ADS5485：快速16位ADC

数据采集系统的A/D转换器选择做电子技术的人经常需要使用A/D转换器，尤其是在数据采集方面，那在选择A/D芯片时，主要需要考虑哪些因素呢？现就A/D芯片的转换速率、分辨率、输入电压范围、输出方式等几个主要参数简单说明一下【1】，以下是以线阵CCD（TCD1304AP）的数据采集为例。

（1）采样频率：A/D的转换速率限制了器件的最高采样速率，器件的采样速率是根据奈奎斯特理论（采样定理）来确定的；本设计的CCD输出信号是经过采样保持的离散模拟信号，频率为0.5MHz；因此A/D器件采样频率只要大于或等于0.5MHz即可。

正常推荐采样率为信号最高频率的5-20倍。

（2）分辨率：A/D的分辨率是由A/D的转换位数决定的。

对于分辨率的要求，需要根据输入信号特性决定（CCD输出信号），综合考虑A/D可能带入的量化噪声进行选择。

TCD1304AP 器件的动态范围为300（最小饱和输出电压与最大暗电压之比），即暗信号噪声幅度为信号幅值的1/300=0.33%。

8位A/D引入的量化噪声为1/28 =0.39%，12位A/D引入的量化噪声为1/212=0.024%，16位A/D引入的量化噪声为1/216=0.0015%。

可见，8位的A/D分辨率不够，而12和16位又有点浪费。

不过这是根据10ms光积分时间的动态范围理论值估计的结果，而实际上光积分时间增加，暗电流幅值也会增大，动态范围相应减小；此外如果A/D 转换时未达到满量程，还需要对A/D量化误差比例进行折算，如8位A/D的量程幅值为3V，实际信号为2V，则量化噪声不再是1/28=0.39%，而是1/28X3/2=0.59%；A/D的差分线性误差也会带入噪声。

综合考虑，选择12的A/D器件最适合系统要求。

（3）输入电压范围：CCD饱和输出电压经过预处理后的输出峰峰值为2V（1.5-3.5V），因此A/D的输入电压范围应该大于2V；另外，电压范围过大，量化误差也越大；综合考虑，A/D 器件的输入电压范围在大于2V的基础上，越小越好。

常用ADC/DAC器件介绍ADC系列：1. ADS5483ADS5483 ADC 拥有高信噪比(SNR) 与无杂散动态范围(SFDR)，可通过第二尼奎斯特区(Nyquist zone) 接收来自DC 的输入频率。

采样速率为135 MSPS 的ADC 在输入频率(IF) 为70 MHz 时可实现78.6 dBFS 的SNR 以及95 dBc 的SFDR，与同类ADC 相比，SNR 高出3.5 dB，SFDR 高出8 dB。

更高性能ADS 5483 能够显著增强设计灵活性，进而使众多应用受益匪浅。

例如，其不仅可针对测量测试系统提供更高的准确度，而且还能凭借更高带宽在包括空中接口等在内的无线通信领域提供更高的灵敏度，如WCDMA、TD-SCDMA、WiMAX、LTE以及多载波3 GSM 等。

2. ADS795x该系列器件不仅可针对高密度应用实现优异的线性与AC 性能，而且优化后还能最大限度地提升诸如手持式医疗仪器、可编程逻辑控制器以及数字电源等电池供电及低电压应用的性能。

ADS795x 产品系列高度集成了众多组件，如GPIO、可编程告警阈值、板上定序器以及SPI 兼容接口等，能最大限度地降低板级空间要求，并简化与主机设备的连接及相关软件。

