AD7705/06 and its applications

    Abstract: This article introduces a 16-bit error-free analog-to-digital conversion chip AD7705/06 based on Σ-Δ conversion technology, and provides a practical hardware interface circuit and software program.

    Keywords: Σ-Δ A/D error-free analog-to-digital conversion chip

1. Introduction

    In the application of intelligent instrumentation, since the traditional sensor signal is an analog signal, for intelligent instruments, an A/D converter is definitely needed to realize the control of the microcontroller. In many applications, high-precision measurement of more than 16 bits is required, but traditional integrating type and successive comparison type A/D are difficult to implement and the cost is high. The Σ-Δ A/D conversion technology that has emerged in recent years can obtain extremely high resolution at a lower cost. AD's AD7705/06 and AD7707 are typical 16-bit A/D conversion chips.

2. Introduction to AD7705/06

    AD7705/06 is a new A/D chip recently launched by the American AD Company. Its overall structure is shown in Figure 1.

    The AD7705/06 chip is a Σ-Δ A/D converter with self-correction function. It consists of multiple analog switches, buffers, programmable gain amplifier (PGA), Σ-Δ modulator, digital filter, reference voltage input, clock circuit and serial interface. The serial interface includes a register group, which consists of communication registers, setting registers, clock registers, data output registers, zero point correction registers and full range correction registers. The chip also includes 2-channel differential inputs (AD7705) and 3 pseudo-differential channel inputs (AD7706).

    The PGA of AD7705/06 can be set by instructions to amplify input signals of different amplitudes by 1, 2, 4, 8, 16, 32, 64 and 128 times. Therefore, the AD7705/06 chip can accept input signals sent from the sensor. Low-level input signals can also accept high-level (10V) signals. It uses Σ-Δ technology to achieve 16-bit error-free performance; its output speed can also be set by instructions, ranging from 20Hz to 500Hz; it can pass instructions Set to calibrate the zero point and full range; the data transmission between AD7705/06 and the microprocessor is carried out through serial mode, using a communication method that saves port lines, occupying at least two port lines of the control machine.

3. The basic connection of AD7705/06 and the interface circuit with the microprocessor

The basic connection

    of AD7705/06 is shown in Figure 2, in which AD780/REF192 provides +2.5V high-precision reference voltage. Since the AD7705 has only 2 channels, it can convert two analog quantities, while the AD7706 has 3 channels, so it can convert three analog quantities. The author fully utilized this when designing the pressure transmitter. The functionality of the chip and the basic connections of the AD7706 are similar to Figure 2.

Interface circuit between AD7705/06 and microcontroller

    The interface between AD7705/06 and microprocessor is very convenient. During its operation, the pins involved in the interface are CS, SCLK, DOUT, DIN and DRDY. It is connected to the microprocessor. The interfaces include three-wire, four-wire, five-wire and multi-wire modes. In the three-wire mode, DOUT, DIN and SCLK pins are usually used for control. DOUT and DIN are connected to the serial port of the microprocessor for data output and input. SCLK is used to input serial clock pulses. CS is always is low level. In four-wire mode, the CS pin can also be controlled by a certain port line of the microprocessor. In five-wire mode, the DRDY pin can also be controlled by a certain port line of the microprocessor. In multi-wire control mode, all interface pins are controlled by the microprocessor. Figure 3 is a simplified circuit diagram of the author using INTEL 8031 ​​to control the AD7706.

    The output signal of AD7706 is directly connected to the RXD (P3.0) terminal of 8031, and the TXD (P3.1) terminal of 8031 ​​provides a clock signal for AD7706. It can be seen that in this connection method, the clock of the A/D converter is Provided by the TXD pin of the 8031. The microcontroller uses the serial port to communicate with the AD7706, and sets the serial port to working mode 0, which is the synchronous shift register mode. In addition, the microcontroller also controls CS through the P1.0 pin and determines DRDY through the P1.1 pin. In this way, in a multi-chip system, we should first select the chip, and the system must first clear the P1.1 port line. When receiving data, first determine the pin level of P1.0. If it is low level, it indicates that there is valid conversion data in the data output register of the chip. In this way, the microcontroller sets REN=1. At this time, Receiving data begins. When 8-bit data is received, the interrupt flag R1 is set. Once the serial reception is completed, the microcontroller automatically stops sending shift pulses. The 8-bit data is read from the serial port buffer into the memory and used by the software. Clear the RI flag, and the microcontroller starts sending shift pulses again until another 8-bit data is received, then another serial reception ends. In this way, the 8-bit data this time is combined with the high-order 8-bit data just received to become 16-bit data, which is the result of an A/D conversion. This interface method directly utilizes the hardware resources of the microcontroller itself, thereby simplifying the circuit design.

    Program 1 AD7706 initialization program

    BEGIN:

    CLR A

    MOV A , #010H; Set the serial working mode 0

    MOV SCON , A

    CLR P1.0; Select the chip AD7706

    MOV A , #20H; Write to CMR, and select the next operation Set CKR

    MOV SBUF, A

    JNB TI, ﹩; After receiving, reset TI

    CLR TI

    MOV A, #0CH; Set CLK

    MOV SBUF, A 

    JNB TI,

    CLR TI

    MOV A, #010H; Write to CMR, next operation Select STR

    MOV SBUF, A

    JNB TI, ﹩

    CLR TI

    MOV A, #40H; set STR

    MOV SBUF, A

    JNB TI, ﹩ 

    CLR TI

    RET

    program 2 Input byte program (judge DRDY pin):

    INB1: CLR C

    JB P1.1, INB1; judge the DRDY pin level

    CLR P1.0; DRDY is 0, valid data, perform read data operation

    MOV A, #38H; perform write operation on CMR, select DOR

    MOV SBUF for the next operation, A 

    MOV A , SBUF; read conversion data

    MOV R3 from AD7706, A; store the high 8 bits in R3 

    JNB TI, 

    CLR TI

    MOV A, SBUF; read conversion data

    MOV R4 from AD7706, A; low 8 bits Store in R4

    JNB TI,﹩ 

    CLR TI

    RET

    program 3 Input byte program (judge the highest bit of CMR):

    INB2: CLR P1.0; operate AD7706

    MOV A, #08H; write to CMR, select CMR for the next operation

    MOV SBUF,A

    MOV SBUF, A; Read the CMR ANL A of AD7706

    , #10000000B; Judge the DRDY bit, if it is 0, there is valid data

    JNZ INB2; Wait for

    MOV A, #38H; Write operation to CMR, next operation Select DOR

    MOV SBUF, A

    MOV A, SBUF

    MOV R3, A

    JNB TI,﹩ 

    CLR TI

    MOV A, SBUF

    MOV R4, A

    JNB TI,﹩ 

    RET

4. Utility program example The

    following is a program for 8031 ​​to control AD7706, including Initialization program (see program listing).

    The converter chip settings are: gain 1, no filter synchronization, bipolar input mode and buffer off mode.

Conclusion

    When designing the intelligent pressure transmitter, the author adopted the wiring method in Figure 3. The three channels of AD7706 receive signals from the pressure (differential pressure) sensor, temperature sensor and static pressure sensor respectively. In this way, the pressure (differential pressure) sensor can be measured. pressure) to compensate, thereby eliminating the effects of temperature and static pressure, and applying the above program to initialize and read conversion data, the effect after use is satisfactory.