Application of 16-bit high-speed A/D converter in high-speed DSP

　　Because DSP has an advanced parallel structure that makes it particularly suitable for signal processing, it has been increasingly used in the field of industrial control and the development and design of various types of instruments. In intelligent instruments and meters composed of digital signal microprocessors, various external analog signals must be converted into digital signals through an A/D converter before they can be sent to the microprocessor chip. BB's high-precision converter ADS7805 has a high performance-price ratio, the highest conversion frequency can reach 100KHz, and is widely used in instrumentation. The ADS7805 chip has a 28-pin double-row in-line or SMD package. It only needs a single-ended 5V power supply to operate normally. The chip contains circuits such as sampling and holding, voltage reference and clock, which greatly simplifies the user's circuit design. Improved system stability. ADS7805 is manufactured using CMOS technology, has low power consumption (maximum power consumption is 100mW), has a single channel input, and the analog input voltage range is ±10V. It adopts the successive approximation working principle. The conversion results are output in parallel by 16-bit data lines to start conversion and Reading the results of the last conversion can be done simultaneously.

　　1. Pins and functions

　　ADS7805 has a total of 28 pins, and Figure 1 is the pin diagram. These pins can be roughly divided into 3 categories.

　　i. Power supply

　　The digital power supply Vdig (pin 28) and the analog power supply Vana (pin 27) are usually connected to the 5V power supply together; the digital ground DGND (pin 14) and the analog grounds AGND1 (pin 2) and AGND2 (pin 5) usually share the same ground; the reference voltage If terminal REF (pin 3) is connected to a 2.2μF tantalum capacitor to ground, a 2.5V reference voltage will be generated inside the chip; reference capacitor terminal CAP (pin 4) is connected to a 2.2μF tantalum capacitor to ground.

　　ii.Signal class

　　Analog signal input terminal Vin (pin 1); D15~D8 (pin 6 to pin 13) and D7~D0 (pin 15 to pin 22) are digital signal parallel output ports.

　　iii.Control signal class

　　Chip select signal input terminal/CS (pin 25); read result/analog-to-digital conversion control signal input terminal R/C (pin 24); conversion result indication output terminal/BUSY (pin 26); control the data read from the bus Is it the high byte or low byte signal input terminal BYTE (pin 23) of the conversion result.

　　2. Start A/D conversion and read the conversion result

　　The timing of starting conversion and reading the conversion result of the ADS78XX series A/D converters produced by BB Company in the United States is relatively special and roughly the same. The conversion timing diagram of ADS7805 is shown in Figure 2.

Figure 2 Conversion timing diagram of ADS7805

　　First, a low-level signal must be input to the R/C pin. The duration of this low-level signal is required to be between 40 and 7000ns. At this time, the BUSY pin level is pulled low to indicate that conversion is in progress. After about After 8μs (that is, the time the /BUSY signal remains low), the conversion is completed, and the BUSY pin level becomes high accordingly; then pull the R/C pin level high, and the ADS7805 will output the conversion result to the data on the bus. Although the conversion result is 16 bits, for a DSP microprocessor with a 16-bit data bus, it does not need to be read in twice, so the BYTE pin can be at the same signal level from beginning to end. When the BYTE pin is high level , Pin6~13 outputs the low byte, and Pin15~22 outputs the high byte; conversely, when the BYTE pin is low level, Pin6~13 outputs the high byte, and Pin6~13 outputs the low byte. If the /CS pin is used to control the conversion of the ADS7805, the timing is similar to the R/C pin, so it will not be explained in detail here. The digital result converted by ADS7805 is given in complement form. Now its ideal data conversion table is given. Table 1 lists the relationship between the analog voltage of ADS7805 and its digital output in complement form. -10V~9.99512V is Measuring range

Table 1 Relationship between analog voltage input and digital signal output

　　3. Interface with DSP

　　TI's 16-bit fixed-point DSP TMS320F206 is a low-power device that adopts an improved Harvard structure. It has 1 program bus and 3 data buses, pipeline operation, and a highly parallel 32-bit arithmetic logic unit, 16 *16-bit parallel hardware multiplier, on-chip memory, on-chip peripherals and highly specialized instruction set. When TMS320F206 is connected to an external 16MHZ crystal oscillator and the operating clock control mode is selected as level, the R/C pin is connected to the expansion output port of the DSP, and the /BUSY signal only needs to be connected to a common input port, such as the IO port of the DSP, the input port of the expansion chip 74LS244, etc. Figure 3 is a typical interface circuit between ADS7805 and TMS320F206.

　　4. C language programming

　　Users of TMS320F206 can write source programs in assembly language or C language. The system developed in C language is easy to maintain, has high reliability and has good portability. Therefore, the development program of ADS7805 written in C language is introduced below.

　　The source program is as follows:

　　#include
　　#include
　　ioport unsigned int port90ac;
　　ioport unsigned int porte0f7;
　　#define ic273n port90ac
　　#define ads7805 porte0f7
　　#define Wait asm(" nop ");
　　int i, ia[8]; double bv_coil,IA;
　　reg_aspcr=0x0e00; reg_iosr=0x0000;
　　asm ( " clrc intm ");
　　ic273n=0x0008; Wait;
　　for(i=0;i<=7;i++)
　　{ ic273n=0x0000;
　　while ((reg_iosr&0x0004)==0x0004)
　　{ if (adt> 2; return() ;)
　　 ic273n=0x0008; Wait;
　　ia[i]=(int)(ads7805&0xffff); }
　　IA=0; for (i=0;i<=7;i++)
　　{ if ia[i]<32767 pr[i] =((double)(ia[i]))*0.000305185;/*10/32767=0.000305185*/ else
　　 pr[i]=((double)(ia[i]-65535))*0.000305185;
　　IA=IA+ pr[i]*pr[i]; }
　　bv_coil = sqrt(IA/8);

　　5. Conclusion

　　ADS7805 is a 16-bit A/D converter. It not only has high resolution and fast conversion speed, but also has a simple and convenient interface and flexible application, so it has a wide range of application prospects. The author used ADS7805 in the GKNDC-H high-voltage switch characteristics precision measuring instrument and achieved good measurement results.