3. AS1538AS1538带有8个12Bit的分辨率的输入通道。

其在50ksps的转换速度下低于850μA(包括内部基准)消耗电流。

在关断模式下，AS1538的供电电流可降至1.5μA以下。

无论是2.5V的内部基准还是1V至5.25V范围内的外部基准，都能对满刻度模拟输入范围进行设定。

AS1538的I2C兼容接口支持速度高达3.4MHz的接口，包括100kHz 和400kHz的行业标准速度。

用户可编程I2C地址可以在一条I2C总线上挂多达4种设备。

4. LTC2262LTC2262 用1.8V 的低压模拟电源工作，极大地节省了功率，而且不牺牲AC 性能。

这个ADC 在基带提供了72.8dB 的信噪比(SNR) 性能和88dB 的无寄生动态范围(SFDR)。

ADS1115-BRVG User GuideCopyright 2020, In Nature Robotics Ltd.IntroductionThe ADS1115-BRVG is a high-precision 16-bit analog to digital converter module, with jumper-selectable and software-selectable gain settings. It uses an Inter-Integrated Circuit (I2C) interface to enable simple communications with computer or microcontroller modules such as the Raspberry Pi™ or Arduino™.Absolute Maximum Ratings(1)(1) Stresses above those listed under Absolute Maximum Ratings may causepermanent damage to the device. Exposure to absolute maximum conditions for extended periods may affect device reliability.Electrical CharacteristicsAll specifications at -40 °C to +125 °C, VDD = 3.3V, and Full-Scale (FS) = ± 2.048V, unless otherwise noted. Typical values are at +25 °C.(1)This parameter expresses the full-scale range of the ADC scaling. In no event should more thanVDD+0.3V be applied to this device.(2)99% of full-scale.(3)Includes all errors from onboard PGA and reference.Jumper Settings For ADC GainThe ADS1115-BRVG comes with jumpers and resistor dividers that can be used to divide the voltage seen by the analog to digital converter. This can be useful for example when monitoring a battery voltage or some other voltage that may be considerably higher than the power supply voltage (VDD) of the ADS1115 analog to digital converter. The following two tables indicate which jumpers correspond to which input channels of the ADS1115 (Table 1) and which jumper settings should be used to achieve a given gain at the ADS1115 (Table 2).Table 1. Jumpers For A to D ChannelsTable 2. Jumper Settings For A to D Input GainPhoenix Connector Screw Terminal WiringElectrical connections to the green Phoenix connector in the top-left corner of the board should be made as follows:Table 3. Phoenix Connector WiringSample CodeArduino Uno Sample CodeThe program below uses the default ADS1115 gain of 2/3, so that the reference voltage is ±4.096V/ (2/3) = ± 6.144V. The program samples the single-ended voltage on all 4 channels approximately once per second, and outputs the results to the serial port at 9600 bps. The program requires the Adafruit ADS1X15 library in order to run. This library is available through the Arduino development software (i.e. cli ck the ‘Tools | Manage Libraries’ menu item, and search for “Adafruit ADS1X15”).#include<Wire.h>#include<Adafruit_ADS1015.h>Adafruit_ADS1115 ads(0x48);float fVoltages[4];void setup() {Serial.begin(9600);Wire.setClock(100000);ads.begin();}void loop() {int16_t adc0;int16_t adc1;int16_t adc2;int16_t adc3;adc0 = ads.readADC_SingleEnded(0);adc1 = ads.readADC_SingleEnded(1);adc2 = ads.readADC_SingleEnded(2);adc3 = ads.readADC_SingleEnded(3);fVoltages[0] = (adc0*0.1875)/1000;fVoltages[1] = (adc1*0.1875)/1000;fVoltages[2] = (adc2*0.1875)/1000;fVoltages[3] = (adc3*0.1875)/1000;Serial.print(fVoltages[0],3);Serial.print("\t");Serial.print(fVoltages[1],3);Serial.print("\t");Serial.print(fVoltages[2],3);Serial.print("\t");Serial.print(fVoltages[3],3);Serial.println();delay(1000);}Raspberry Pi Sample CodeThe program below also uses the default ADS1115 gain of 2/3, so that the reference voltage is ±4.096V/ (2/3) = ± 6.144V. The program samples the single-ended voltage on all 4 channels and prints out the results. The program requires the “AToD.h” and “AToD.cpp” library files from In Nature Robotics Ltd. Links to these files are available from the ADS1115-BRVG product page at .#include<iostream>#include<string>#include<stdio.h>#include<stdlib.h>#include<unistd.h>#include<memory>#include"AToD.h"using namespace std;int main (void){const int NUM_READINGS = 100;char *i2c_filename = (char*)"/dev/i2c-1";const unsigned char A_TO_D_ADDRESS = 0x48;AToD atod(i2c_filename, A_TO_D_ADDRESS);//constructorfor (int i=0;i<NUM_READINGS;i++){double channel_voltages[4] = {0.0,0.0,0.0,0.0};for (int j=0;j<4;j++) {atod.GetMeasurement(j+1,0,1.0,channel_voltages[j]);}printf("Voltages: %.3f, %.3f, %.3f, %.3f\n",channel_voltages[0],channel_vol tages[1],channel_voltages[2],channel_voltages[3]);}return0 ;}。