® ADS7843 TOUCH SCREEN CONTROLLER FEATURES DESCRIPTION ● 4-WIRE TOUCH SCREEN INTERFACE The ADS7843 is a 12-bit sampling analog-to-digital converter (ADC) with a synchronous serial interface and low on-resistance switches for driving touch screens. Typical power dissipation is 750µW at a 125kHz throughput rate and a +2.7V supply. The reference voltage (VREF) can be varied between 1V and +VCC, providing a corresponding input voltage range of 0V to VREF. The device includes a shutdown mode which reduces typical power dissipation to under 0.5µW. The ADS7843 is guaranteed down to 2.7V operation. ● RATIOMETRIC CONVERSION ● SINGLE SUPPLY: 2.7V to 5V ● UP TO 125kHz CONVERSION RATE ● ● ● ● SERIAL INTERFACE PROGRAMMABLE 8- OR 12-BIT RESOLUTION 2 AUXILIARY ANALOG INPUTS FULL POWER-DOWN CONTROL Low power, high speed, and on-board switches make the ADS7843 ideal for battery operated systems such as personal digital assistants with resistive touch screens and other portable equipment. The ADS7843 is available in a 16-lead SSOP package and is guaranteed over the –40°C to +85°C temperature range. APPLICATIONS ● ● ● ● ● PERSONAL DIGITAL ASSISTANTS PORTABLE INSTRUMENTS POINT-OF-SALES TERMINALS PAGERS TOUCH-SCREEN MONITORS PENIRQ +VCC X+ X– SAR DCLK Y+ Y– Four Channel Multiplexer CS Comparator CDAC IN3 IN4 Serial Interface and Control DIN DOUT BUSY VREF 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/ • FAXLine: (800) 548-6133 (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © 1997 Burr-Brown Corporation PDS-1441C Printed in U.S.A. June, 1998 SPECIFICATIONS At TA =–40°C to +85°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, fCLK = 16 • fSAMPLE = 2MHz, 12-bit mode, and digital inputs = GND or +VCC, unless otherwise noted. ADS7843E PARAMETER ANALOG INPUT Full-Scale Input Span Absolute Input Range CONDITIONS MIN Positive Input - Negative Input Positive Input Negative Input 0 –0.2 –0.2 Capacitance Leakage Current TYP MAX UNITS VREF +VCC +0.2 +0.2 V V V pF µA 25 0.1 SYSTEM PERFORMANCE Resolution No Missing Codes Integral 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 Channel-to-Channel Isolation 12 11 0.1 0.1 30 70 12 5 6 Ω Ω 125 VIN = 2.5Vp-p at 50kHz 1.0 CS = GND or +VCC +VCC 5 13 2.5 0.001 fSAMPLE = 12.5kHz CS = +VCC DIGITAL INPUT/OUTPUT Logic Family Logic Levels, Except PENIRQ VIH VIL VOH VOL PENIRQ VOL Data Format 500 30 100 100 Clk Cycles Clk Cycles kHz ns ns ps dB 3 SWITCH DRIVERS On-Resistance Y+, X+ Y–, X– REFERENCE INPUT Range Resistance Input Current ±2 ±6 1.0 ±4 1.0 Bits Bits LSB(1) LSB LSB LSB LSB µVrms dB 40 3 V GΩ µA µA µA CMOS | IIH | ≤ +5µA | IIL | ≤ +5µA IOH = –250µA IOL = 250µA +VCC • 0.7 –0.3 +VCC • 0.8 +VCC +0.3 +0.8 TA = 0°C to +85°C, 100kΩ Pull-Up 0.4 V V V 0.8 V 3.6 650 3 V µA µA µA 1.8 mW +85 °C Straight Binary POWER SUPPLY REQUIREMENTS Specified Performance +VCC Quiescent Current 2.7 280 220 fSAMPLE = 12.5kHz Shut Down Mode with DCLK = DIN = +VCC +VCC = +2.7V Power Dissipation TEMPERATURE RANGE Specified Performance –40 NOTE: (1) LSB means Least Significant Bit. With VREF equal to +2.5V, one LSB is 610µV. 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. ® ADS7843 2 PIN CONFIGURATION PIN DESCRIPTION Top View SSOP +VCC 1 16 DCLK X+ 2 15 CS Y+ 3 14 DIN X– 4 13 BUSY 12 DOUT ADS7843 Y– 5 GND 6 11 PENIRQ IN3 7 10 +VCC IN4 8 9 VREF ABSOLUTE MAXIMUM RATINGS(1) PIN NAME 1 2 3 4 5 6 7 8 9 10 11 +VCC X+ Y+ X– Y– GND IN3 IN4 VREF +VCC PENIRQ 12 DOUT 13 BUSY 14 DIN 15 CS 16 DCLK +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 .......................................................................... 