(4) PRODUCT ADS7818E PACKAGE MSOP-8 PACKAGE DRAWING NUMBER(1) 337 SPECIFICATION TEMPERATURE RANGE –40°C to +85°C PACKAGE MARKING(2) A18 ORDERING NUMBER(3) ADS7818E/250 ADS7818E/2K5 ADS7818EB/250 ADS7818EB/2K5 ADS7818P ADS7818PB TRANSPORT MEDIA Tape and Reel Tape and Reel Tape and Reel Tape and Reel Rails Rails " ADS7818EB " ±1 " ±1 " MSOP-8 " 337 " –40°C to +85°C " A18 " ADS7818P ADS7818PB " ±2 ±1 " N/S(4) ±1 " Plastic DIP-8 " 006 " –40°C to +85°C " ADS7818P ADS7818PB " " " 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 ”ADS7818E/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. (4) N/S = Not Specified, typical only. However, 12-Bits no missing codes is guaranteed over temperature. ® 3 ADS7818 TYPICAL PERFORMANCE CURVES At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified. CHANGE IN FULL-SCALE ERROR vs TEMPERATURE 2 0 Delta from 25°C (LSB) 2.0 1.6 CHANGE IN OFFSET vs TEMPERATURE –2 –4 –6 –8 –10 –40 Delta from 25°C (LSB) 1.2 0.8 0.4 0.0 –0.4 –40 –20 0 20 40 60 80 100 –20 0 20 40 60 80 100 Temperature (°C) Temperature (°C) POWER-DOWN SUPPLY CURRENT vs TEMPERATURE 470 SUPPLY CURRENT vs TEMPERATURE 2.3 2.2 Power-down Supply Current (µA) 460 Supply Current (mA) 450 440 430 420 410 400 390 –40 2.1 2.0 1.9 1.8 1.7 1.6 –40 fSAMPLE = 500kHz fSAMPLE = 125kHz –20 0 20 40 60 80 100 –20 0 20 40 60 80 100 Temperature (°C) Temperature (°C) SUPPLY CURRENT vs SAMPLE RATE 2.4 2.3 0.4 CHANGE IN INTEGRAL LINEARITY and DIFFERENTIAL LINEARITY vs SAMPLE RATE Delta from fSAMPLE = 500kHz (LSB) 0.3 0.2 0.1 0.0 –0.1 –0.2 –0.3 100 Change in Differential Linearity (LSB) 200 300 400 500 600 Change in Integral Linearity (LSB) Supply Current (mA) 2.2 2.1 2.0 1.9 1.8 1.7 100 200 300 400 500 600 Sample Rate (kHz) Sample Rate (kHz) ® ADS7818 4 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified. CHANGE IN FULL-SCALE ERROR vs EXTERNAL REFERENCE VOLTAGE 0.2 Delta from VREF = 2.5V (mV) CHANGE IN OFFSET vs EXTERNAL REFERENCE VOLTAGE 0.5 0.4 Delta from VREF = 2.5V (mV) 0.0 –0.2 –0.4 –0.6 –0.8 –1.0 –1.2 2.0 0.3 0.2 0.1 0.0 –0.1 –0.2 –0.3 –0.4 1.9 2.1 2.2 2.3 2.4 2.5 2.1 2.3 2.5 2.7 2.9 3.1 External Reference Voltage (V) External Reference Voltage (V) PEAK-TO-PEAK NOISE vs EXTERNAL REFERENCE VOLTAGE 0.70 POWER SUPPLY REJECTION vs POWER SUPPLY RIPPLE FREQUENCY 30 Power Supply Rejection (mV/V) Peak-to-Peak Noise (LSB) 0.65 0.60 0.55 0.50 0.45 0.40 2.0 25 20 15 10 5 0 2.1 2.2 2.3 2.4 2.5 1 10 100 1k 10k 100k 1M External Reference Voltage (LSB) Power Supply Ripple Frequency (Hz) 0 –20 FREQUENCY SPECTRUM (4096 Point FFT; fIN = 10.9kHz, –0.2dB) 0 –20 Amplitude (dB) FREQUENCY SPECTRUM (4096 Point FFT; fIN = 102kHz, –0.2dB) Amplitude (dB) –40 –60 –80 –100 –120 0 62.5 125 Frequency (kHz) 187.5 250 –40 –60 –80 –100 –120 0 62.5 125 Frequency (kHz) 187.5 250 ® 5 ADS7818 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified. 0 –20 Amplitude (dB) FREQUENCY SPECTRUM (4096 Point FFT; fIN = 247kHz, –0.2dB) SIGNAL-TO-NOISE RATIO and SIGNAL-TO-(NOISE+DISTORTION) vs INPUT FREQUENCY 76 74 SNR and SINAD (dB) SNR –40 –60 –80 –100 –120 0 62.5 125 Frequency (kHz) 187.5 250 72 SINAD 70 68 66 64 1k 10k 100k 1M Input Frequency (Hz) SPURIOUS FREE DYNAMIC RANGE AND TOTAL HARMONIC DISTORTION vs INPUT FREQUENCY 90 SFDR 85 THDh SFDR (dB) THD (dB) –90 –85 Delta from +25°C (dB) CHANGE IN SIGNAL-TO-NOISE RATIO and SIGNAL-TO-(NOISE+DISTORTION) vs TEMPERATURE 0.3 0.2 SNR 0.1 0.0 –0.1 –0.2 fIN = 100kHz, –0.2dB –0.3 –40 –20 0 20 40 60 80 100 Temperature (°C) 80 –80 –75 –70 h First SINAD 75 70 nine harmonics of the input frequency 65 1k 10k 100k 1M Input Frequency (Hz) –65 CHANGE IN SPURIOUS FREE DYNAMIC RANGE AND TOTAL HARMONIC DISTORTION vs TEMPERATURE 1.0 fIN = 100kHz, –0.2dB –1.0 –0.5 THDh 0.0 0.5 SFDR –1.0 –1.5 –2.0 –40 1.0 1.5 2.0 20 40 60 80 100 SFDR Delta from +25°C (dB) 0.0 –0.5 h First nine harmonics of the input frequency –20 0 Temperature (°C) ® ADS7818 THD Delta from +25°C (dB) 0.5 6 THEORY