Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

In industrial production process control, it is often necessary to accurately measure certain control variables, and then use a microcomputer to calculate the correction amount for control. The traditional NYQUIST rate ADC (such as integrating type, successive comparison type, flickering type, etc.) cannot meet the accuracy requirements. Up to 24-bit resolution.Since the delta-sigma principle adopts technologies such as oversampling, noise formation and digital decimation, it can achieve high resolution at low cost, with low noise and strong anti-interference ability, so it is especially suitable for low frequency, high

In industrial production process control, it is often necessary to accurately measure certain control variables, and then use a microcomputer to calculate the correction amount for control. The traditional NYQUIST rate ADC (such as integrating type, successive comparison type, flickering type, etc.) cannot meet the accuracy requirements. Up to 24-bit resolution. Since the delta-sigma principle adopts technologies such as oversampling, noise formation and digital decimation, it can achieve high resolution at low cost, with low noise and strong anti-interference ability, so it is especially suitable for low frequency, high resolution, A/D conversion with wide dynamic range.

1 Main features of AD7711A

・High precision, 24 bits without missing codes, output nonlinearity of ±0.0015%.

・Adopt delta-sigma conversion structure, low cost, low noise and strong anti-interference ability.

・Built-in programmable gain amplifier, the gain range is 1~128, which can be directly connected with the sensor. The input is dual-channel, which can be switched.

・Built-in programmable digital low-pass filter.

・Built-in self-calibration circuit, there are 8 optional calibration modes, and the self-calibration register can be directly read and written, which effectively removes zero drift and gain error.

・Low power consumption (25mW in typical application) with power saving standby mode.

・Bidirectional serial interface can be easily connected with microprocessor and DSP chip.

The built-in programmable gain amplifier allows the AD7711A to connect directly to the sensor. When the reference Voltage is 2.5V and the built-in amplifier gain changes from 1 to 128, the acceptable signal range changes from 0 to 20mV to 0 to 2.5V (single-ended). Provides a constant current source for RTD excitation, which simplifies the circuit design of RTD temperature measurement. The 24-bit read-write control register in AD7711A enables the microprocessor or DSP chip to conveniently control the cut-off frequency of the digital filter, the gain of the input amplifier, the channel selection and the self-calibration mode. The AD7711A has a typical master clock frequency of 10MHz and can be powered by a single or dual power supply. The A/D conversion rate is equal to the frequency at the first notch of the digital filter (ie programmable).

2 Internal structure of AD7711A

The internal structure of AD7711A is shown in Figure 1. It includes a Δ-Σ ADC, digital filter, programmable amplification, clock generator, 24-bit control/data/calibration register, and 400μA constant current source. Among the pins of AD7711A, SCLK is the serial clock input terminal; MCLKIN and MCLKOU are the connection terminals of the main clock frequency; A0 is the register address selection, when A0 is set low, the control register is selected, and when it is set high, the data or self-calibration register is selected; SYNC pin It is the reset terminal of the digital filter; MODE selects the clock mode of data transmission (external clock or internal clock); AIN1+, AIN-, AIN2+, AIN2- are the input terminals of two signals respectively; SDATA is the output/input terminal of serial data ;DRDY is the A/D conversion completion terminal, and the low level is active; RFS and TFS are the input or output frame synchronization terminals respectively; IOUT is the output terminal of the 400μA constant current source, which can be used as the excitation current of the RTD; REFOUT is the reference voltage ( 2.5V output terminal, REFIN- and REFIN+ are external reference voltage input terminals. The digital filter in AD7711A is

Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

The first notch frequency is determined by the value of the 12th to 23rd bits in the control register. The 3dB cutoff frequency of the digital filter is 0.262 times the first notch frequency, and is equal to the A/D conversion rate. The function description of the 24-bit readable and writable control register of AD7711A is shown in Table 1. The MSB is BITO and the LSB is BIT23.

Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

3 Application of AD7711A

3.1 Introduction of high-precision rubber vulcanization temperature control system

The vulcanization of rubber is a key link in rubber production. The entire vulcanization process requires high temperature. The overshoot from room temperature to the set temperature does not exceed ±0.3°C? The vulcanization temperature is stable within the range of ±0.3°C of the set temperature. When the temperature changes caused by other disturbances such as feeding, the reconstruction time of the stable temperature of the system is required to be within 45 seconds. The system measures the temperature of the vulcanization reaction chamber through RTD, and controls the heating device through the PID regulator, so as to achieve precise control of the vulcanization temperature. The traditional A/D converter cannot meet the resolution requirements, because the temperature signal is a slowly changing signal, but the high-stability RTD excitation constant current source integrated in the AD7711A chip makes the AD7711A an ideal choice. Figure 2 is a block diagram of the entire vulcanization temperature control system.

Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

3.2 Interface between AD7711A and MCU

Because the data serial output format of AD7711A is opposite to that of 8751 single-chip microcomputer, the serial port of 8751 is not used in this design, but the P1 port is directly connected to AD7711A. P1.0 is connected to A0 to select the register, the clock signal for reading and writing data is given by P1.2, the serial data is read or written by P1.3,Performance characteristics, structure and application analysis of high-resolution A/D device AD7711AIt is connected with INT1, after the data conversion is completed, the data reading program is activated by interrupt mode. The temperature signal is single-ended input, and the other channel is used to measure the torque of the stirrer, which is measured by the bridge balance method. The specific connection is shown in Figure 3.

Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

3.3 Read and write timing and microcontroller code of AD7711A

Read and write data, control, and calibration registers are serially read and written through the SDATA data line. After the data A/D conversion is completed, DRDY is set low, causing an interrupt, A0 selects the data register, RFS is set low to make the read data valid, and one bit of data is read every time SCLK rises. The read and write timing is shown in Figure 4.

Performance characteristics, structure and application analysis of high-resolution A/D device AD7711A

Read data program code:

RD: SETB A0; read data register

SETB TFS;

CLR RFS; set to 0 to validate data

CLR SCLK;

MOV R1, #3;

RDD: MOV R2, #8;

RDDD: SETB SCLK; clock set high

MOV C, SDATA; read 1 bit

CLR SCLK;

RLC A;

DJNZ R2, RDDD; whether to finish reading 1BYTE

MOV R0, [email protected]; data is stored in @R0 area

INC RO;

DJNZ R1, RDD;

Write control register program code:

WR: CLR A0; write control register

SETB-RFS;

CLR TFS; make write data valid

CLR SCLK;

MOV R1, #3;

WRR: MOV R2, #8

MOV [email protected]

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