Establishment of the hottest high performance data

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Establishment of high-performance data acquisition system

a data acquisition system may be as complex as a distributed sensor network connected to a central computer, or as simple as a sensor that displays measurement data on an instrument. For each system, its cost/benefit must be evaluated and weighed. From the perspective of cost alone, simple solutions may be preferred, but other factors still need to be considered

system requirements

for the basic system, the requirements are determined one by one according to the comparison of performance and cost. The system must be more than 16 bits, capable of processing 2.5V differential input, and can work without a single 5VDC power supply, and the cost must be controlled within $10. The requirements of performance and cost are often contradictory, but only a little creativity can meet its requirements, or even do better: that is, integrate the decision-making function into the sensor circuit without increasing the final material cost. This complex function can be added by comparing the measured value with the value within the predetermined range and identifying the out of range state

the basic building block of this kind of data acquisition (DA) system is the analog conversion and processing function. The simplest form of the system includes: an ADC processing sensor input conversion, and a microcontroller processing data. Today's ADC and microcontroller performance has been greatly improved than before. These improvements make the integration of the above systems possible, but they also pay a price, because they not only improve the performance, but also increase the complexity of integrating these two functions. This problem can be solved only by developing a truly integrated single-chip system solution. This paper describes a low-cost, high-performance DA system, which can make full use of the characteristics of each device

flexible device characteristics

the core of the system is an ADC, that is, burr Brown can play an important role in promoting n's 24 bit to converter ads1212, and microchip technology's microcontroller pic16c54c

- ADC can provide the high resolution and good linearity required by many sensor applications. The effective resolution can be measured directly from the output data of the converter and expressed in the root mean square value of digits. If the noise is white, 2.7 bits must be subtracted from the effective value to convert to peak to peak or no fluctuation bit value

it has not achieved breakthrough development in China since then. In order to achieve the best resolution, the device should work at the highest device frequency and the lowest data output rate. The device frequency and the turbo mode rate control the sampling frequency of the modulator (the frequency of the modulator can be set in the acceleration mode). The sampling rate can control the data output rate. If the converter works at 2.5MHz, the acceleration mode is to realize the full digital adjustment of data collection and control process, and the data output rate is 100Hz, the effective resolution is about 22 significant bits, and the converted non fluctuation bit value is about 19.3 bits. If the device frequency drops to 1MHz, the acceleration mode is 16, and the data output rate is 100Hz, the resolution drops to 21 significant bits, and the non fluctuation bit value is about 18.3 bits. In fact, it is better to work at low frequency, which can reduce system power consumption and switching noise

with this flexibility, we can determine the device frequency that can meet the system requirements. At the same time, in order to improve or reduce the effective resolution, the software can also make a compromise between the data output rate and the accelerated mode rate. In addition, ads1212 can conduct internal self calibration to eliminate the errors introduced by components, thus improving the effective resolution. System offset and gain errors can be corrected by system calibration

other requirements

for the 2.5V differential input signal, the fourth-order gain is required, so the on-board programmable gain amplifier (PGA) must be used. It should be noted that the product of the acceleration mode rate and PGA should not exceed 16. In this case, the PGA is limited to 4, and the acceleration mode rate is also limited to 4. The digital filter in ads1212 is a sinc ▲ 3 ▲ low-pass filter, whose -3db cut-off frequency is equal to 0.262 data rate. Since the input signal is a DC signal, it is not a problem that the frequency response of the signal decreases. An anti aliasing low-pass filter can be added at the input to isolate ADC and buffer

the input impedance of the converter depends on the PGA, the number of acceleration modes and the device frequency, and is given by the following formula:

ain=[(1mhz/f ▼ Xin ▼) *20 ▲ 6 ▲/(pga*tmr)] (1)

where: tmr= acceleration mode rate

therefore, the input impedance is 1.2m, which is sufficient for most sensor applications

the internal reference voltage of the device can meet the requirements of the system because customers can also return to work one after another to provide about 20 significant bits

after further standardizing the analog requirements, now let's see how the analog part and the digital part are combined. In this respect, several important characteristics of microcontroller play an important role

the power consumption of the device is very small during operation, and the working current is usually less than 2mA. To further reduce power consumption, both the microcontroller and ADC have sleep mode. In extremely low-power applications, the converter can even be powered through the i/o pin of the microcontroller. For example, the microcontroller can stop supplying power to the converter and enter the sleep state. Therefore, the only current is the standby current of the microcontroller, which reduces power consumption and extends battery life

The time needed by the

microcontroller can be obtained from a simple RC circuit. This clock is output at the clkout pin and can be used to drive the Xin of the converter. This not only saves two separate crystal oscillators, but also ensures the synchronization between the microcontroller and the clock. However, it should be noted that if a simple RC circuit is used as the system clock, its frequency will change with the change of voltage and temperature. This is not a problem, unless you need to filter out power frequency interference, and you can also use one of the key features of ads1212 to solve it

The digital filter in the

converter can calculate the output value according to the latest result of the - modulator. The number of modulator results used depends on the setting of the sampling rate in the instruction register

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