OF OPERATION The ADS7818 is a high speed successive approximation register (SAR) analog-to-digital converter (A/D) with an internal 2.5V bandgap reference. The architecture is based on capacitive redistribution which inherently includes a sample/hold function. The converter is fabricated on a 0.6µ CMOS process. See Figure 1 for the basic operating circuit for the ADS7818. The ADS7818 requires an external clock to run the conversion process. This clock can vary between 200kHz (12.5Hz throughput) and 8MHz (500kHz throughput). The duty cycle of the clock is unimportant as long as the minimum HIGH and LOW times are at least 50ns and the clock period is at least 125ns. The minimum clock frequency is set by the leakage on the capacitors internal to the ADS7818. 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 range of the analog input is set by the voltage on the VREF pin. With the internal 2.5V reference, the input range is 0 to 5V. An external reference voltage can be placed on VREF, overdriving the internal voltage. The range for the external voltage is 2.0V to 2.55V, giving an input voltage range of 4.0V to 5.1V. The digital result of the conversion is provided in a serial manner, synchronous to the CLK input. The result is provided most significant bit first and represents the result of the conversion currently in progress—there is no pipeline delay. By properly controlling the CONV and CLK inputs, it is possible to obtain the digital result least significant bit first. are common to both inputs. Thus, the –IN input is best used to sense a remote ground point near the source of the +IN signal. If the source driving the +IN signal is nearby, the –IN should be connected directly to ground. The input current into the analog input depends on input voltage and sample rate. Essentially, the current into the device must charge the internal hold capacitor 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 to a 12-bit settling level within the sample period—which can be as little as 350ns in some operating modes. While the converter is in the hold mode or after the sampling capacitor has been fully charged, the input impedance of the analog input is greater than 1GΩ. Care must be taken regarding the input voltage on the +In and –IN pins. To maintain the linearity of the converter, the +In input should remain within the range of GND – 200mV to +VCC + 200mV. The –IN input should not drop below GND – 200mV or exceed GND + 200mV. Outside of these ranges, the converter’s linearity may not meet specifications. REFERENCE The reference voltage on the VREF pin directly sets the fullscale range of the analog input. The ADS7818 can operate with a reference in the range of 2.0V to 2.55V, for a fullscale range of 4.0V to 5.1V. The voltage at the VREF pin is internally buffered and this buffer drives the capacitor DAC portion of the converter. This is important because the buffer greatly reduces the dynamic load placed on the reference source. However, the voltage at VREF will still contain some noise and glitches from the SAR conversion process. These can be reduced by carefully bypassing the VREF pin to ground as outlined in the sections that follow. INTERNAL REFERENCE The ADS7818 contains an on-board 2.5V reference, resulting in a 0V to 5V input range on the analog input. The specification table gives the various specifications for the ANALOG INPUT The +IN and –IN input pins allow for a differential input signal to be captured on the internal hold capacitor when the converter enters the hold mode. The voltage range on the –IN input is limited to –0.2V to 0.2V. Because of this, the differential input can be used to reject only small signals that +5V + + 2.2µF 0.1µF 1 0 to 5V Analog Input 2 3 4 ADS7818 VREF +In –In GND +VCC CLK DATA CONV 8 7 6 5 0.1µF 10µF Serial Clock Serial Data Convert Start from Microcontroller or DSP FIGURE 1. Basic Operation of the ADS7818. ® 