Principle and application of 18-bit high-precision analog-to-digital converter FS511

　　1 Overview

　　FS511 is a high-precision, low-power analog-to-digital converter launched by Fujing Semiconductor . Its core is an 18-bit precision △-∑ ADC. In addition to the △-∑ ADC, it also integrates an operational amplifier, a low-pass filter, and a digital filter, which can filter the input signal multiple times to achieve high-precision A/D conversion. At a working voltage of 5 V, the power consumption of FS511 is only 1.2 mA. The device uses the SPI bus to interface with an external microprocessor. In addition, there are also a variety of control registers inside, which users can configure according to their needs to obtain different A/D conversion frequencies, output rates, A/D accuracy, etc.

　　FS511 can be widely used in electronic scales, sensor measurement devices, high-precision data acquisition systems, etc.

　　2 Main features and pin functions of FS511

　　The main features of FS511 are as follows:

　　△-∑ADC, 18-bit conversion accuracy, 10 Hz output rate (programmable);
　　linear error: +0.005%;
　　operating voltage range: 4.5 V ~ 6 V;
　　4 MHz crystal oscillator;
　　operating current is less than 1.2 mA, and the sleep mode current is about 1 μA;
　　the interface with the microprocessor uses the SPI bus;
　　low-noise operational amplifier.

　　FS511 uses a 20-pin DIP or SOP package, and its pin arrangement is shown in Figure 1. The functions of each pin are as follows:

　　VDD (12): Positive pole of analog power supply. Connect to 5 V DC power supply, connect a 0.1 μF capacitor to bypass to analog ground, and connect a 10 μF capacitor in parallel to analog ground;
　　VSSA (11): Analog ground;
　　VCC (14): Positive pole of digital power supply. Connect a 0.1 μF capacitor to bypass to digital ground, and connect a 10 μF capacitor in parallel to digital ground;
　　VSS (13): Digital ground;
　　XTAL0, XTAU (15, 16): Connect to 4 MHz crystal oscillator, and connect a 1 MΩ resistor in parallel to both ends of the crystal oscillator. The internal clock circuit obtains a clock signal of 83.33 KHz as the conversion frequency of the internal ∆-∑ ADC.

　　CS, Stock, DI, DO/IROO (20, 19, 18, 17): Analog SPI bus to transmit data with the microprocessor. CS is chip select, low level is effective; Stock is SPI clock input, which can be input by external clock signal or simulated by microprocessor; DI is SH data input, external microprocessor inputs initialization command to FS511 through DI port; DO/IRQO is SPI data output or interrupt request output, when FS511 completes A/D conversion, the conversion result is output to external microprocessor through DO;

　　OPP, OPN, OPO (5, 4, 3): correspond to the positive input, negative input and output of operational amplifier integrated in FS511 respectively. External analog signal is differentially input to FS511 through OPP and OPN, and then output from OPO single-ended. By building external circuits, different multiples of signal amplification

　　can be achieved; VRL, VRH (6, 8): positive and negative reference voltages of the internal ADC module, powered by its internal integrated power supply (1 V), or by an external circuit;

　　FTE, FTC (1, 2): positive and negative outputs of the internal integrated low-pass filter, connected in parallel with a 27 nF capacitor;

　　SGND (7): signal ground, the internal integrated power supply is 0.5 V), or it can be powered by an external circuit;

　　AGND (9): analog ground, when the internal register ENVS is set to 1, AGND = 2.5 V;

　　VS (10): internal voltage source, when the internal register ENVS is set to 1, VS is connected to VDD.

　　3 Working principle

　　The internal working principle diagram of FS511 is shown in Figure 2. The device mainly consists of four parts: △-∑ ADC, operational amplifier, low-pass filter, and digital filter. △-∑ ADC realizes A/D conversion, has two positive and negative input terminals, and the reference voltage can be selected from external or internal power supplies. When SVR0=1, the internal reference voltage is selected. When VDD=5 V, VRH-VRL=1 V; when SVR0=0, the external reference voltage is selected. When the external input signal is small, it should be amplified by the operational amplifier to make it close to the reference voltage of the △-∑ ADC to improve the conversion accuracy. When high precision is required, the register needs to be configured so that the input signal is filtered multiple times through the low-pass filter and the digital filter.
　　In Figure 2, the external signal is input to FS511 from OPP and OPN in differential form, first amplified by the operational amplifier, and then output from OPO single-ended. If high precision is required, SINH=00 can be configured, the low-pass filter is input to OPO, the OPO output signal is first filtered by the low-pass filter, and then enters the △-∑ ADC for A/D conversion. After the conversion is completed, it is further filtered by the digital filter, and finally the conversion result (24 bits) is output. Since FS511 performs A/D conversion on the input signal, amplifies it and filters it multiple times, a conversion accuracy of up to 18 bits can be obtained.

　　4 Application Circuit

　　As shown in Figure 3, the application circuit of FS511 is a data acquisition system that can realize: the force sensor collects force signals and converts them into voltage signals. Then the analog-to-digital conversion is performed by FS511, and the results are transmitted to the C8051F330 microcontroller through the D0 pin of FS511 for further processing, and finally sent to the PC through the serial port to display the conversion results.

　　In this application system, FS511 uses 5 V power supply and configures ENVS=1, so that VS=VDD=5 V. △-∑ADC uses external reference voltage, VRH-VRL=1 V, VS can provide 5 V voltage to the force sensor (full-scale signal sensitivity is 2 mV/V), then the force sensor output signal (SENA-SENB) ranges from 0 mV to 10 mV, this signal is much smaller than the reference voltage of 1 V of △-∑ADC, in order to obtain high conversion accuracy, the sensor output signal should be amplified to make it close to the reference force voltage of 1 V. The force sensor output signals SENA and SENB are input to FS511 in differential form, amplified 100 times by the operational amplifier, and then output from the OPO single-ended. At this time, the OPO output signal (i.e., the positive input signal of △-∑ADC) varies from 0 V to 1 V, close to the reference voltage of 1 V of △-∑ADC, so that it can be converted with high accuracy.

　　After FS511 conversion, the conversion result is sent to the microcontroller C8051F330 through the D0 port, and the data is transmitted by SPI bus. It can be realized by software programming according to the timing diagram of FS511. The timing diagram of FS511 refers to the FS511 data sheet. After experimental verification, the force measurement accuracy of this application system is as high as 0.01%.

　　5 Conclusion The analog-to-

　　digital converter FS511 has high precision, low power consumption, and high cost performance, and is suitable for applications requiring high precision.