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 DESCRIPTION Power Supply, 2.7V to 5V. X+ Position Input. ADC input Channel 1. Y+ Position Input. ADC input Channel 2. X– Position Input. Y– Position Input. Ground Auxiliary Input 1. ADC input Channel 3. Auxiliary Input 2. ADC input Channel 4. Voltage Reference Input Power Supply, 2.7V to 5V. Pen Interrupt. Open anode output (requires 10kΩ to 100kΩ pull-up resistor externally). Serial Data Output. Data is shifted on the falling edge of DCLK. This output is high impedance when CS is HIGH. Busy Output. This output is high impedance when CS is HIGH. Serial Data Input. If CS is LOW, data is latched on rising edge of DCLK. Chip Select Input. Controls conversion timing and enables the serial input/output register. External Clock Input. This clock runs the SAR conversion process and synchronizes serial data I/O. 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. 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. PACKAGE/ORDERING INFORMATION PRODUCT MAXIMUM INTEGRAL LINEARITY ERROR (LSB) ADS7843E " PACKAGE PACKAGE DRAWING NUMBER(1) SPECIFICATION TEMPERATURE RANGE ±2 16-Lead SSOP 322 –40°C to +85°C " " " " ORDERING NUMBER(2) TRANSPORT MEDIA ADS7843E ADS7843E/2K5 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 “ADS7843E/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. ® 3 ADS7843 TYPICAL PERFORMANCE CURVES At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. POWER DOWN SUPPLY CURRENT vs TEMPERATURE 400 140 350 120 Supply Current (nA) Supply Current (µA) SUPPLY CURRENT vs TEMPERATURE 300 250 200 100 80 60 40 150 20 100 –40 –20 0 20 60 40 80 –40 100 –20 0 20 60 40 80 100 Temperature (˚C) Temperature (˚C) SUPPLY CURRENT vs +VCC MAXIMUM SAMPLE RATE vs +VCC 320 1M 300 Sample Rate (Hz) Supply Current (µA) fSAMPLE = 12.5kHz 280 VREF = +VCC 260 240 220 100k 10k VREF = +VCC 200 180 1k 2 2.5 3.5 3 4 4.5 5 2 2.5 3 +VCC (V) 4 4.5 5 CHANGE IN OFFSET vs TEMPERATURE CHANGE IN GAIN vs TEMPERATURE 0.15 0.6 0.10 0.4 Delta from +25˚C (LSB) Delta from +25˚C (LSB) 3.5 +VCC (V) 0.05 0.00 –0.05 0.2 0.0 –0.2 –0.4 –0.10 –0.6 –0.15 –40 –20 0 20 40 60 80 –40 100 0 20 40 Temperature (˚C) Temperature (˚C) ® ADS7843 –20 4 60 80 100 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 TEMPERATURE 18 12 16 Reference Current (µA) Reference Current (µA) REFERENCE CURRENT vs SAMPLE RATE 14 10 8 6 4 14 12 10 8 2 0 6 0 25 50 75 100 125 –40 –20 0 20 40 60 80 Sample Rate (kHz) Temperature (˚C) SWITCH ON RESISTANCE vs +VCC (X+, Y+: +VCC to Pin; X–, Y–: Pin to GND) SWITCH ON RESISTANCE vs TEMPERATURE (X+, Y+: +VCC to Pin; X–, Y–: Pin to GND) 8 8 7 7 6 6 X– 100 Y– RON (Ω) X– 4 Y+ 5 X+ Y+ 4 X+ 3 3 2 2 1 1 2 2.5 3 3.5 4 4.5 5 –40 –20 0 +VCC (V) 20 40 60 80 100 Temperature (˚C) MAXIMUM SAMPLING RATE vs RIN 2 1.8 INL: R = 2k INL: R = 500 DNL: R = 2k DNL: R = 500 1.6 1.4 LSB Error RON (Ω) Y– 5 1.2 1 0.8 0.6 0.4 0.2 0 20 40 60 80 100 120 140 Sampling Rate (kHz) 160 180 200 ® 5 ADS7843 THEORY OF OPERATION ANALOG INPUT Figure 2 shows a block diagram of the input multiplexer on the ADS7843, the differential input of the A/D converter, and the converter’s differential reference. Table I and Table II show the relationship between the A2, A1, A0, and SER/DFR control bits and the configuration of the ADS7843. The control bits are provided serially via the DIN pin—see the Digital Interface section of this data sheet for more details. The ADS7843 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 ADS7843 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 1V 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 ADS7843. 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 input current on the analog inputs depends on the conversion rate of the device. During the sample period, the source must charge the internal sampling capacitor (typically 25pF). After the capacitor has been fully charged, there is no further input current. The rate of charge transfer from the analog source to the converter is a function of conversion rate. The analog input to the converter is provided via a fourchannel multiplexer. A unique configuration of low onresistance switches allows an unselected ADC input channel to provide power and an accompanying pin to