Interface design between 16-bit Σ-Δ A/D converter AD7705 and microcontroller

　　AD7705 is a newly launched 16-bit Σ-Δ A/D converter by AD Company. The device includes a front-end analog conditioning circuit composed of a buffer and a programmable gain amplifier (PGA), a Σ-Δ modulator, a programmable digital filter and other components. It can directly perform A/D conversion on multiple small signals measured by the sensor. This device also has the characteristics of high resolution, wide dynamic range, self-calibration, excellent anti-noise performance and low voltage and low power consumption. It is very suitable for applications in instrument measurement, industrial control and other fields. It uses a three-wire serial interface, has two fully differential input channels, and can achieve 0.003% non-linear 16-bit error-free data output. Its gain and data output update rate can be programmed, and an input analog buffer can also be selected. As well as self-calibration and system calibration methods. Operating voltage 3 V or 5 V. At 3 V, the maximum power consumption is 1 mW, and the supply current in standby mode is only 8 μA. 　

　　1 Internal structure

　　The AD7705 is a complete 16-bit A/D converter. The internal structure is shown in Figure 1. If an external crystal oscillator, precision reference source and a small amount of decoupling capacitors are connected, A/D conversion can be performed continuously. It uses low-cost but extremely high-resolution Σ-Δ conversion technology and can obtain 16-bit error-free data output. This is very suitable for applications that require high resolution but not high requirements for digital conversion, such as digital audio products and smart instrumentation products. The following is a brief description of several important parts and characteristics of this device. 

　　The AD7705 gain programmable amplifier includes two fully differential analog input channels. The on-chip gain programmable amplifier PGA can select one of eight gains: 1, 2, 4, 8, 16, 32, 64, and 128, which can amplify various input signals with different swing ranges to close to the A/D converter full-scale voltage before performing A/D conversion, which will help improve the conversion quality. When the supply voltage is 5 V and the reference voltage is 2.5 V, the device can directly accept unipolar signals in the swing range from 0 to 20 mV to 0 to 2.5 V and from 0 to ±20 mV to 0 to ±2.5 V. bipolar signal. It must be pointed out that the negative polarity voltage here is relative to the AIN(-) pin, and these two pins should be biased to the appropriate positive potential. Applying a signal with a negative voltage relative to GND on any pin of the device is not allowed. The input analog signal is continuously sampled by the A/D converter, and the sampling frequency fS is determined by the main clock frequency fCLK and the selected gain. Gain (16 to 128) is obtained through multiple sampling and using the ratio of the reference capacitance to the input capacitance.

　　Digital Filtering and Output Update Rate The analog signal is converted by a Σ-Δ modulator into a digital pulse train whose duty cycle is modulated (widthed) by the analog voltage, which is then interpreted into a 16-bit binary number using an on-chip low-pass digital filter. And filter out noise to complete A/D conversion. AD7705 uses a (sinNx/sinx)3 function low-pass digital filter, whose amplitude-frequency characteristics are as follows:

　　 In the formula: N is the ratio of modulation rate to output update rate.
It
　　should be pointed out that the noise source generated by the device mainly comes from semiconductor noise and quantization noise. The lower the PGA amplification amount and the first notch frequency of the filter, the smaller the output semiconductor noise and quantization noise. The actual resolution of the A/D converter The higher.

　　Calibration and self-calibration In order to improve the quality of A/D conversion, the AD7705 provides two function options: self-calibration and system calibration. Whenever the ambient temperature and operating voltage change, or the working status of the device changes, such as input channel switching, gain or digital filter first notch frequency change, signal input range change, etc., a calibration must be performed. For self-calibration mode, the calibration process is completed once inside the device. AD7705 internally sets the AIN(+) terminal and AIN(-) terminal to the same bias voltage to calibrate zero scale; full-scale calibration is performed under an internally generated VREF voltage and selected gain conditions. System calibration is to calibrate the gain error and offset error of the entire system, including internal errors of the device. Under the selected gain, apply zero-scale voltage and full-scale voltage to the AIN(+) terminal externally, first calibrate the zero-scale point, and then calibrate the full-scale point. Based on the zero-scale and full-scale calibration data, the on-chip microcontroller calculates the offset and gain slope of the converter's input-output transfer function to compensate for the error.

　　The serial data interface of the digital interface AD7705 includes 5 interfaces, among which the chip select input CS, the serial clock input SCLK, the data input DIN, and the conversion data output port DOUT are used to transmit data, and the status signal output port is used to indicate when to output data. The register data is ready. When it is low, the conversion data is available; when it is high, the output register is updating data and data cannot be read. The A/D conversion process of the device is performed continuously according to the set data output update rate. Any operation requires sending new programming instructions to the corresponding on-chip registers.

　　The on-chip register AD7705 includes 8 registers, all of which are accessed through the device serial port. The first is the communication register. Its content determines which register the next operation is to read or write, and controls which input channel is sampled. All communication with the device must first write to the communications register. After power-on or reset, the device's default state is to wait for instruction data to be written to the communication register. Its register selection bits RS2~RS0 determine which register is accessed in the next operation, while the input channel selection bits CH1 and CH0 determine which input channel performs A/D conversion or accesses calibration data. The second is the setting register, which is a read/write 8-bit register used to set the working mode, calibration method, gain, etc. The third is the clock register, which is also a readable/writable 8-bit register used to set the parameters related to the AD7705 operating frequency and the A/D conversion output update rate. The fourth is the data register, which is a 16-bit read-only register that stores the latest conversion results of AD7705. It is worth noting that although the data sheet states that it is a 16-bit register, it is actually composed of two 8-bit storage units. The MSB is first when outputting. If the receiving microcontroller requires the LSB first , such as the 8051 series, when reading, it should be read in two times, each time reading 8 bits in reverse order, rather than the entire 16 bits in reverse order. The other registers are test registers, zero-scale calibration registers, full-scale calibration registers, etc., which are used for testing and storing calibration data, and can be used to analyze noise and conversion errors. 

　　2 Microcontroller interface application examples

　　AD7705 adopts SPI/QSPI compatible three-wire serial interface, which can be easily connected to various microcontrollers and DSPs. It also greatly saves the I/O port of the CPU compared with the parallel interface method. In the application circuit shown in the figure below, 80C51 is used to control the AD7705 to perform analog-to-digital conversion of the bridge sensing signal. This solution uses a second-wire connection to send and receive data. CS of AD7705 is connected to low level. The status of DRDY is obtained by monitoring P32 connected to the DRDY line (it can also be judged by accessing the DRDY bit of the communication register to save an I/O port). In this application, the same power supply is used to generate the sensor bridge excitation voltage and the base reference voltage of the AD7705, so they are affected by the same proportion when the voltage changes, and no system error will occur, thus reducing the requirements for voltage stability. . This also replaces expensive high-precision voltage reference circuits

　　without degrading performance. 80C51 is configured as serial interface mode 0 working mode. Its data serial port line RXD (P30) is connected to the DIN and DOUT pins of AD7705, and connected to a 10 kΩ pull-up resistor. The clock interface TXD is connected to SCLK (P31) of AD7705 to provide a clock for transmitting data. When there is no data transmission, TXD is idle at high level.

　　It should be noted that similar to the read operation, in the write operation mode, the data output of the 80C51 is LSB first, while the AD7705 hopes that the MSB is first, so the data must be written in reverse order. Below are several key C51 functions. 