Typical power dissipation is ? The device includes a shutdown mode which reduces typical power dissipation to under 0. The ADS is guaranteed down to 2. Low power, high speed, and on-board switches make the ADS ideal for battery operated systems such as personal digital assistants with resistive touch screens and other portable equipment.

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Typical power dissipation is ? The device includes a shutdown mode which reduces typical power dissipation to under 0. The ADS is guaranteed down to 2. Low power, high speed, and on-board switches make the ADS ideal for battery operated systems such as personal digital assistants with resistive touch screens and other portable equipment.

Street Address: S. Tucson Blvd. Tel: ? Telex: ? FAX: ? Immediate Product Info: ? 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. ADC input Channel 1. 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. Pen Interrupt. Open anode output requires 10k? Serial Data Output. Data is shifted on the falling edge of DCLK. Busy Output. Serial Data Input. Chip Select Input. External Clock Input. Exposure to absolute maximum conditions for extended periods may affect device reliability.

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. A —40 —20 0 20 40 60 80 Temperature? C 80 60 40 20 —40 —20 0 20 40 60 80 Temperature? C LSB 0.

A 10 8 6 4 2 0 0 25 50 75 Sample Rate kHz 14 12 10 8 6 —40 —20 0 20 40 60 80 Temperature? The converter is fabricated on a 0. The basic operation of the ADS is shown in Figure 1. The device requires an external reference and an external clock. It operates from a single supply of 2. 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 ADS 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. The control bits are provided serially via the DIN pin—see the Digital Interface section of this data sheet for more details. 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. F Optional ADS 0. F k? Basic Operation of the ADS Simplified Diagram of Analog Input.

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 For example, if the offset of a given converter is 2 LSBs with a 2.

In each case, the actual offset of the device is the same, 1. 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. Typically, the input current is 13? 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.

There is also a critical item regarding the reference when making measurements where the switch drivers are on. This particular application shows the device being used to digitize a resistive touch screen. 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. 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.

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. The next twelve clock cycles accomplish the actual analog-to-digital conversion.

A thirteenth clock cycle is needed for the last bit of the conversion result. These will be ignored by the converter. Control Byte Also shown in Figure 5 is the placement and order of the control bits within the control byte. The differential mode is also referred to as the ratiometric conversion mode. 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 ADS, but they might not be able to supply enough current for the external load such as a resistive touch screen. 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. Order of the Control Bits in the Control Byte.

A new control byte can start every 15th clock cycle in bit conversion mode or every 11th clock cycle in 8-bit conversion mode. Channel Select Bits. 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.

Power-Down Mode Select Bits. See Table V for details. Descriptions of the Control Bits within the Control Byte. In differential mode, the reference voltage is the difference between the currently enabled switches. 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.

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.



This allows for easy upgrade of current applications to the new version. Only software changes are required to take advantage of the added features of direct battery measurement, temperature measurement, and touch-pressure measurement. The ADS also has an on-chip 2. The reference can also be powered down when not used to conserve power. The internal reference operates down to 2. All trademarks are the property of their respective owners.


ADS7843 Datasheet



XPT2046 (ADS7843) - Touch Screen Decoder IC


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