A7799之STM32程式——STM32測試高精度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,效能引數如下
•均方根(RMS)噪聲:
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)
•省電模式:最大1 µA
•低噪聲可程式設計增益儀表放大器
•更新速率:4.17 Hz至470 Hz;3個差分輸入(相比ADS1232多一個通道)
•內部時鐘振盪器
•50 Hz/60 Hz同時抑制
•基準電壓檢測
•低端電源開關
•可程式設計數字輸出
•熔斷電流控制
•電源電壓:2.7 V至5.25 V
•高達23.5個有效位
AD7799價效比很高,非常適用於靜態變數的測試,如電子秤、應變計、氣體分析、儀器儀表、壓力感測器、血液分析、工業過程控制等應用。本人對AD7799做了一次比較測試,分享下測試的結果
2. 硬體設計分析
從結構圖可以看出來,AD7799是模擬區域與數字區域完全獨立的ADC,即AVDD給模擬區域供電,DVDD給數字區域供電,在原理圖設計方面按照官方指導文件,需要對兩個區域做獨立的佈線與隔離處理,才能讓信噪比最佳。另可靠的基準電壓是高精度ADC命根,本次試驗選擇TI公司推出的REF5025作基準參考,REF5025可低於3µVpp/V 噪聲、3ppm/°C 漂移,效能是十分出色的。
由於經常做高頻類專案,十分討厭杜邦線/飛線測試方式,在高精度的領域,24位ADC梯度值2的2416777216,如果接入基準電壓是2.5v,理論解析度可達到0.149μV,做過高頻的工程師深知杜邦線的罪惡,根據上面的技術分析,哪怕線路被引入1μV的干擾,也可以讓精度打上一定折扣。為了讓ADS1232效能得以充分體現,特意做了一個測試載板,載板的設計也是很關鍵,分割模擬數字區域同時,連線地方大量使用鉭電容做旁路電路,以把波紋抑制到最小,合理的佈局與佈線也很重要,敷銅區域也需要模數分離,以磁珠或者0-5R/電感隔開。
3. 時序圖解說
由時序圖看出來,AD7799 讀寫是簡單的3線序列讀數方式,屬於Microwire序列介面,STM32的SPI介面可以
完美的與之匹配,當然也可以採用軟仿SPI替代STM32的硬體SPI,這樣的程式更具移植性。SPI時序實現也相對簡單,
AD7799的CS線僅僅只是做片選使用(上圖所示),而不用過多管理,保持低電平即可。特別需要注意的是在空閒時
候,SCLK時鐘訊號需要保持高電平,在SCLK半個週期當DIN接收到0x58後轉換的資料才傳入到DOUT匯流排,這時候
才能讀取資料。
4. 核心原始碼
暫存器列表(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)
初始化程式
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);
}
讀取暫存器值 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; }
寫暫存器
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;
}
復位
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); / 不使用內部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);// 連續轉換模式
AD7799_SetRegisterValue(AD7799_REG_MODE,command,2);
}
5. 測試結果
測試條件Vref=2.5v(REF5025調理過後輸出)、Gain=1、Updata Reat=4.17Hz測試源是AVDD電阻分壓後的電壓值,實際加入電壓是:2.217452v(8位半表實測),通過誤差曲線的分析,
擺幅穩定在±4µV,效果還是很理想的,官方測試條件Gain=64,加入片內放大器後噪聲干擾還是不小的(下圖,Y軸
單位LBS),有將近60個LBS的波動,當然主抗匹配也有一定的原因。因此在使用前零度和滿度校準是十分必要的。
6. 總結
作為一款低速高精度的ADC,AD7799是個不錯的選擇,相比於ADS1232雖然價格上是高了,但是他比
ADS1232多了一個差分通道,轉換速率可以到達470Hz,在本次測試看來,就效能上看起來稍比ADS1232好一點,
但是相差無幾,同樣,超高的價效比和出色的效能讓它在同級別的ADC中也有很強的競爭能力。歡迎大家多交流技術
,Q Q:1625874998,可提供部分資料,供大家設計參考。