7 ADS7818 internal reference. This reference can be used to supply a small amount of source current to an external load, but the load should be static. Due to the internal 10kΩ resistor, a dynamic load will cause variations in the reference voltage, and will dramatically affect the conversion result. Note that even a static load will reduce the internal reference voltage seen at the buffer input. The amount of reduction depends on the load and the actual value of the internal “10kΩ” resistor. The value of this resistor can vary by ±30%. The VREF pin should be bypassed with a 0.1µF capacitor placed as close as possible to the ADS7818 package. In addition, a 2.2 µF tantalum capacitor should be used in parallel with the ceramic capacitor. Placement of this capacitor is not as critical. EXTERNAL REFERENCE The internal reference is connected to the VREF pin and to the internal buffer via a 10kΩ series resistor. Thus, the reference voltage can easily be overdriven by an external reference voltage. The voltage range for the external voltage is 2.0V to 2.55V, corresponding to an analog input range of 4.0V to 5.1V. While the external reference will not source significant current into the VREF pin, it does have to drive the series 10kΩ resistor that is terminated into the 2.5V internal reference (the exact value of the resistor will vary up to ±30% from part to part). In addition, the VREF pin should still be bypassed to ground with at least a 0.1 µF ceramic capacitor (placed as close to the ADS7818 as possible). The reference will have to be stable with this capacitive load. Depending on the particular reference and A/D conversion speed, additional bypass capacitance may be required, such as the 2.2µF tantalum capacitor shown in Figure 1. Reasons for choosing an external reference over the internal reference vary, but there are two main reasons. One is to achieve a given input range. For example, a 2.048V reference provides for a 0V to 4.095V input range—or 1mV per LSB. The other is to provide greater stability over temperature. The internal reference is typically 20ppm/°C which translates into a full-scale drift of roughly 1 output code for every 12°C (this does not take into account other sources of full-scale drift). If greater stability over temperature is needed, then an external reference with lower temperature drift will be required. DIGITAL INTERFACE Figure 2 shows the serial data timing and Figure 3 shows the basic conversion timing for the ADS7818. The specific timing numbers are listed in Table I. There are several important items in Figure 3 which give the converter additional capabilities over typical 8-pin converters. First, the transition from sample mode to hold mode is synchronous to the falling edge of CONV and is not dependent on CLK. Second, the CLK input is not required to be continuous during the sample mode. After the conversion is complete, the CLK may be kept LOW or HIGH. tCKP tCKH CLK tCKDS DATA tCKL tCKDH FIGURE 2. Serial Data and Clock Timing. SYMBOL tACQ tCONV tCKP tCKL tCKH tCKDH tCKDS tCVL tCVH tCKCH tCKCS tCKDE tCKDD tCKSP tCKPD tCVHD tCVSP tCVPU tCVDD tCVPD tDRP DESCRIPTION Acquisition Time Conversion Time Clock Period Clock LOW Clock HIGH Clock Falling to Current Data Bit No Longer Valid Clock Falling to Next Data Valid CONV LOW CONV HIGH CONV Hold after Clock Falls(1) CONV Setup to Clock Falling(1) Clock Falling to DATA Enabled Clock Falling to DATA High Impedance Clock Falling to Sample Mode Clock Falling to Power-down Mode CONV Falling to Hold Mode (Aperture Delay) CONV Rising to Sample Mode CONV Rising to Full Power-up CONV Changing State to DATA High Impedance CONV Changing State to Power-down Mode CONV Falling to Start of CLK (for hold droop < 0.1 LSB) 40 40 10 10 20 70 5 50 5 5 50 70 50 5 100 50 100 MIN 350 1.5 125 50 50 5 15 30 50 5000 TYP MAX UNITS ns µs ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns µs Note: (1) This timing is not required under some situations. See text for more information. TABLE I. Timing Specifications (TA = –40°C to +85°C, CLOAD = 30pF). The asynchronous nature of CONV to CLK raises some interesting possibilities, but also some design considerations. Figure 3 shows that CONV has timing restraints in relation to CLK (tCKCH and tCKCS). However, if these times are violated (which could happen if CONV is completely asynchronous to CLK), the converter will perform a conversion correctly, but the exact timing of the conversion is indeterminate. Since the setup and hold time between CONV and CLK has been violated in this example, the start of conversion could vary by one clock cycle. (Note that the start of conversion can be detected by using a pull-up resistor on DATA. When DATA drops out of high-impedance and goes LOW, the conversion has started and that clock cycle is this first of the conversion.) In addition if CONV is completely asynchronous to CLK and CLK is continuous, then there is possibility that CLK will transition just prior to CONV going LOW. If this occurs ® ADS7818 8 faster than the 10ns indicated by tCKCH, then there is a chance that some digital feedthrough may be coupled onto the hold capacitor. This could cause a small offset error for that particular conversion. Thus, there are two basic ways to operate the ADS7818. CONV can be synchronous to CLK and CLK can be continuous. This would be the typical situation when interfacing the converter to a digital signal processor. The second method involves having CONV asynchronous to CLK and gating the operation of CLK (a non-continuous clock). This method would be more typical of an SPI-like interface on a microcontroller. This method would also allow CONV to be generated by a trigger circuit and to initiate (after some delay) the start of CLK. These two methods are covered under DSP Interfacing and SPI Interfacing. POWER-DOWN TIMING The conversion timing shown in Figure 3 does not result in the ADS7818 going into the power-down mode. If the conversion rate of the device is high (approaching 500kHz), then there is very little power that can be saved by using the power-down mode. However, since the power-down mode incurs no conversion penalty (the very first conversion is valid), at lower sample rates, significant power can be saved by allowing the device to go into power-down mode between conversions. Figure 4 shows the typical method for placing the A/D into the power-down mode. If CONV is kept LOW during the conversion and is LOW at the start of the 13 clock cycle, then the device enters the power-down mode. It remains in this mode until the rising edge of CONV. Note that CONV must be HIGH for at least tACQ in order to sample the signal properly as well as to power-up the internal nodes. There are two different methods for clocking the ADS7818. The first involves scaling the CLK input in relation to the conversion rate. For example, an 8MHz input clock and the timing shown in Figure 3 results in a 500kHz conversion rate. Likewise, a 1.6MHz clock would result in a 100kHz conversion rate. The second method involves keeping the clock input as close to the maximum clock rate as possible and starting conversions as needed. This timing is similar to that shown in Figure 4. As an example, a 50kHz conversion rate would require 160 clock periods per conversion instead of the 16 clock periods used at 500kHz. The main distinction between the two is the amount of time that the ADS7818 remains in power down. In the first mode, the converter only remains in power down for a small number of clock periods (depending on how many clock periods there are per each conversion). As the conversion rate scales, the converter always spends the same percentage of time in power down. Since less power is drawn by the digital logic, there is a small decrease in power consumption, but it is very slight. This