Description
PPC907BE101 Protective devices
PPC907BE101 Protective devices
Module Clips Drive controller servo moto
PPC907BE101 Protective devicesSuitable for steel manufacturing, thermal power generation, hydroelectric power generation, nuclear power generation, wind power generation, gas supply, and other glass manufacturing, paper mills, mechanical manufacturing, electronic manufacturing, automotive manufacturing, aviation manufacturing, chemical manufacturing, coal mining and selection, oil and natural gas mining and selection, metal mining and selection, and non-metallic mining and selection
3 Basic characteristics and typical applications of ADS7846
3.1 Basic features
ADS7846 is a 4-wire touch screen controller produced by Analog Devices. It is currently widely used in resistive touch screen input systems.
The ADS7846 digital converter operates in a 12-bit successive approximation comparison register (SAR) ADC architecture integrating low-pass
impedance switches for driving touch screens. These devices do not use an internal reference voltage and consume less than 1.4mW maximum
power when operating at a throughput rate greater than 125kp/s. They also have 10keV to 12keV analog input ESD protection for enhanced ESD immunity to
avoid damage to critical internal system components. Operates from a single 2.2V to 5.25V supply. A complete operation of the AD7846 serial interface requires
24 DCLKs., former 8 pulses receive 8-bit commands, and start sampling on the rising edge of the 6th pulse. Starting from the 9th pulse, it enters the conversion phase, outputs a
12-bit sample value, and enters the idle phase after the conversion. Until the end of 24 DCLKs, CS Set to high level, one measurement ends.
3.2 Interface circuit and coordinate value acquisition of resistive touch screen
Figure 3 shows the interface circuit between ADS7846 and PhilipsARM7 chip LPC2210.
The ADS7846 chip switches the on-chip analog electronic switch, connects the X+ (Y+) terminal to the positive power supply VCC
, connects the X- (Y-) terminal to the ground, and connects the X+ (Y+) and X- (Y-) terminals to A in differential form /D converter input.
When a pen is used to click on different positions of the touch screen, due to the different voltages input to the A/D converter (see the previous voltage dividing principle), the output
value of the pen touch point is obtained after A/D conversion, and the output value is the same as the pen touch point. position has an approximately linear relationship. Therefore, the
ADS7846 can obtain the relative position of the pen touch point on the touch screen.
3.3 Causes of errors and elimination methods
The main reasons that affect the accuracy of coordinate values are:
① The uniformity of the resistive material of the touch screen itself may cause uneven voltage division due to material uniformity issues, which will inevitably affect the accuracy of the output.
②The jitter problem of touch during pressing and releasing.
③The internal resistance of the ADS7846 analog switch and the conversion accuracy of the A/D converter itself.
④ESD interference problem.
Problems ① and ③ are inherent problems of the device and cannot be eliminated. For the second problem, the key vibration can be reduced through software. A better method is to use two
key value comparisons. The specific working principle is to continuously measure the X and Y coordinate values twice, and then compare them. If they are the same or the difference is
within the allowed error range, it is considered a valid key, otherwise it is invalid. key.
Regarding the ESD problem, this is the main problem faced during design. The specific measures are mainly as follows:
(1) The analog ground of ADS7846 is not connected to the digital ground of the system at one point. Digital interference is coupled to the analog ground of the ADS7846 by the common
impedance, causing interference and causing jitter. The solution is to connect analog ground to digital ground at one point.
(2) The successive comparison A/D converter of the ADS7846 is very sensitive to power supply and digital writing. The solution is to place a 10μF bypass capacitor near the power pin and a
0.1μF bypass capacitor at the reference voltage input terminal. circuit capacitance.
(3) In situations where electromagnetic interference is relatively strong, in order to prevent the high-frequency interference pulses generated by the touch screen pins from interfering with the
ADS7846, a 0.001μF high-frequency rectifier capacitor should be connected to the key pins DCLK, DIN, and DOUT to ground . When wiring the PCB, the connection from the screen to the chip should also be short and thick.
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