ads8689: can not get correct data

You can use RVS to know when the device is done wtith the conversation phase, note that RVS can depend on the output protocol used.

I would suggest looking at the datasheet, starting on Page 10 for timing requirements of the device. The frequency you use depends on how you are using the device, just be sure to be within the bounds of the device's capabilities.

Also, if you want to learn more about ADCs' there is a great resouce, Precision Labs-ADCs that can really help

Regards

Cynthia

Hi Biao,

Your schematic looks fine for me, but there are some concerns:

1.  Your are splitting the ground and your ground connection point by R22 is far from ADC, also you are using only two-layer pcb board and there is very small analog ground area. You have to make sure the voltage difference between GND and AGND/REFGND is less than +/-300mV, so a solid ground plane is strongly recommended and this is also mentioned in datasheet.

2.  Two-Layer pcb layout is not easy to achieve 16-bit ADC's performance, also your board size is small and all circuits including DC-DC and MCU digital circuit are put together. It's important to have a clean layout to avoid any interference from digital/power to analog circuit.

3.  ADC's AVDD is used for internal signal acquisition and conversion, so it's important to have a clean and well-bypassed power supply. A low noise LDO is a good solution to provide a supply to the ADC, for example, TPS7A47xx, this is a very common device to be used in our EVM boards.

Please refer to Layout Guidelines for more details in ADS8689 datasheet page 61.

Except above layout suggestions, can you please let us know the value for resistors and capacitors in your schematic? also please upload your timing screenshot from a scope so that we can check if your timing is correct, like Figure 3 in datasheet.

The test conditions I mean are: 

Did you apply the test signal on Q2? What's the voltage for test and where is signal from(precision signal generator or others)? What's the output conversion code you got from ADC? etc.

Thanks.

Best regards

Dale

ADS868X_CS=0;
delay_ms(1); //延时ms
SPI2_ReadWriteByte(0xD014);
SPI2_ReadWriteByte(0x0009);
ADS868X_CS=1;

float adc_filter(u16 *data,u16 num)
{
u8 i=0;
u16 max=0,min=0;
u32 sum=0;
float average=0;

max = *(data+0);
min = *(data+0);
sum += *(data+0);

for (i=1; i<num; i++)
{
if (max < *(data+i))
{
max = *(data+i);
}
if (min > *(data+i))
{
min = *(data+i);
}
sum += *(data+i);
}

sum = sum - max - min;
average = (float)((float)sum/(float)(num-2));

return average;
}

void TIM3_IRQHandler(void) //TIM3中断
{
static u16 ADC_RateDataGroup[ADC_FILTER_NUM]= {0};
static u16 ADC_RateDataCount=0;
if (TIM_GetITStatus(TIM3, TIM_IT_Update) != RESET) //检查TIM3更新中断发生与否
{
if(ADC_RateDataRedy==0)
{
ADS868X_CS=0;
ADC_RateDataGroup[ADC_RateDataCount]=SPI2_ReadWriteByte(0x0000);//发送读取ID命令
SPI2_ReadWriteByte(0x0000);//发送读取ID命令
ADS868X_CS=1;
ADC_RateDataCount++;
if(ADC_RateDataCount==ADC_FILTER_NUM)
{

ADC_RateData =adc_filter(ADC_RateDataGroup,ADC_FILTER_NUM)*10.24/65535; //100-600
ADC_RateDataRedy=1;
ADC_RateDataCount=0;
}
}
TIM_ClearITPendingBit(TIM3, TIM_IT_Update); //清除TIMx更新中断标志

}
}

/*设置量程0-2.5*4.096*/
ADS868X_CS=0;
delay_ms(1); //延时ms
SPI2_ReadWriteByte(0xD014);
SPI2_ReadWriteByte(0x0009);
ADS868X_CS=1;

/*设置量程0-2.5*4.096*/
ADS868X_CS=0;
delay_ms(1); //延时ms
SPI2_ReadWriteByte(0xD014);
SPI2_ReadWriteByte(0x0009);
ADS868X_CS=1;


float adc_filter(u16 *data,u16 num)
{
u8 i=0;
u16 max=0,min=0;
u32 sum=0;
float average=0;

max = *(data+0);
min = *(data+0);
sum += *(data+0);

for (i=1; i<num; i++)
{
if (max < *(data+i))
{
max = *(data+i);
}
if (min > *(data+i))
{
min = *(data+i);
}
sum += *(data+i);
}

sum = sum - max - min;
average = (float)((float)sum/(float)(num-2));

return average;
}


void TIM3_IRQHandler(void) //TIM3中断
{
static u16 ADC_RateDataGroup[ADC_FILTER_NUM]= {0};
static u16 ADC_RateDataCount=0;
if (TIM_GetITStatus(TIM3, TIM_IT_Update) != RESET) //检查TIM3更新中断发生与否
{
if(ADC_RateDataRedy==0)
{
ADS868X_CS=0;
ADC_RateDataGroup[ADC_RateDataCount]=SPI2_ReadWriteByte(0x0000);//发送读取ID命令
SPI2_ReadWriteByte(0x0000);//发送读取ID命令
ADS868X_CS=1;
ADC_RateDataCount++;
if(ADC_RateDataCount==ADC_FILTER_NUM)
{

ADC_RateData =adc_filter(ADC_RateDataGroup,ADC_FILTER_NUM)*10.24/65535; //100-600
ADC_RateDataRedy=1;
ADC_RateDataCount=0;
}
}
TIM_ClearITPendingBit(TIM3, TIM_IT_Update); //清除TIMx更新中断标志

}
}

Thanks.

the value for resistors and capacitors in my schematic

output conversion code i got from ADC: I get the average of 10 data.

/*设置量程0-2.5*4.096*/
ADS868X_CS=0;
delay_ms(1); //延时ms
SPI2_ReadWriteByte(0xD014);
SPI2_ReadWriteByte(0x0009);
ADS868X_CS=1;

float adc_filter(u16 *data,u16 num)
{
u8 i=0;
u16 max=0,min=0;
u32 sum=0;
float average=0;

max = *(data+0);
min = *(data+0);
sum += *(data+0);

for (i=1; i<num; i++)
{
if (max < *(data+i))
{
max = *(data+i);
}
if (min > *(data+i))
{
min = *(data+i);
}
sum += *(data+i);
}

sum = sum - max - min;
average = (float)((float)sum/(float)(num-2));

return average;
}

void TIM3_IRQHandler(void) //TIM3中断
{
static u16 ADC_RateDataGroup[ADC_FILTER_NUM]= {0};
static u16 ADC_RateDataCount=0;
if (TIM_GetITStatus(TIM3, TIM_IT_Update) != RESET) //检查TIM3更新中断发生与否
{
if(ADC_RateDataRedy==0)
{
ADS868X_CS=0;
ADC_RateDataGroup[ADC_RateDataCount]=SPI2_ReadWriteByte(0x0000);//发送读取ID命令
SPI2_ReadWriteByte(0x0000);//发送读取ID命令
ADS868X_CS=1;
ADC_RateDataCount++;
if(ADC_RateDataCount==ADC_FILTER_NUM)
{

ADC_RateData =adc_filter(ADC_RateDataGroup,ADC_FILTER_NUM)*10.24/65535; //100-600
ADC_RateDataRedy=1;
ADC_RateDataCount=0;
}
}
TIM_ClearITPendingBit(TIM3, TIM_IT_Update); //清除TIMx更新中断标志

}
}

Hi Biao,

I would like to communicate with you by email and call, I will contact you directly.

Thanks

Best regards

Dale