effect can be seen in the typical performance curve “Supply Current vs Sample Rate.” tCVL CONV tCKCS tCKCH CLK 14 (1) tCVCK 15 16 1 2 3 4 11 12 13 14 15 16 1 tCKDE DATA D11 (MSB) D10 D9 D2 D1 D0 (LSB) tCKDD tACQ tCVHD SAMPLE/HOLD MODE SAMPLE (2) tCKSP HOLD SAMPLE HOLD tCONV INTERNAL CONVERSION STATE IDLE CONVERSION IN PROGRESS IDLE(3) NOTES: (1) Clock periods 14 and 15 are shown for clarity, but are not required for proper operation of the ADS7818, provided that the minimum tACQ time is met. The CLK input may remain HIGH or LOW during this period. (2) The transition from sample mode to hold mode occurs on the falling edge of CONV. This transition is not dependent on CLK. (3) The device remains fully powered when operated as shown. If the sample time is longer than 3 clock periods, power consumption can be reduced by allowing the device to enter a power down mode. See the power down timing for more information. FIGURE 3. Basic Conversion Timing. ® 9 ADS7818 CONV CLK 1 2 3 12 13 DATA D11 (MSB) D10 D1 D0 (LSB) tCVSP tACQ SAMPLE (3) SAMPLE/HOLD SAMPLE MODE HOLD HOLD INTERNAL CONVERSION STATE IDLE CONVERSION IN PROGRESS tCKPD IDLE tCVPU LOW POWER (1) (2) POWER MODE FULL POWER FULL POWER NOTES: (1) The low power mode (“power-down”) is entered when CONV remains LOW during the conversion and is still LOW at the start of the 13th clock cycle. (2) The low power mode is exited when CONV goes HIGH. (3) When in power-down, the transition from hold mode to sample mode is initiated by CONV going HIGH. FIGURE 4. Power-down Timing. tCVH CONV tCKCH CLK 1 2 3 12 13 14 tCKCS DATA D11 (MSB) D10 D1 D0 (LSB) (1) 23 24 tCVDD D1 D10 D11 (MSB) LOW... (2) SAMPLE/HOLD SAMPLE MODE HOLD INTERNAL CONVERSION STATE IDLE CONVERSION IN PROGRESS tCVPD IDLE POWER MODE FULL POWER (3) LOW POWER NOTES: (1) The serial data can be transmitted LSB first by pulling CONV LOW during the 13th clock cycle. (2) After the MSB has been transmitted, the DATA output pin will remain LOW until CONV goes HIGH. (3) When CONV is taken LOW to initiate the LSB first transfer, the converter enters the power-down mode. FIGURE 5. Serial Data “LSB-First” Timing. In contrast, the second method (clocking at a fixed rate) means that each conversion takes X clock cycles. As the time between conversions get longer, the converter remains in power-down an increasing percentage of time. This reduces total power consumption by a considerable amount. For example, a 50kHz conversion rate results in roughly 1/10 of the power (minus the reference) that is used at a 500kHz conversion rate. ® ADS7818 10 Table II offers a look at the two different modes of operation and the difference in power consumption. POWER WITH CLK = 16 • fSAMPLE 11mW 10mW 9mW POWER WITH CLK = 8MHz 11mW 7mW 4mW fSAMPLE 500kHz 250kHz 100kHz the conversion will terminate immediately, before all 12-bits have been decided. This can be a very useful feature when a resolution of 12-bits is not needed. An example would be when the converter is being used to monitor an input voltage until some condition is met. At that time, the full resolution of the converter would then be used. Short-cycling the conversion can result in a faster conversion rate or lower power dissipation. There are several very important items shown in Figure 6. The conversion currently in progress is terminated when CONV is taken HIGH during the conversion and then taken LOW prior to tCKCH before the start of the 13th clock cycle. Note that if CONV goes LOW during the 13th clock cycle, then the LSB first mode will be entered (Figure 5). Also, when CONV goes LOW, the DATA output immediately transitions to high impedance. If the output bit that is present during that clock period is needed, CONV must not go LOW until the bit has been properly latched into the receiving logic. DATA FORMAT The ADS7818 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. TABLE II. Power Consumption versus CLK