provide ground for an external device. By maintaining a differenital input to the converter and a differential reference architecture, it is possible to negate the switch’s on-resistance error (should this be a source of error for the particular measurement). A2 A1 A0 X+ 0 1 0 1 0 0 1 1 1 1 0 0 +IN Y+ IN3 IN4 –IN(1) X SWITCHES Y SWITCHES +REF(1) –REF(1) +IN GND GND GND GND OFF ON OFF OFF ON OFF OFF OFF +VREF +VREF +VREF +VREF GND GND GND GND +IN +IN NOTE: (1) Internal node, for clarification only—not directly accessible by the user. TABLE I. Input Configuration, Single-Ended Reference Mode (SER/DFR HIGH). A2 A1 A0 X+ 0 1 0 1 0 0 1 1 1 1 0 0 +IN Y+ IN3 IN4 –IN(1) X SWITCHES Y SWITCHES +REF(1) –REF(1) +IN –Y –X GND GND OFF ON OFF OFF ON OFF OFF OFF +Y +X +VREF +VREF –Y –X GND GND +IN +IN NOTE: (1) Internal node, for clarification only—not directly accessible by the user. TABLE II. Input Configuration, Differential Reference Mode (SER/DFR LOW). +2.7V to +5V 1µF + to 10µF (Optional) ADS7843 0.1µF Touch Screen Auxiliary Inputs DCLK 16 +VCC 2 X+ CS 15 3 Y+ DIN 14 4 X– BUSY 13 Converter Status 5 Y– DOUT 12 Serial Data Out 6 GND 7 IN3 +VCC 10 8 IN4 VREF PENIRQ 11 9 0.1µF FIGURE 1. Basic Operation of the ADS7843. ® ADS7843 6 Serial/Conversion Clock 1 Chip Select Serial Data In Pen Interrupt 100kΩ (optional) PENIRQ +VCC VREF A2-A0 (Shown 001B) SER/DFR (Shown HIGH) X+ X– Y+ +IN +REF CONVERTER Y– –IN –REF IN3 IN4 GND FIGURE 2. Simplified Diagram of Analog Input. REFERENCE INPUT the ADS7843 as shown in Figure 1. This particular application shows the device being used to digitize a resistive touch screen. A measurement of the current Y position of the pointing device is made by connecting the X+ input to the A/D converter, turning on the Y+ and Y– drivers, and digitizing the voltage on X+ (see Figure 3 for a block diagram). For this measurement, the resistance in the X+ lead does not affect the conversion (it does affect the settling time, but the resistance is usually small enough that this is not a concern). The voltage difference between +REF and –REF (see Figure 2) sets the analog input range. The ADS7843 will operate with a reference in the range of 1V to +VCC. 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, it will typically be 5 LSBs with a 1V reference. In each case, the actual offset of the device is the same, 1.22mV. With a lower reference voltage, more care must 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. VREF +VCC Y+ The voltage into the VREF input is not buffered and directly drives the capacitor digital-to-analog converter (CDAC) portion of the ADS7843. Typically, the input current is 13µA with VREF = 2.7V and fSAMPLE = 125kHz. This value will vary by a few 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. X+ +REF +IN Converter –IN –REF Y– GND There is also a critical item regarding the reference when making measurements where the switch drivers are on. For this discussion, it’s useful to consider the basic operation of FIGURE 3. Simplified Diagram of Single-Ended Reference (SER/DFR HIGH, Y Switches Enabled, X+ is Analog Input). ® 7 ADS7843 However, since the resistance between Y+ and Y– is fairly low, the on-resistance of the Y drivers does make a small difference. Under the situation outlined so far, it would not be possible to achieve a zero volt input or a full-scale input regardless of where the pointing device is on the touch screen because some voltage is lost across the internal switches. In addition, the internal switch resistance is unlikely to track the resistance of the touch screen, providing an additional source of error. This situation can be remedied as shown in Figure 4. By setting the SER/DFR bit LOW, the +REF and –REF inputs are connected directly to Y+ and Y–. This makes the analogto-digital conversion ratiometric. The result of the conversion is always a percentage of the external resistance, regardless of how it changes in relation to the on-resistance of the internal switches. Note that there is an important consideration regarding power dissipation when using the ratiometric mode of operation, see the Power Dissipation section for more details. 