TYPICAL CHARACTERISTICS (Cont.) At TA=+25C.+Vcc =+2.7V,VREF=External +2.5V,fsAMPLE=125kHz,and fcu=16.fsAMPLE=2MHz.unless otherwise noted. REFERENCE CURRENT vs SAMPLE RATE REFERENCE CURRENT vs TEMPERATURE 18 16 10 14 12 6 10 2 8 0 6 0 25 50 75 100 125 40 -20 0 20 40 60 80 100 Sample Rate (kHz) Temperature (C) SWITCH-ON RESISTANCE vs +Vcc SWITCH-ON RESISTANCE vs TEMPERATURE (X+,Y+:+Vcc to Pin;X-,Y-:Pin to GND) (X+,Y+:+Vcc to Pin;X-.Y-:Pin to GND) 8 7 7 X- Y 6 5 X- g X+ Y+ 4 4 Y+ X+ 3 3 2 2 1 2.0 2.5 3.0 3.5 4.0 4.5 5.0 40 -20 0 20 40 60 80 100 +Vcc (V) Temperature (C) MAXIMUM SAMPLING RATE VS RIN INTERNAL VREF VS TEMPERATURE 2.0 2.4920 .8 ◆一INL:R=2k 2.4915 1 INL:R=500 ·DNLR=2k 2.4910 米-DNLR=500 2.4905 2.4900 0 2.4895 2.4890 0.4 2.4885 0.2 2.4880 0 2.4875 20 40 60 80100120140160180200 导号吊号8号号号○8ee88房8导导8388尺2品8 Sampling Rate (kHz) Temperature (C) TEXAS 6 INSTRUMENTS ADS7846 www.ti.com SBAS125H
ADS7846 6 www.ti.com SBAS125H TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, +VCC = +2.7V, VREF = External +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. REFERENCE CURRENT vs SAMPLE RATE 0 125 25 50 100 75 Sample Rate (kHz) Reference Current (µA) 14 12 10 8 6 4 2 0 REFERENCE CURRENT vs TEMPERATURE –40 100 –20 0 40 20 Temperature (°C) Reference Current (µA) 18 16 14 12 10 8 6 60 80 SWITCH-ON RESISTANCE vs TEMPERATURE (X+, Y+: +VCC to Pin; X–, Y–: Pin to GND) –40 100 –20 20 X+ Y+ X– Y– 40 Temperature (°C) RON (Ω) 1 8 7 6 5 4 3 2 0 60 80 2.4920 2.4915 2.4910 2.4905 2.4900 2.4895 2.4890 2.4885 2.4880 2.4875 Internal VREF (V) Temperature (°C) INTERNAL VREF vs TEMPERATURE –40 –35 –30 –25 –20 –15 –10 –05 0 05 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 SWITCH-ON RESISTANCE vs +VCC (X+, Y+: +VCC to Pin; X–, Y–: Pin to GND) 2.0 5.0 2.5 3.5 Y+ X+ Y– X– 4.0 +VCC (V) RON (Ω) 1 8 7 6 5 4 3 2 3.0 4.5 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 Error (LSB) 20 40 60 80 100 120 140 160 180 200 Sampling Rate (kHz) MAXIMUM SAMPLING RATE vs RIN INL: R = 2k INL: R = 500 DNL: R = 2k DNL: R = 500
TYPICAL CHARACTERISTICS (Cont.) At TA=+25C,+Vcc =+2.7V,VREF External +2.5V,fsAMPLE =125kHz,and fcLk 16.fsAMPLE 2MHz,unless otherwise noted. INTERNAL VREF VS VcC INTERNAL VREE Vs TURN-ON TIME 2.4865 100 2.4860 80 No Cap E 2.4855 60 (52μS) 12.Bt 1uF Cap Settling (1110uS) 2.4850 40 12-Bit Settling 2.4845 20 2.4840 0 9 9N恩&85g将99 0 200 400 600 800 1000 1200 Turn-On Time (uS) Vcc (V) TEMP DIODE VOLTAGE vs TEMPERATURE(2.7V SUPPLY) TEMPO DIODE VOLTAGE vs VsUPPLY(25C) 850 620 618 102.7mV 616 132.25mV 614 TEMPO 612 450 610 导号吊8闲#骨°8P988号8导导8888尺品品 2.7 3.0 3.3 Temperature(C) VsUPPLY (V) TEMP1 DIODE VOLTAGE vs VSUPPLY(25C) 732 730 728 726 724 722 2.7 3.0 3.3 VsUPPLY (V) ADS7846 TEXAS INSTRUMENTS SBAS125H www.ti.com
ADS7846 7 SBAS125H www.ti.com TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, +VCC = +2.7V, VREF = External +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted. 2.4865 2.4860 2.4855 2.4850 2.4845 2.4840 VREF (V) 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 VCC (V) INTERNAL VREF vs VCC 850 800 750 700 650 600 550 500 450 TEMP Diode Voltage (mV) Temperature (°C) –40 –35 –30 –25 –20 –15 –10 –05 0 05 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 TEMP DIODE VOLTAGE vs TEMPERATURE (2.7V SUPPLY) 102.7mV 132.25mV TEMP0 TEMP1 100 80 60 40 20 0 Internal VREF (%) 0 200 400 600 800 1000 1200 INTERNAL VREF vs TURN-ON TIME Turn-On Time (µS) 1µF Cap (1110µS) 12-Bit Settling No Cap (52µS) 12-Bit Settling 620 618 616 614 612 610 2.7 3.0 3.3 VSUPPLY (V) TEMP0 DIODE VOLTAGE vs VSUPPLY (25°C) TEMP0 Diode Voltage (mV) 732 730 728 726 724 722 2.7 3.0 3.3 VSUPPLY (V) TEMP1 DIODE VOLTAGE vs VSUPPLY (25°C) TEMP1 Diode Voltage (mV)
