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STM32L1 OPAMP1 Gain Error

Question asked by Tom on Apr 2, 2014
Latest reply on Apr 2, 2014 by Tom
Hello All,

I connected DAC1 output (S6 closed, S5 opened)  to the internal OPAMP1 AIN+ which is in noninvering configuration by two external resistors (Rf=150k, Rg (to ground)=50k) which should yeld G=x4 amplification.
When I change DAC output (Uin) from0 - 0,5V I get on the OPAMP1 ouput (ADC Input Channel 3),  Vout :

Uin=0,5V      -> Vout=2,035V   which is ca. G=4,0
Uin=0,2V      -> Vout=0,853V   which is ca. G=4,2
Uin=0,1V      -> Vout=0,450V   which is ca. G=4,5
Uin=0,025V  -> Vout=0,157V   which is ca. G=6,2 !!!!

The lower the Uin to the opamp AIN+ the greater the OPAMP Gain error.

These are my configs:

void OPAMP1_Config(void)
  /*  (OPAMP1 output) in analog mode Configure  (OPAMP1 positive input) and  (OPAMP1 negative input) in analog mode */
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_1 | GPIO_Pin_2 | GPIO_Pin_3;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
  GPIO_Init(GPIOA, &GPIO_InitStructure);
  /* COMP Peripheral clock enable: COMP and OPAMP share the same Peripheral clock enable */
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_COMP, ENABLE);
  /* Enable OPAMP1 */
  /* Close S4 and S5 swicthes */

void DAC_Config(void)
  /* Enable GPIOA clock */
  /* Configure PA.04 (DAC_OUT1) in analog mode -------------------------*/
  GPIO_InitStructure.GPIO_Mode  = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_PuPd  = GPIO_PuPd_NOPULL;
  GPIO_InitStructure.GPIO_Pin = PinDAC1Out;
  GPIO_Init(PortDAC1Out, &GPIO_InitStructure);
  /* Enable DAC clock */
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_DAC, ENABLE);
  /* DAC channel1 Configuration */
  DAC_InitStructure.DAC_Trigger = DAC_Trigger_None;
  DAC_InitStructure.DAC_WaveGeneration = DAC_WaveGeneration_None;
  DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Disable;
  /* DAC Channel1 Init */
  DAC_Init(DAC_Channel_1, &DAC_InitStructure);
  /* Enable DAC Channel1 */
  DAC_Cmd(DAC_Channel_1, ENABLE);

What could be the reason for this error?

thanks in advance