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, switches, and reference inputs appropriately, the converter enters the acquisition (sample) mode and, if needed, the internal switches are turned on. 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 and the internal switches may turn off. The next twelve clock cycles accomplish the actual analog-to-digital conversion. If the conversion is ratiometric (SER/DFR LOW), the internal switches are on during the 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 +VCC Also shown in Figure 5 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 ADS7843 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). The MODE bit determines the number of bits for each conversion, either 12 bits (LOW) or 8 bits (HIGH). Y+ X+ +IN +REF Converter –IN –REF The SER/DFR bit controls the reference mode: either singleended (HIGH) or differential (LOW). (The differential mode is also referred to as the ratiometric conversion mode.) In Y– GND FIGURE 4. Simplified Diagram of Differential Reference (SER/DFR LOW, Y Switches Enabled, X+ is Analog Input). Bit 7 (MSB) Bit 6 Bit 5 Bit 4 S A2 A1 A0 Bit 2 MODE SER/DFR Bit 1 Bit 0 (LSB) PD1 PD0 TABLE III. Order of the Control Bits in the Control Byte. As a final note about the differential reference mode, it must be used with +VCC as the source of the +REF voltage and cannot be used with VREF. It is possible to use a high precision reference on VREF and single-ended reference mode for measurements which do not need to be ratiometric. Or, in some cases, it could be possible to power the converter directly from a precision reference. Most references can provide enough power for the ADS7843, but they might not be able to supply enough current for the external load (such as a resistive touch screen). BIT 7 NAME Figure 5 shows the typical operation of the ADS7843’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. Each communication between the processor and the converter consists of eight clock cycles. One DESCRIPTION S Start Bit. Control byte starts with first HIGH bit on DIN. A new control byte can start every 15th clock cycle in 12-bit conversion mode or every 11th clock cycle in 8-bit conversion mode. 6-4 A2 - A0 Channel Select Bits. Along with the SER/DFR bit, these bits control the setting of the multiplexer input, switches, and reference inputs, as detailed in Tables I and II. 3 MODE 12-Bit/8-Bit Conversion Select Bit.This bit controls the number of bits for the following conversion: 12bits (LOW) or 8-bits (HIGH). 2 SER/DFR Single-Ended/Differential Reference Select Bit. Along with bits A2 - A0, this bit controls the setting of the multiplexer input, switches, and reference inputs, as detailed in Tables I and II. PD1 - PD0 Power-Down Mode Select Bits. See Table V for details. DIGITAL INTERFACE 1-0 TABLE IV. Descriptions of the Control Bits within the Control Byte. ® ADS7843 Bit 3 8 single-ended mode, the converter’s reference voltage is always the difference between the VREF and GND pins. In differential mode, the reference voltage is the difference between the currently enabled switches. See Tables I and II and Figures 2 through 4 for more information. The last two bits (PD1 - PD0) select the power- down 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. There are two power-down modes: one where PENIRQ is disabled and one where it is enabled. PD1 PD0 PENIRQ DESCRIPTION 0 0 Enabled 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. The Y– switch is on while in power-down. 0 1 Disabled Same as mode 00, except PENIRQ is disabled. The Y– switch is off while in power-down mode. 1 0 Disabled Reserved for future