THEORY OF OPERATION possible to negate the error from each touch panel driver switch's on-resistance (if this is a source of error for the The ADS7846 is a classic successive approximation register particular measurement). (SAR)analog-to-digital converter (ADC).The architecture is based on capacitive redistribution which inherently includes ANALOG INPUT a sample-and-hold function.The converter is fabricated on a 0.6um CMOS process. See Figure 2 for a block diagram of the input multiplexer on the ADS7846,the differential input of the ADC,and the The basic operation of the ADS7846 is shown in Figure 1. differential reference of the converter.Table I and Table ll The device features an internal 2.5V reference and an external clock.Operation is maintained from a single supply show the relationship between the A2.A1,A0,and SER/DFR of 2.7V to 5.25V.The internal reference can be overdriven control bits and the configuration of the ADS7846.The with an external,low impedance source between 1V and control bits are provided serially via the DIN pin-see the +Vcc.The value of the reference voltage directly sets the Digital Interface section of this data sheet for more details. input range of the converter. When the converter enters the hold mode,the voltage difference between the +IN and-IN inputs(see Figure 2)is The analog input (X-,Y-,and Z-position coordinates,auxil- captured on the internal capacitor array.The input current iary input,battery voltage,and chip temperature)to the converter is provided via a multiplexer.A unique configura- into the analog inputs depends on the conversion rate of the tion of low on-resistance touch panel driver switches allows device.During the sample period,the source must charge an unselected ADC input channel to provide power and its the internal sampling capacitor (typically 25pF).After the capacitor has been fully charged,there is no further input accompanying pin to provide ground for an external device, current.The rate of charge transfer from the analog source such as a touch screen.By maintaining a differential input to the converter and a differential reference architecture,it is to the converter is a function of conversion rate. +2.7V1o+5V 1uF ADS7846 10uF 0.1uf (Optional） DCLK 16 X+ cS 15 Chip Select Y+ DIN 14 Serial Data In Touch X- BUSY 13 Converter Status Screen Y- DOUT Serial Data Out To Battery 6 GND PENIRO 11 <Pen Interrupt VBAT +Vcc 10 Auxiliary Input O AUX 9 50k2 Voltage Regulator FIGURE 1.Basic Operation of the ADS7846. A2 A1 A0 VBAT AUXIN TEMP Y- Y+ Y-POSITION X-POSITION Z,-POSITION Z,-POSITION X-DRIVERS Y-DRIVERS 0 0 +IN (TEMPO) Off Off 0 +IN Measure Off On 0 1 0 Off Off 0 +lN Measure X-,On Y+, On 0 +IN Measure X-, On Y+, 0 01 +IN Measure On Off 1 1 +IN f Off +IN (TEMP1) O开 Off TABLE I.Input Configuration(DIN),Single-Ended Reference Mode(SER/DFR high). A2 A1 A0 +REF -REF Y- X+ Y+ Y-POSITION X-POSITION Z,-POSITION Z2-POSITION DRIVERS ON 0 1 Y+ Measure Y+,Y- 0 1 Y+ X- N Measure Y+,X- 1 0 0 Y+ X- +IN Measure Y+,X- 1 0 X+ X- +IN Measure X+,X- TABLE II.Input Configuration(DIN),Differential Reference Mode(SER/DFR low). TEXAS 8 INSTRUMENTS ADS7846 www.ti.com SBAS125H
ADS7846 8 www.ti.com SBAS125H THEORY OF OPERATION The ADS7846 is a classic successive approximation register (SAR) analog-to-digital converter (ADC). The architecture is based on capacitive redistribution which inherently includes a sample-and-hold function. The converter is fabricated on a 0.6µm CMOS process. The basic operation of the ADS7846 is shown in Figure 1. The device features an internal 2.5V reference and an external clock. Operation is maintained from a single supply of 2.7V to 5.25V. The internal reference can be overdriven with an external, low impedance source between 1V and +VCC. The value of the reference voltage directly sets the input range of the converter. The analog input (X-, Y-, and Z-position coordinates, auxiliary input, battery voltage, and chip temperature) to the converter is provided via a multiplexer. A unique configuration of low on-resistance touch panel driver switches allows an unselected ADC input channel to provide power and its accompanying pin to provide ground for an external device, such as a touch screen. By maintaining a differential input to the converter and a differential reference architecture, it is possible to negate the error from each touch panel driver switch’s on-resistance (if this is a source of error for the particular measurement). ANALOG INPUT See Figure 2 for a block diagram of the input multiplexer on the ADS7846, the differential input of the ADC, and the differential reference of the converter. Table I and Table II show the relationship between the