A7799之STM32程序——STM32测试高精度ADC篇（二）

1．  AD7799概述AD7799是ADI公司早期推出一款高精度低速率的ADC，性能参数如下•均方根(RMS)噪声： 27 nV（4.17 Hz、AD7799） 65 nV（16.7 Hz、AD7799） 40 nV（4.17 Hz、AD7798） 85 nV（16.7 Hz、AD7798）•功耗：380 µA（典型值）•省电模式：最大1 µA•低噪声可编程增益仪表放大器•更新速率：4.17 Hz至470 Hz；3个差分输入（相比ADS1232多一个通道）•内部时钟振荡器•50 Hz/60 Hz同时抑制•基准电压检测•低端电源开关•可编程数字输出•熔断电流控制•电源电压：2.7 V至5.25 V•高达23.5个有效位AD7799性价比很高，非常适用于静态变量的测试，如电子秤、应变计、气体分析、仪器仪表、压力传感器、血液分析、工业过程控制等应用。本人对AD7799做了一次比较测试，分享下测试的结果2．  硬件设计分析        从结构图可以看出来，AD7799是模拟区域与数字区域完全独立的ADC，即AVDD给模拟区域供电，DVDD给数字区域供电，在原理图设计方面按照官方指导文档，需要对两个区域做独立的布线与隔离处理，才能让信噪比最佳。另可靠的基准电压是高精度ADC命根，本次试验选择TI公司推出的REF5025作基准参考，REF5025可低于3µVpp/V 噪声、3ppm/°C 漂移，性能是十分出色的。             由于经常做高频类项目，十分讨厌杜邦线/飞线测试方式，在高精度的领域，24位ADC梯度值2的2416777216，如果接入基准电压是2.5v，理论分辨率可达到0.149μV，做过高频的工程师深知杜邦线的罪恶，根据上面的技术分析，哪怕线路被引入1μV的干扰，也可以让精度打上一定折扣。为了让ADS1232性能得以充分体现，特意做了一个测试载板，载板的设计也是很关键，分割模拟数字区域同时，连接地方大量使用钽电容做旁路电路，以把波纹抑制到最小，合理的布局与布线也很重要，敷铜区域也需要模数分离，以磁珠或者0-5R/电感隔开。3．  时序图解说

