Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsData collecting ICsData acquisition IC - Touchscreen controller 12 Bit 1x TSC input, Pins, Encoded SSOP 16

ADS7845

FEATURES

● 5-WIRE TOUCH SCREEN INTERFACE

● RATIOMETRIC CONVERSION

● SINGLE SUPPLY: 2V to 5V

● UP TO 125kHz CONVERSION RATE

● SERIAL INTERFACE

● PROGRAMMABLE 8- OR 12-BIT RESOLUTION

● AUXILIARY ANALOG INPUTS

● FULL POWER-DOWN CONTROL

DESCRIPTION

The ADS7845 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 (V

REF

) can be varied between 1V and

, providing a corresponding input voltage range

of 0V to V

REF

. The device includes a shutdown mode

which reduces typical power dissipation to under

0.5µW. The ADS7845 is guaranteed down to 2.7V

operation.

Low power, high speed, and on-board switches make

the ADS7845 ideal for battery-operated systems such

as personal digital assistants with resistive touch screens

and other portable equipment. The ADS7845 is avail-

able 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

1998 Burr-Brown Corporation PDS-1497A Printed in U.S.A. December, 1998

TOUCH SCREEN CONTROLLER

CDAC

SAR

ADS7845

Comparator

MUX

Serial

Data

Interface

and

Control

DOUT

BUSY

DCLK

DIN

AUX IN

WIPER

PENIRQ

REF

GND

Driver

GND

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

ADS7845

SBAS104

Summary of content (16 pages)

PAGE 1
® ADS ADS7845 784 5 TOUCH SCREEN CONTROLLER FEATURES DESCRIPTION ● 5-WIRE TOUCH SCREEN INTERFACE The ADS7845 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.
PAGE 2
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. ADS7845E PARAMETER CONDITIONS MIN RESOLUTION TYP MAX 12 ANALOG INPUT Full-Scale Input Span Absolute Input Range Positive Input - Negative Input Positive Input Negative Input 0 –0.2 –0.2 Capacitance Leakage Current Bits VREF +VCC +0.2 +0.2 25 0.
PAGE 3
PIN CONFIGURATION PIN DESCRIPTION Top View SSOP PIN NAME DESCRIPTION 1 2 3 +VCC UL UR Power Supply, 2.0V to 5V. Upper Left Panel Driver (VCC ON/OFF) Upper Right Panel Driver (switch between VCC and GND) Lower Left Panel Driver (switch between GND and VCC) Lower Right Panel Driver (GND ON/OFF) Ground Panel Input Auxiliary Input Voltage Reference Input Power Supply, 2.0V to 5V. Pen Interrupt. Open anode output (requires 10kΩ to 100kΩ pull-up resistor externally). Serial Data Output.
PAGE 4
TYPICAL PERFORMANCE CURVES At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
PAGE 5
TYPICAL PERFORMANCE CURVES (CONT) At TA = +25°C, +VCC = +2.7V, VREF = +2.5V, fSAMPLE = 125kHz, and fCLK = 16 • fSAMPLE = 2MHz, unless otherwise noted.
PAGE 6
THEORY OF OPERATION differential 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). The ADS7845 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.
PAGE 7
+VCC PENIRQ VREF UR SER/DFR (Shown LOW) UL WIPER +IN +REF Drivers A/D CONVERTER AUXIN –IN –REF LL LR GND FIGURE 2. Simplified Diagram of ADS7845 Input and Panel Drivers. REFERENCE INPUT 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 the ADS7845 as shown in Figure 1. This particular application shows the device being used to digitize a resistive touch screen.
PAGE 8
processor and the converter consists of 8 clock cycles. One complete conversion can be accomplished with three serial communications, for a total of 24 clock cycles on the DCLK input. 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.
PAGE 9
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.
PAGE 10
16 Clocks per Conversion Programmable Gate Arrays (FPGAs) or Application Specific Integrated Circuits (ASICs). This effectively increases the maximum conversion rate of the converter beyond the values given in the Specification table, 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.
PAGE 11
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 ADS7845 output data is in Straight Binary format as shown in Figure 9.
PAGE 12
PENIRQ Output 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 pen interrupt output function is detailed in Figure 11.
PAGE 13
PACKAGE OPTION ADDENDUM www.ti.
PAGE 14
PACKAGE MATERIALS INFORMATION www.ti.com 11-Mar-2008 TAPE AND REEL INFORMATION *All dimensions are nominal Device ADS7845E/2K5 Package Package Pins Type Drawing SSOP/ QSOP DBQ 16 SPQ Reel Reel Diameter Width (mm) W1 (mm) 2500 330.0 12.4 Pack Materials-Page 1 A0 (mm) B0 (mm) K0 (mm) P1 (mm) 6.4 5.2 2.1 8.0 W Pin1 (mm) Quadrant 12.
PAGE 15
PACKAGE MATERIALS INFORMATION www.ti.com 11-Mar-2008 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) ADS7845E/2K5 SSOP/QSOP DBQ 16 2500 346.0 346.0 29.
PAGE 16
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