use. 1 1 Disabled No power-down between conversions, device is always powered. TABLE V. Power-Down Selection. CS tACQ DCLK 1 DIN S 8 A2 A1 1 8 1 8 A0 MODE SER/ DFR PD1 PD0 (START) Idle Acquire Conversion Idle BUSY DOUT 11 10 9 8 7 6 5 4 3 2 (MSB) X/Y SWITCHES(1) (SER/DFR HIGH) OFF X/Y SWITCHES(1, 2) (SER/DFR LOW) 1 0 Zero Filled... (LSB) ON OFF OFF ON OFF NOTES: (1) Y Drivers are on when X+ is selected input channel (A2 - A0 = 001B), X Drivers are on when Y+ is selected input channel (A2 - A0 = 101B). Y– will turn on when power-down mode is entered and PD1, PD0 = 00B. (2) Drivers will remain on if power-down mode is 11B (no power-down) until selected input channel, reference mode, or power-down mode is changed. FIGURE 5. Conversion Timing, 24-Clocks per Conversion, 8-bit Bus Interface. No DCLK Delay Required with Dedicated Serial Port. CS DCLK 1 DIN 8 1 8 S 1 8 1 S CONTROL BITS CONTROL BITS BUSY DOUT 11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 FIGURE 6. Conversion Timing, 16-Clocks per Conversion, 8-bit Bus Interface. No DCLK Delay Required with Dedicated Serial Port. ® 9 ADS7843 16-Clocks per Conversion 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. 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 6. 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. Note that the ADS7843 is fully powered while other serial communications are taking place during a conversion. SYMBOL DESCRIPTION MIN tACQ Acquisition Time 1.5 TYP MAX UNITS tDS DIN Valid Prior to DCLK Rising 100 ns tDH DIN Hold After DCLK HIGH 10 ns µs tDO DCLK Falling to DOUT Valid 200 ns tDV CS Falling to DOUT Enabled 200 ns tTR CS Rising to DOUT Disabled Digital Timing tCSS CS Falling to First DCLK Rising Figure 7 and Table VI provide detailed timing for the digital interface of the ADS7843. tCSH CS Rising to DCLK Ignored 0 ns tCH DCLK HIGH 200 ns tCL DCLK LOW 200 15-Clocks per Conversion tBD DCLK Falling to BUSY Rising 200 ns tBDV CS Falling to BUSY Enabled 200 ns tBTR CS Rising to BUSY Disabled 200 ns Figure 8 provides the fastest way to clock the ADS7843. 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 200 ns 100 ns ns TABLE VI. Timing Specifications (+VCC = +2.7V and Above, TA = –40°C to +85°C, CLOAD = 50pF). CS tCSS tCL tCH tBD tBD tCSH tD0 DCLK tDS tDH DIN PD0 tBDV tBTR BUSY tDV tTR DOUT 11 10 FIGURE 7. Detailed Timing Diagram. CS DCLK 15 1 DIN S SGL/ A2 A1 A0 MODE DIF PD1 PD0 1 S 15 SGL/ A2 A1 A0 MODE DIF PD1 PD0 1 S A2 5 4 A1 A0 BUSY DOUT 11 10 9 8 7 6 5 4 FIGURE 8. Maximum Conversion Rate, 15-Clocks per Conversion. ® ADS7843 10 3 2 1 0 11 10 9 8 7 6 3 2 Data Format Figure 10 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 conversions 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). The ADS7843 output data is in Straight Binary format as shown in Figure 9. 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) 1 LSB = VREF(1)/4096 1 LSB 1000 11...111 fCLK = 16 · fSAMPLE 11...101 Supply Current (µA) Output Code 11...110 00...010 00...001 00...000 100 fCLK = 2MHz 10 TA = 25˚C +VCC = +2.7V FS – 1 LSB 0V 1 Input Voltage(2) (V) 1k 10k 100k 1M fSAMPLE (Hz) NOTES: (1) Reference voltage at converter: +REF–(–REF). See Figure 2. (2) Input voltage at converter, after multiplexer: +IN–(–IN). See Figure 2 FIGURE 10. Supply Current vs Directly Scaling the Frequency of DCLK with Sample Rate or Keeping DCLK at the Maximum Possible Frequency. FIGURE 9. Ideal Input Voltages and Output Codes. 