由时序图看出来，AD7799读写是简单的3线串行读数方式，属于Microwire串行接口，STM32的SPI接口可以

完美的与之匹配，当然也可以采用软仿SPI替代STM32的硬件SPI，这样的程序更具移植性。SPI时序实现也相对简单，

AD7799的CS线仅仅只是做片选使用（上图所示），而不用过多管理，保持低电平即可。特别需要注意的是在空闲时

候，SCLK时钟信号需要保持高电平，在SCLK半个周期当DIN接收到0x58后转换的数据才传入到DOUT总线，这时候

才能读取数据。

4．  核心源码寄存器列表（官方）#define AD7799_CS_LOW AD_CS_0()#define AD7799_CS_HIGH AD_CS_1()#define ADC_RDY_DAT (AD_DO)/*AD7799 Registers*/#define AD7799_REG_COMM 0 /* Communications Register(WO, 8-bit) */#define AD7799_REG_STAT 0 /* Status Register (RO, 8-bit) */#define AD7799_REG_MODE 1 /* Mode Register (RW, 16-bit */#define AD7799_REG_CONF 2 /* Configuration Register (RW, 16-bit)*/#define AD7799_REG_DATA 3 /* Data Register (RO, 16-/24-bit) */#define AD7799_REG_ID 4 /* ID Register (RO, 8-bit) */#define AD7799_REG_IO 5 /* IO Register (RO, 8-bit) */#define AD7799_REG_OFFSET 6 /* Offset Register (RW, 24-bit */#define AD7799_REG_FULLSALE 7 /* Full-Scale Register (RW, 24-bit *//* Communications Register Bit Designations (AD7799_REG_COMM) */#define AD7799_COMM_WEN (1 << 7) /* Write Enable */#define AD7799_COMM_WRITE (0 << 6) /* Write Operation */#define AD7799_COMM_READ (1 << 6) /* Read Operation */#define AD7799_COMM_ADDR(x) (((x) & 0x7) << 3) /* Register Address */#define AD7799_COMM_CREAD (1 << 2) /* Continuous Read of Data Register *//* Status Register Bit Designations (AD7799_REG_STAT) */#define AD7799_STAT_RDY (1 << 7) /* Ready */#define AD7799_STAT_ERR (1 << 6) /* Error (Overrange, Underrange) */#define AD7799_STAT_CH3 (1 << 2) /* Channel 3 */#define AD7799_STAT_CH2 (1 << 1) /* Channel 2 */#define AD7799_STAT_CH1 (1 << 0) /* Channel 1 *//* Mode Register Bit Designations (AD7799_REG_MODE) */#define AD7799_MODE_SEL(x) (((x) & 0x7) << 13) /* Operation Mode Select */#define AD7799_MODE_PSW(x) (1 << 12) /* Power Switch Control Bit */#define AD7799_MODE_RATE(x) ((x) & 0xF) /* Filter Update Rate Select *//* AD7799_MODE_SEL(x) options */#define AD7799_MODE_CONT 0 /* Continuous Conversion Mode */#define AD7799_MODE_SINGLE 1 /* Single Conversion Mode */#define AD7799_MODE_IDLE 2 /* Idle Mode */#define AD7799_MODE_PWRDN 3 /* Power-Down Mode */#define AD7799_MODE_CAL_INT_ZERO 4 /* Internal Zero-Scale Calibration */#define AD7799_MODE_CAL_INT_FULL 5 /* Internal Full-Scale Calibration */#define AD7799_MODE_CAL_SYS_ZERO 6 /* System Zero-Scale Calibration */#define AD7799_MODE_CAL_SYS_FULL 7 /* System Full-Scale Calibration *//* Configuration Register Bit Designations (AD7799_REG_CONF) */#define AD7799_CONF_BO_EN (1 << 13) /* Burnout Current Enable */#define AD7799_CONF_UNIPOLAR (1 << 12) /* Unipolar/Bipolar Enable */#define AD7799_CONF_GAIN(x) (((x) & 0x7) << 8) /* Gain Select */#define AD7799_CONF_REFDET(x) (((x) & 0x1) << 5) /* Reference detect function */#define AD7799_CONF_BUF (1 << 4) /* Buffered Mode Enable */#define AD7799_CONF_CHAN(x) ((x) & 0x7) /* Channel select *//* AD7799_CONF_GAIN(x) options */#define AD7799_GAIN_1 0#define AD7799_GAIN_2 1#define AD7799_GAIN_4 2#define AD7799_GAIN_8 3#define AD7799_GAIN_16 4#define AD7799_GAIN_32 5#define AD7799_GAIN_64 6#define AD7799_GAIN_128 7/* AD7799_CONF_REFDET(x) options */#define AD7799_REFDET_ENA 1#define AD7799_REFDET_DIS 0/* AD7799_CONF_CHAN(x) options */#define AD7799_CH_AIN1P_AIN1M 0 /* AIN1(+) - AIN1(-) */#define AD7799_CH_AIN2P_AIN2M 1 /* AIN2(+) - AIN2(-) */#define AD7799_CH_AIN3P_AIN3M 2 /* AIN3(+) - AIN3(-) */#define AD7799_CH_AIN1M_AIN1M 3 /* AIN1(-) - AIN1(-) */#define AD7799_CH_AVDD_MONITOR 7 /* AVDD Monitor *//* ID Register Bit Designations (AD7799_REG_ID) */#define AD7799_ID 0x9#define AD7799_ID_MASK 0xF/* IO (Excitation Current Sources) Register Bit Designations (AD7799_REG_IO) */#define AD7799_IOEN (1 << 6)#define AD7799_IO1(x) (((x) & 0x1) << 4)#define AD7799_IO2(x) (((x) & 0x1) << 5)