A2, A1, A0, and SER/DFR control bits and the configuration of the ADS7846. The control bits are provided serially via the DIN pin—see the Digital Interface section of this data sheet for more details. When the converter enters the hold mode, the voltage difference between the +IN and –IN inputs (see Figure 2) is captured on the internal capacitor array. The input current into 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. FIGURE 1. Basic Operation of the ADS7846. A2 A1 A0 VBAT AUXIN TEMP Y– X+ Y+ Y-POSITION X-POSITION Z1-POSITION Z2-POSITION X-DRIVERS Y-DRIVERS 0 00 +IN (TEMP0) Off Off 0 0 1 +IN Measure Off On 0 1 0 +IN Off Off 0 1 1 +IN Measure X–, On Y+, On 1 0 0 +IN Measure X–, On Y+, On 1 0 1 +IN Measure On Off 1 1 0 +IN Off Off 1 11 +IN (TEMP1) Off Off TABLE I. Input Configuration (DIN), Single-Ended Reference Mode (SER/DFR high). TABLE II. Input Configuration (DIN), Differential Reference Mode (SER/DFR low). A2 A1 A0 +REF –REF Y– X+ Y+ Y-POSITION X-POSITION Z1-POSITION Z2-POSITION DRIVERS ON 0 0 1 Y+ Y– +IN Measure Y+, Y– 0 1 1 Y+ X– +IN Measure Y+, X– 1 0 0 Y+ X– +IN Measure Y+, X– 1 0 1 X+ X– +IN Measure X+, X– +VCC X+ Y+ X– Y– GND VBAT AUX 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 DCLK CS DIN BUSY DOUT PENIRQ +VCC VREF Serial/Conversion Clock Chip Select Serial Data In Converter Status Serial Data Out + 1µF to 10µF (Optional) +2.7V to +5V ADS7846 Auxiliary Input To Battery Voltage Regulator Touch Screen 0.1µF Pen Interrupt 50kΩ
PENIRO +Vcc VREF TEMP1 、TEMPO A2-A0 SER/DFR (Shown 001B) (Shown High) X+ O Ref On/Off +REF +IN Y- Converter N 2.5V -REF Reference 75k2 2.5k Battery ●n AUX GND FIGURE 2.Simplified Diagram of Analog Input. INTERNAL REFERENCE Reference The ADS7846 has an intemnal 2.5V voltage reference that can Power Down be tumned on or off with the control bit,PD1=1(see Table V and Figure 3).Typically,the intemal reference voltage is only used in the single-ended mode for battery monitoring,temperature measurement,and for using the auxiliary input.Optimal touch screen performance is achieved when using the differential mode.The internal reference voltage of the ADS7846 must be Band Gap Buffer commanded to be off to maintain compatibility with the ADS7843. Therefore,after power-up,a write of PD1 =0 is required to insure the reference is off(see the Typical Characteristics for Optional To power-up time of the reference from power-down). CDAC FIGURE 3.Simplified Diagram of the Internal Reference. REFERENCE INPUT The voltage difference between +REF and-REF(shown in bit)size and is equal to the reference voltage divided by 4096 Figure 2)sets the analog input range.The ADS7846 oper- in 12-bit mode.Any offset or gain error inherent in the ADC ates with a reference in the range of 1V to +Vcc.There are appears to increase,in terms of LSB size,as the reference several critical items concerning the reference input and its voltage is reduced.For example,if the offset of a given wide voltage range.As the reference voltage is reduced,the converter is 2LSBs with a 2.5V reference,it is typically analog voltage weight of each digital output code is also 5LSBs with a 1V reference.In each case,the actual offset of reduced.This is often referred to as the LSB(least significant the device is the same,1.22mV.With a lower reference ADS7846 TEXAS INSTRUMENTS 9 SBAS125H www.ti.com
ADS7846 9 SBAS125H www.ti.com INTERNAL REFERENCE The ADS7846 has an internal 2.5V voltage reference that can be turned on or off with the control bit, PD1 = 1 (see Table V and Figure 3). Typically, the internal reference voltage is only used in the single-ended mode for battery monitoring, temperature measurement, and for using the auxiliary input. Optimal touch screen performance is achieved when using the differential mode. The internal reference voltage of the ADS7846 must be commanded to be off to maintain compatibility with the ADS7843. Therefore, after power-up, a write of PD1 = 0 is required to insure the reference is off (see the Typical Characteristics for power-up time of the reference from power-down). REFERENCE INPUT The voltage difference between +REF and –REF (shown in Figure 2) sets the analog input range. The ADS7846 operates 