The latest method: How to improve DSP sampling accuracy

1 Main Features of AD7858

AD7858 is a 12-bit serial port, high-speed, low-power, successive approximation AD converter launched by AD. It can operate at a voltage of 3 to 5.5 V, and its data throughput rate is as high as 200 kSPS. The chip contains a low-noise, wide-band tracking/holding amplifier that can process broadband signals up to 200 kHz.

The AD7858 is easy to interface with a microprocessor or DSP. The input signal is sampled starting from the falling edge of CONVST (this bit can be operated through a hardware pin or software bit), and the conversion is started from this point. The busy signal line is high at the start of the conversion, and then jumps to a low level after 400μs to indicate the end of the conversion.

The main features of the AD7858 are as follows:

◇Support 3~5.5 V voltage power supply;

◇When powered on, it has the functions of system automatic calibration (checking whether the chip itself or chip peripherals have errors) and automatic self-calibration (checking whether the calibration register has errors);

◇With high-speed serial interface;

◇Low power consumption. At 3 V voltage, the power consumption is only 12 mW;

◇The chip integrates a high-performance sampling and holding amplifier, and the input signal can be single-ended or differential input;

◇The AD7858 chip can support AD conversion started by hardware or software;

◇After the conversion is completed, it automatically enters the sleep mode, and the power consumption during sleep is 25μw;

◇ Available in various package forms such as 24-pin DIP, SOIC or TSSOP.

2 AD7858 Pin Functions

The main pin functions of AD7858 are as follows:

CONVST: Conversion start bit. When the falling edge of the clock arrives, the system is in hold mode and starts the digital-to-analog conversion. If this pin is not used, it should be connected to DVDD;

BUSY: Busy signal output terminal. Used to indicate the state of the device. After the falling edge of CONVST or the rising edge of CAL, the busy signal becomes high and remains high during the conversion. Once the conversion is completed and the conversion result is stored in the output register, the terminal is reset to low;

SLEEP: Sleep or low power mode. When this pin is low, the device is in sleep mode. If there is no digital-to-analog conversion or calibration work at this time, then all circuits (including internal reference voltage) are in shutdown mode; if this pin is 1, the device operates normally. If the sleep mode is only controlled by software, then this pin is always high;

REFIN/REFOUT: Reference input/output. This pin can be used as the internal reference voltage for analog-to-digital conversion and can be connected to the internal circuit through a series resistor;

3 Hardware Design

The connection method between DSP and AD7858 chip is shown in Figure 2. During design, analog power supply and analog ground, digital power supply and digital ground should be decoupled through capacitor network to reduce power supply input noise. In order to obtain a better signal-to-noise ratio, AGND and DGND can be connected together, and single-ended input is adopted, and two separate channels AIN1 and AIN4 are selected at the same time; the SLEEP pin is connected to DVDD, indicating that the sleep mode at this time is only controlled by software; the CAL pin can be connected to DGND through 0.01μF, and the conversion result will be output from DOUT in 16-bit binary and sent back to DSP through SPISOMI. The first 4 bits of the data are always 0. When the BUSY bit is low, the DSP will send the CONVST signal to AD7858 and start the conversion at the falling edge of CONVST. The given main pulse frequency of this circuit is 4MHz, and the system can send the synchronization signal SYNC to AD7858 through the I/O port.

4. Software Design

The main task of software design is to complete data acquisition. In this design, TI's TMS320LF2407A chip is used as the host. The system debugging environment is CCS2.0 version. And C language is used for programming.

5 Conclusion

The highest sampling voltage of the DSP is 3.3 V, and the AD converter (AD7858) is a 12-bit converter. Therefore, removing the highest bit (sign bit), the circuit has an 11-bit sampling accuracy.

It should also be pointed out that in practical applications, analog ground and digital ground are best in the form of ground layers, and the analog ground and digital ground should be grounded at a single point under the chip. Since the AD7858 analog/digital converter uses a serial port and outputs data in serial mode, and this data output port has fewer pins than a parallel port, it can save the space occupied by the device on the PCB board and is more suitable for various small devices.