 |
Motore di ricerca datesheet componenti elettronici |
|
|
Motore di ricerca datesheet componenti elettronici |
|
AD7873 Scheda tecnica(PDF) 16 Page - Analog Devices |
|
||||||||||||||||||||||||||||||
AD7873 Scheda tecnica(HTML) 16 Page - Analog Devices |
|
|
16 / 29 page  Data Sheet AD7873 Rev. F | Page 15 of 28 Acquisition Time The track-and-hold amplifier enters tracking mode on the falling edge of the fifth DCLK after the start bit is detected (see Figure 35). The time required for the track-and-hold amplifier to acquire an input signal depends on how quickly the 37 pF input capacitance is charged. With zero source impedance on the analog input, three DCLK cycles are always sufficient to acquire the signal to the 12-bit level. With a source impedance (RIN) on the analog input, the actual acquisition time required is calculated using the formula: ( ) pF 37 Ω 100 4 . 8 × + × = IN ACQ R t where RIN is the source impedance of the input signal, and 100 Ω, 37 pF is the input RC. Depending on the frequency of DCLK used, three DCLK cycles may or may not be sufficient to acquire the analog input signal with various source impedance values. Touch Screen Settling In some applications, external capacitors could be required across the touch screen to filter noise associated with it, for example, noise generated by the LCD panel or backlight circuitry. The value of these capacitors causes a settling time requirement when the panel is touched. The settling time typically appears as a gain error. There are several methods for minimizing or eliminating this issue. The problem can be that the input signal, reference, or both, have not settled to their final value before the sampling instant of the ADC. Additionally, the reference voltage could still be changing during the conversion cycle. One option is to stop or slow down the DCLK for the required touch screen settling time. This allows the input and reference to stabilize for the acquisition time, resolving the issue for both single-ended and differential modes. The other option is to operate the AD7873 in differential mode only for the touch screen, and program the AD7873 to keep the touch screen drivers on and not go into power-down (PD0 = PD1 = 1). Several conversions could be required, depending on the settling time required and the AD7873 data rate. Once the required number of conversions have been made, the AD7873 can then be placed in a power-down state on the last measurement. The last method is to use the 15-DCLK cycle mode, maintaining the touch screen drivers on until it is commanded by the processor to stop. Internal Reference The AD7873 has an internal reference voltage of 2.5 V. The internal reference is available on the VREF pin for external use in the system; however, it must be buffered before it is applied elsewhere. The on-chip reference can be turned on or off with the power-down address, PD1 = 1 (see Table 8 and Figure 27). Typically, the reference voltage is only used in 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 power-up time of the 2.5 V reference is typically 10 µs without a load; however, a 0.1 µF capacitor on the VREF pin is recommended for optimum performance because it affects the power-up time (see Figure 20). Figure 27. On-Chip Reference Circuitry Reference Input The voltage difference between +REF and −REF (see Figure 26) sets the analog input range. The AD7873 operates with a refer- ence input in the range of 1 V to +VCC. Figure 27 shows the on-chip reference circuitry on the AD7873. The internal reference on the AD7873 can be overdriven with an external reference; for best performance, however, the internal reference should be disabled when an external reference is applied, because SW1 in Figure 27 opens on the AD7873 when the internal reference is disabled. The on-chip reference always is available at the VREF pin as long as the reference is enabled. The input impedance seen at the VREF pin is approximately 260 Ω when the internal reference is enabled. When it is disabled, the input impedance seen at the VREF pin is in the GΩ region. When making touch screen measurements, conversions can be made in differential (ratiometric) mode or single-ended mode. If the SER/DFR bit is set to 1 in the control register, then a single-ended conversion is performed. Figure 28 shows the configuration for a single-ended Y coordinate measurement. The X+ input is connected to the analog-to-digital converter, the Y+ and Y− drivers are turned on, and the voltage on X+ is digitized. The conversion is performed with the ADC referenced from GND to VREF. This VREF is either the on-chip reference or the voltage applied at the VREF pin externally, and is determined by the setting of the power management Bit PD0 and Bit PD1 (see Table 7). The advantage of this mode is that the switches that supply the external touch screen can be turned off once the acquisition is complete, resulting in a power savings. However, the on resistance of the Y drivers affects the input voltage that can be acquired. The full touch screen resistance could be in the order of 200 Ω to 900 Ω, depending on the manufacturer. Thus, if the on resistance of the switches is approximately 6 Ω, true full-scale and zero-scale voltages cannot be acquired, regardless of where the pen/stylus is on the touch screen. Note that the minimum touch screen resistance recommended for use with X+ Y+ VREF 260 Ω SW1 3-TO-1 MUX 2.5V REF ADC BUF |
Codice articolo simile - AD7873_17 |
|
Descrizione simile - AD7873_17 |
|
|
Link URL |
| Privacy Policy |
| ALLDATASHEETIT.COM |
| Lei ha avuto il aiuto da alldatasheet? [ DONATE ] |
Di alldatasheet | Richest di pubblicita | contatti | Privacy Policy | scambio Link | Ricerca produttore All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
Italian ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