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 in 12-bit mode. Any offset or gain error inherent in the ADC appears to increase, in terms of LSB size, as the reference voltage is reduced. For example, if the offset of a given converter is 2LSBs with a 2.5V reference, it is typically 5LSBs with a 1V reference. In each case, the actual offset of the device is the same, 1.22mV. With a lower reference FIGURE 2. Simplified Diagram of Analog Input. Converter –REF +REF +IN –IN VBAT AUX Battery On GND A2-A0 (Shown 001B) 2.5V Reference Ref On/Off SER/DFR (Shown High) X+ X– +VCC TEMP1 PENIRQ Y+ Y– VREF TEMP0 7.5kΩ 2.5kΩ Buffer Band Gap Reference Power Down To CDAC Optional VREF FIGURE 3. Simplified Diagram of the Internal Reference
voltage,more care must be taken to provide a clean layout +Vcc including adequate bypassing,a clean(low-noise,low-ripple) power supply,a low-noise reference(if an external reference is used),and a low-noise input signal. The voltage into the VREF input directly drives the capacitor ● digital-to-analog converter(CDAC)portion of the ADS7846. Y+ Therefore,the input current is very low(typically 13uA). There is also a critical item regarding the reference when making measurements where the switch drivers are on.For 了WW- IN +REF this discussion,it is useful to consider the basic operation of Converter the ADS7846 (see Figure 1).This particular application -IN shows the device being used to digitize a resistive touch -REF screen.A measurement of the current Y position of the pointing device is made by connecting the X+input to the Y ADC,turning on the Y+and Y-drivers,and digitizing the voltage on X+(Figure 4 shows a block diagram).For this GND O measurement.the resistance in the X+lead does not affect the conversion (it does affect the settling time,but the FIGURE 5.Simplified Diagram of Differential Reference resistance is usually small enough that this is not a concem). (SER/DFR Low.Y Switches Enabled.X+is However,since the resistance between Y+and Y-is fairly Analog Input). low,the on-resistance of the Y drivers does make a small difference.Under the situation outlined so far,it is not As a final note about the differential reference mode,it must possible to achieve a OV input or a full-scale input regardless be used with +Vcc as the source of the +REF voltage and of where the pointing device is on the touch screen,because cannot be used with VREF.It is possible to use a high some voltage is lost across the internal switches.In addition. precision reference on VREF and single-ended reference the internal switch resistance is unlikely to track the resis- mode for measurements which do not need to be ratiometric. tance of the touch screen,providing an additional source of error In some cases,it is possible to power the converter directly from a precision reference.Most references can provide enough power for the ADS7846,but might not be able to +Vcc supply enough current for the external load (such as a resistive touch screen). TOUCH SCREEN SETTLING In some applications,extemal capacitors may be required across the touch screen for filtering noise picked up by the touch screen(for example,noise generated by the LCD panel WWx° N +REF or backlight circuitry).These capacitors provide a low-pass Converter filter to reduce the noise,but cause a settling time requirement REF when the panel is touched that typically shows up as a gain error.The problem is that the input and/or reference has not Y settled to the final steady-state value prior to the ADC sam- pling the input(s)and providing the digital output.Additionally. GND O the reference voltage may still be changing during the mea- surement cycle.There are several methods for minimizing or FIGURE 4.Simplified Diagram of Single-Ended Reference eliminating this issue.Option 1 is to stop or slow down the (SER/DFR High,Y Switches Enabled,X+is ADS7846 DCLK for the required touch screen settling time. Analog Input). This allows the input and reference to have stable values for the Acquire period(3 clock cycles of the ADS7846;see Figure This situation can be remedied as shown in Figure 5.By 9).This works for both the single-ended and the differential setting the