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AD7873 датащи(PDF) 20 Page - Analog Devices |
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AD7873 датащи(HTML) 20 Page - Analog Devices |
20 / 29 page Data Sheet AD7873 Rev. F | Page 19 of 28 CONTROL REGISTER The control word provided to the ADC via the DIN pin is shown in Table 7. This provides the conversion start, channel addressing, ADC conversion resolution, configuration, and power-down of the AD7873. Table 7 provides detailed information on the order and description of these control bits within the control word. Initiate START The first bit, the S bit, must always be set to 1 to initiate the start of the control word. The AD7873 ignores any inputs on the DIN line until the start bit is detected. Channel Addressing The next three bits in the control register, A2, A1, and A0, select the active input channel(s) of the input multiplexer (see Table 6 and Figure 26), touch screen drivers, and the reference inputs. Mode The MODE bit sets the resolution of the analog-to-digital converter. With a 0 in this bit, the following conversion has 12 bits of resolution. With a 1 in this bit, the following conversion has eight bits of resolution. SER/DFR The SER/DFR bit controls the reference mode, set to either single-ended or differential when a 1 or a 0 is written to this bit, respectively. The differential mode is also referred to as the ratiometric conversion mode. This mode is optimum for X-position, Y-position, and pressure-touch measurements. The reference is derived from the voltage at the switch drivers, which is almost the same as the voltage to the touch screen. In this case, a separate reference voltage is not needed because the reference voltage to the ADC is the voltage across the touch screen. In single-ended mode, the reference voltage to the converter is always the difference between the VREF and GND pins. See Table 6 and Figure 26 through Figure 29 for further information. If X-position, Y-position, and pressure touch are measured in single-ended mode, an external reference voltage or +VCC is required for maximum dynamic range. The internal reference can be used for these single-ended measurements; however, a loss in dynamic range is incurred. If an external reference is used, the AD7873 should also be powered from the external reference. Because the supply current required by the device is so low, a precision reference can be used as the supply source to the AD7873. It might also be necessary to power the touch screen from the reference, which can require 5 mA to 10 mA. A REF19x voltage reference can source up to 30 mA, and, as such, could supply both the ADC and the touch screen. Care must be taken, however, to ensure that the input voltage applied to the ADC does not exceed the reference voltage and therefore the supply voltage. See the Absolute Maximum Ratings section. Note that the differential mode can only be used for X-position, Y-position, and pressure touch measurements. All other measurements require single-ended mode. PD0 and PD1 The power management options are selected by programming the power management bits, PD0 and PD1, in the control register. Table 8 summarizes the options available and the internal reference voltage configurations. The internal reference can be turned on or off independent of the analog-to-digital converter, allowing power saving between conversions using the power management options. On power-up, PD0 defaults to 0, while PD1 defaults to 1. MSB LSB S A2 A1 A0 MODE SER/DFR PD1 PD0 Table 7. Control Register Bit Function Description Bit No. Mnemonic Comment 7 S Start Bit. The control word starts with the first high bit on DIN. A new control word can start every 15th DCLK cycle when in the 12-bit conversion mode or every 11th DCLK cycle when in 8-bit conversion mode. 6 to 4 A2 to A0 Channel Select Bits. These three address bits along with the SER/DFR bit control the setting of the multiplexer input, switches, and reference inputs, as detailed in Table 6. 3 MODE 12-Bit/8-Bit Conversion Select Bit. This bit controls the resolution of the following conversion. With a 0 in this bit, the conversion has 12-bit resolution or, with a 1 in this bit, 8-bit resolution. 2 SER/DFR Single-Ended/Differential Reference Select Bit. Together with Bit A2 to Bit A0, this bit controls the setting of the multiplexer input, switches, and reference inputs as described in Table 6. 1, 0 PD1, PD0 Power Management Bits. These two bits decode the power-down mode of the AD7873 as shown in Table 8. |
Аналогичный номер детали - AD7873_17 |
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Аналогичное описание - AD7873_17 |
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