8-Bit Conversion Another important consideration for power dissipation is the reference mode of the converter. In the single-ended reference mode, the converter’s internal switches are on only when the analog input voltage is being acquired (see Figure 5). Thus, the external device, such as a resistive touch screen, is only powered during the acquisition period. In the differential reference mode, the external device must be powered throughout the acquisition and conversion periods (see Figure 5). If the conversion rate is high, this could substantially increase power dissipation. The ADS7843 provides an 8-bit conversion mode that can be used when faster throughput is needed and the digital result is not as critical. By switching to the 8-bit mode, a conversion is complete four clock cycles earlier. This could be used in conjunction with serial interfaces that provide 12bit transfers or two conversions could be accomplished with three 8-bit transfers. Not only does this shorten each conversion by four bits (25% faster throughput), but each conversion can actually occur at a faster clock rate. This is because the internal settling time of the ADS7843 is not as critical— settling to better than 8 bits is all that is needed. The clock rate can be as much as 50% faster. The faster clock rate and fewer clock cycles combine to provide a 2x increase in conversion rate. LAYOUT The following layout suggestions should provide the most optimum performance from the ADS7843. However, many portable applications have conflicting requirements concerning power, cost, size, and weight. In general, most portable devices have fairly “clean” power and grounds because most of the internal components are very low power. This situation would mean less bypassing for the converter’s power and less concern regarding grounding. Still, each situation is unique and the following suggestions should be reviewed carefully. POWER DISSIPATION There are two major power modes for the ADS7843: full power (PD1 - PD0 = 11B) and auto power-down (PD1 - PD0 = 00B). When operating at full speed and 16-clocks per conversion (as shown in Figure 6), the ADS7843 spends most of its time acquiring or converting. There is little time for auto powerdown, assuming that this mode is active. Therefore, 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 conversions are simply done less often, the difference between the two modes is dramatic. For optimum performance, care should be taken with the physical layout of the ADS7843 circuitry. 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 ® 11 ADS7843 large external transient voltages can easily affect the conversion result. Such glitches might originate from switching power supplies, nearby digital logic, and high power devices. The degree of error in the digital output depends on the reference voltage, layout, and the exact timing of the external event. The error can change if the external event changes in time with respect to the DCLK input. The ADS7843 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, voltage variation due to line frequency (50Hz or 60Hz) can be difficult to remove. With this in mind, power to the ADS7843 should be clean and well bypassed. A 0.1µF ceramic bypass capacitor should be placed as close to the device as possible. A 1µF to 10µF capacitor may also be needed if the impedance of the connection between +VCC and the power supply is high. 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 or battery connection point. The ideal layout will include an analog ground plane dedicated to the converter and associated analog circuitry. The reference should be similarly bypassed with a 0.1µF capacitor. If the reference voltage originates from an op amp, make sure that it can drive the bypass capacitor without oscillation. The ADS7843 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). In the specific case of use with a resistive touch screen, care should be taken with the connection between the converter and the touch screen. Since resistive touch screens have fairly low resistance, the interconnection should be as short and robust as possible. Longer connections will be a source of error, much like the on-resistance of the internal switches. Likewise, loose connections can be a source of error when the contact resistance changes with flexing or vibrations. ® ADS7843 12