I have problem with the result from ADC. I divide it into two problems and will try to describe them good below. First some facts about the design, software and ADC configuration.
Device: STM32G0B1CCU6
Software using: STM32CubeG0 MCU Firmware Package V1.5.0
Config: HSE=24MHz SYSCLK=40MHz HCLK=APB=20MHz PLLQ=ToUSB=48MHz
SYSCLK=40MHz->ToADC(async) ADC=async/2=20MHz tS=3.5 tCONV=tS+12.5=16 fS=1.25MSps
25000 samples via DMA
PA3 connected to a light sensor, OPT101 via a RC filter (100ohm 1nF).
void HAL_MspInit(void)
{
__HAL_RCC_SYSCFG_CLK_ENABLE();
__HAL_RCC_PWR_CLK_ENABLE();
HAL_SYSCFG_VREFBUF_VoltageScalingConfig(SYSCFG_VREFBUF_VOLTAGE_SCALE1); // Internal voltage reference 2.5V
HAL_SYSCFG_EnableVREFBUF();
HAL_SYSCFG_VREFBUF_HighImpedanceConfig(SYSCFG_VREFBUF_HIGH_IMPEDANCE_DISABLE);
HAL_SYSCFG_StrobeDBattpinsConfig(SYSCFG_CFGR1_UCPD1_STROBE | SYSCFG_CFGR1_UCPD2_STROBE);
}
void HAL_ADC_MspInit(ADC_HandleTypeDef* hadc)
{
GPIO_InitTypeDef GPIO_InitStruct = {0};
if (hadc->Instance == ADC1) {
__HAL_RCC_ADC_CLK_ENABLE();
__HAL_RCC_GPIOA_CLK_ENABLE();
GPIO_InitStruct.Pin = GPIO_PIN_3; // PA3
GPIO_InitStruct.Mode = GPIO_MODE_ANALOG;
GPIO_InitStruct.Pull = GPIO_NOPULL;
HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);
hDmaAdc.Instance = DMA1_Channel1;
hDmaAdc.Init.Request = DMA_REQUEST_ADC1;
hDmaAdc.Init.Direction = DMA_PERIPH_TO_MEMORY;
hDmaAdc.Init.PeriphInc = DMA_PINC_DISABLE;
hDmaAdc.Init.MemInc = DMA_MINC_ENABLE;
hDmaAdc.Init.PeriphDataAlignment = DMA_PDATAALIGN_HALFWORD;
hDmaAdc.Init.MemDataAlignment = DMA_MDATAALIGN_HALFWORD;
hDmaAdc.Init.Mode = DMA_NORMAL;
hDmaAdc.Init.Priority = DMA_PRIORITY_HIGH;
if (HAL_DMA_Init(&hDmaAdc) != HAL_OK) {
Error_Handler();
}
__HAL_LINKDMA(hadc, DMA_Handle, hDmaAdc);
}
}
static void InitAdc(void)
{
ADC_ChannelConfTypeDef sConfig = {0};
hAdc.Instance = ADC1;
hAdc.Init.ClockPrescaler = ADC_CLOCK_ASYNC_DIV2; // ADC Clock Mux
hAdc.Init.Resolution = ADC_RESOLUTION_12B;
hAdc.Init.DataAlign = ADC_DATAALIGN_RIGHT;
hAdc.Init.ScanConvMode = ADC_SCAN_SEQ_FIXED;
hAdc.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
hAdc.Init.LowPowerAutoWait = DISABLE;
hAdc.Init.LowPowerAutoPowerOff = DISABLE;
hAdc.Init.ContinuousConvMode = ENABLE;
hAdc.Init.NbrOfConversion = 1;
hAdc.Init.DiscontinuousConvMode = DISABLE;
hAdc.Init.ExternalTrigConv = ADC_SOFTWARE_START;
hAdc.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
hAdc.Init.DMAContinuousRequests = DISABLE;
hAdc.Init.Overrun = ADC_OVR_DATA_PRESERVED;
hAdc.Init.SamplingTimeCommon1 = ADC_SAMPLETIME_3CYCLES_5;
hAdc.Init.OversamplingMode = DISABLE;
hAdc.Init.TriggerFrequencyMode = ADC_TRIGGER_FREQ_HIGH;
if (HAL_ADC_Init(&hAdc) != HAL_OK) {
Error_Handler();
}
sConfig.Channel = ADC_CHANNEL_3;
sConfig.Rank = ADC_RANK_CHANNEL_NUMBER;
if (HAL_ADC_ConfigChannel(&hAdc, &sConfig) != HAL_OK) {
Error_Handler();
}
}#1 Slow rising
I early discover that the first sample seems to be lower than the others. However after looking at curves I noted that is was more than the first sample. Signal is slowly rising. I have made a workaround for this, by sample 1000 extra sample and discard them. The picture show with different serie resistor on the analog input. This is from prototype no 1. The second have different DC level (around 200).
#2 DC offset
It is some kind of DC offset. DC level measured with oscilloscope isn't that low from the ADC. I think that this mysterious DC offset level keep the ADC from see low level signal and therefor give curves like this:
#3 Overshoot
It will for some level of signals be a heavy overshoot in the curves from the ADC. Note that the 50 Hz square wave signal to the ADC isn't straight square wave. It is bandwidth limited by the light sensor. So the upper corner to the left of the rising signal shouldn't be sharp, it will round off little like the curves down to right in the pictures. However overshoot and ringing as in these curves shouldn't exist. These cuvres are from prototype number 2 and the DC offset differ from previous curves above.
What have I done wrong?
//Michael
1 ACCEPTED SOLUTION
Accepted Solutions
More investigation of the DC offset. I remove the light sensor and instead mounted a resistor of 1kohm from PA3 to GND. This give a level close to zero, as can seen in picture below. I added a resistor to VDD and got a straight line at 2683-2696, which corresponds to 1.64 volt. It seems good, half of 3.3 volt.
Now I got an idea of measure with an oscilloscope at the analog input (with only a resistor to ground). It looks like this, while the ADC is working:
Is it this that destroy my measurements. Can it build up a DC level. Let us mount a capacitor over the resistor at PA3. The result can be seen in picture above at the third row of curves. Yes, a DC level is measured by the ADC.
How can I avoid this?
//Michael
I mounted an OP and got better results. Now low levels are visible and no overshoots. It looks like curves I'm used to see from this sensor, then it is connected to an oscilloscope. I connected the OP (MAX44259) as a buffer between the sensor and the ADC. Between OP output and ADC input is a resistor. I believe this resistor is needed to reduce the peak current at sample time. I mounted 470 ohm and think that is good, since datasheet specify max 680 ohm for the configuration I use. I already have 100 ohm resistors in the design and was on the they to select it for this position also. However I think it is better with higher value, to reduce the current. I will experiment with different value to see if it effects the sample values. I will also test to load the OPT101 output. Right now it only goes directly to the OP input.
THANKS!
//Michael