Input. LSB FIRST DATA TIMING Figure 5 shows a method to transmit the digital result in a least-significant bit (LSB) format. This mode is entered when CONV is pulled HIGH during the conversion (before the end of the 12th clock) and then pulled LOW during the 13th clock (when D0, the LSB, is being transmitted). The next 11 clocks then repeat the serial data, but in an LSB first format. The converter enters the power-down mode during the 13th clock and resumes normal operation when CONV goes HIGH. SHORT-CYCLE TIMING The conversion currently in progress can be “short-cycled” with the technique shown in Figure 6. This term means that (1) tCVL CONV tCVH CLK 1 2 3 4 5 6 7 tCVDD DATA D11 (MSB) D10 D9 D8 D7 D6 SAMPLE/HOLD MODE SAMPLE HOLD INTERNAL CONVERSION STATE IDLE CONVERSION IN PROGRESS tCVPD IDLE POWER MODE FULL POWER LOW POWER NOTE: (1) The conversion currently in progress can be stopped by pulling CONV LOW during the conversion. This must occur at least tCKCS prior to the start of the 13th clock cycle. The DATA output pin will tri-state and the device will enter the power-down mode when CONV is pulled LOW. FIGURE 6. Short-cycle Timing. ® 11 ADS7818 FS = Full-Scale Voltage = 2 • VREF 1 LSB = FS/4096 1 LSB Output Code 11...111 11...110 11...101 microcontrollers form various manufacturers. CONV would be tied to a general purpose I/O pin (SPI) or to a PCX pin (QSPI), CLK would be tied to the serial clock, and DATA would be tied to the serial input data pin such as MISO (master in slave out). Note the time tDRP shown in Figure 9. This represents the maximum amount of time between CONV going LOW and the start of the conversion clock. Since CONV going LOW places the sample and hold in the hold mode and because the hold capacitor looses charge over time, there is a requirement that time tDRP be met as well as the maximum clock period (tCKP). 00...010 00...001 00...000 0V Input Voltage (V) (2) 4.999V(1) NOTES: (1) For external reference, value is 2 • VREF – 1 LSB. (2) Voltage at converter input: +IN–(–IN). LAYOUT For optimum performance, care should be taken with the physical layout of the ADS7818 circuitry. This is particularly true if the CLK input is approaching the maximum input rate. 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 CLK input. FIGURE 7. Ideal Input Voltages and Output Codes. DSP INTERFACING Figure 8 shows a timing diagram that might be used with a typical digital signal processor such as a TI DSP. For the buffered serial port (BSP) on the TMS320C54X family, CONV would tied to BFSX, CLK would be tied to BCLKX, and DATA would be tied to BDR. SPI/QSPI INTERFACING Figure 9 shows the timing diagram for a typical serial peripheral interface (SPI) or queued serial peripheral interface (QSPI). Such interfaces are found on a number of CONV CLK 15 16 1 2 3 12 13 14 15 16 1 2 3 4 DATA D11 (MSB) D10 D1 D0 (LSB) D11 (MSB) D10 D9 FIGURE 8. Typical DSP Interface Timing. tDRP CONV tACQ CLK 1 2 3 4 13 14 15 16 1 2 3 DATA D11 (MSB) D10 D1 D0 (LSB) D11 (MSB) FIGURE 9. Typical SPI/QSPI Interface Timing. ® ADS7818 12 With this in mind, power to the ADS7818 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 is recommended. If needed, an even larger capacitor and a 5Ω or 10Ω series resistor my be used to lowpass filter a noisy supply. The ADS7818 draws very little current from an external reference on average as the reference voltage is internally buffered. However, glitches from the conversion process appear at the VREF input and the reference source must be able to handle this. Whether the reference is internal or external, the VREF pin should be bypassed with a 0.1µF capacitor. An additional larger capacitor may also be used, if desired. If the reference voltage is external and originates from an op-amp, make sure that it can drive the bypass capacitor or capacitors without oscillation. 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. ® 13 ADS7818