初始化程序
unsigned char AD7799_Init(void)
{
unsigned char status = 0x1;
u32 ID=AD7799_GetRegisterValue(AD7799_REG_ID, 1);
if( (ID& 0x0F) != AD7799_ID)
{
status = 0x0;
}

return(status);
}

读取寄存器值unsigned long AD7799_GetRegisterValue(unsigned char regAddress, unsigned char size){unsigned char data[5] = {0x00, 0x00, 0x00, 0x00, 0x00};unsigned long receivedData = 0x00;data[0] = AD7799_COMM_READ | AD7799_COMM_ADDR(regAddress);AD7799_CS_LOW;SPI_Write(data,1);SPI_Read(data,size);AD7799_CS_HIGH;if(size == 1){receivedData += (data[0] << 0);}if(size == 2){receivedData += (data[0] << 8);receivedData += (data[1] << 0);}if(size == 3){receivedData += (data[0] << 16);receivedData += (data[1] << 8);receivedData += (data[2] << 0);}return receivedData;}

写寄存器
void AD7799_SetRegisterValue(unsigned char regAddress,
unsigned long regValue,
unsigned char size)
{
unsigned char data[5] = {0x03, 0x00, 0x00, 0x00, 0x00};
data[0] = AD7799_COMM_WRITE |  AD7799_COMM_ADDR(regAddress);
if(size == 1)
{
data[1] = (unsigned char)regValue;
}
if(size == 2)
{
data[2] = (unsigned char)((regValue & 0x0000FF) >> 0);
data[1] = (unsigned char)((regValue & 0x00FF00) >> 8);
}
if(size == 3)
{
data[3] = (unsigned char)((regValue & 0x0000FF) >> 0);
data[2] = (unsigned char)((regValue & 0x00FF00) >> 8);
data[1] = (unsigned char)((regValue & 0xFF0000) >> 16);
}
AD7799_CS_LOW;
SPI_Write(data,(1 + size));
AD7799_CS_HIGH;
}

复位
void AD7799_Reset(void)
{
unsigned char dataToSend[5] = {0x03, 0xff, 0xff, 0xff, 0xff};
AD7799_CS_LOW;
SPI_Write(dataToSend,4);
AD7799_CS_HIGH;
}

初始化：
void AD7799_INIT(void)
{
unsigned long command;
command = AD7799_GetRegisterValue(AD7799_REG_CONF,2);
command &= ~AD7799_CONF_GAIN(0xFF);
command |= AD7799_CONF_GAIN(1); / 不使用内部PGA
AD7799_SetRegisterValue(AD7799_REG_CONF,command,2);
AD7799_SetReference();
command = AD7799_GetRegisterValue(AD7799_REG_CONF,2);
command &= ~AD7799_CONF_CHAN(0xFF);
command |= AD7799_CONF_CHAN(2); // 第三通道 AIN3+ —— AIN3-
AD7799_SetRegisterValue(AD7799_REG_CONF,command,2);
command = AD7799_GetRegisterValue(AD7799_REG_MODE,2);
command &= ~AD7799_MODE_SEL(0xFF);
command |= AD7799_MODE_SEL(0);// 连续转换模式
AD7799_SetRegisterValue(AD7799_REG_MODE,command,2);
}

5．  测试结果测试条件Vref=2.5v（REF5025调理过后输出）、Gain=1、Updata Reat=4.17Hz测试源是AVDD电阻分压后的电压值，实际加入电压是：2.217452v（8位半表实测），通过误差曲线的分析，摆幅稳定在±4µV，效果还是很理想的，官方测试条件Gain=64，加入片内放大器后噪声干扰还是不小的（下图，Y轴单位LBS），有将近60个LBS的波动，当然主抗匹配也有一定的原因。因此在使用前零度和满度校准是十分必要的。

6．  总结作为一款低速高精度的ADC，AD7799是个不错的选择，相比于ADS1232虽然价格上是高了，但是他比ADS1232多了一个差分通道，转换速率可以到达470Hz，在本次测试看来，就性能上看起来稍比ADS1232好一点，但是相差无几，同样，超高的性价比和出色的性能让它在同级别的ADC中也有很强的竞争能力。欢迎大家多交流技术，Q Q：1625874998，可提供部分资料，供大家设计参考。