SER/DFR bit low,the +REF and-REF inputs are modes.Option 2 is to operate the ADS7846 in the differential connected directly to Y+and Y-,respectively,which makes mode only for the touch screen measurements and command the analog-to-digital conversion ratiometric.The result of the the ADS7846 to remain on(touch screen drivers on)and not conversion is always a percentage of the external resistance, go into power-down(PDO 1).Several conversions are made regardless of how it changes in relation to the on-resistance of depending on the settling time required and the ADS7846 data the internal switches.Note that there is an important consid- rate.Once the required number of conversions have been eration regarding power dissipation when using the ratiometric made,the processor commands the ADS7846 to go into the mode of operation (see the Power Dissipation section for power-down state on the last measurement.This process is more details). TEXAS 10 INSTRUMENTS ADS7846 www.ti.com SBAS125H
ADS7846 10 www.ti.com SBAS125H 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 (if an external reference is used), and a low-noise input signal. The voltage into the VREF input directly drives the capacitor digital-to-analog converter (CDAC) portion of the ADS7846. Therefore, the input current is very low (typically < 13µA). There is also a critical item regarding the reference when making measurements where the switch drivers are on. For this discussion, it is useful to consider the basic operation of the ADS7846 (see 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 ADC, turning on the Y+ and Y– drivers, and digitizing the voltage on X+ (Figure 4 shows 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). 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 is not possible to achieve a 0V 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. FIGURE 4. Simplified Diagram of Single-Ended Reference (SER/DFR High, Y Switches Enabled, X+ is Analog Input). This situation can be remedied as shown in Figure 5. By setting the SER/DFR bit low, the +REF and –REF inputs are connected directly to Y+ and Y–, respectively, which makes the analog-to-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). FIGURE 5. Simplified Diagram of Differential Reference (SER/DFR Low, Y Switches Enabled, X+ is Analog Input). Converter +IN +REF Y+ +VCC X+ Y– GND –REF –IN 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. In some cases, it is possible to power the converter directly from a precision reference. Most references can provide enough power for the ADS7846, but might not be able to supply enough current for the external load (such as a resistive touch screen). TOUCH SCREEN SETTLING In some applications, external capacitors may be required across the touch screen for filtering noise picked up by the touch screen (for example, noise generated by the LCD panel or backlight circuitry). These capacitors provide a low-pass filter to reduce the noise, but cause a settling time requirement when the panel is touched that typically shows up as a gain error. The problem is that the input and/or reference has not settled to the final steady-state value prior to the ADC sampling the input(s) and providing the digital output. Additionally, the reference voltage may still be changing during the measurement cycle. There are several methods for minimizing or eliminating this issue. Option 1 is to stop or slow down the ADS7846 DCLK for the required touch screen settling time. This allows the input and reference to have stable values for the Acquire period (3 clock cycles of the ADS7846; see Figure 9). This works for both the single-ended and the differential modes. Option 2 is to operate the ADS7846 in the differential mode only for the touch screen measurements and command the ADS7846 to remain on (touch screen drivers on) and not go into power-down (PD0 = 1). Several conversions are made depending on the settling time required and the ADS7846 data rate. Once the required number of conversions have been made, the processor commands the ADS7846 to go into the power-down state on the last measurement. This process is Converter +IN +REF Y+ +VCC VREF X+ Y– GND –REF –IN