index are changing with the voltage apply to the adc

i am using STM DSP library for FFT. FFT Apply on My ADC Count data. I am not sure fft are properly work or not. My Problem is the " when i apply 210mV(Frequency can be varied) to ADC (ADC MCP3911) the index of the high magnitude index are not exactly but near about the apply frequency and also that is linearly from 20 HZ to 2000 HZ .But when Apply Less than or greter than the 210 mV the index are not matched with apply frequency.

So what's am i going wrong.

My code is given below.

Thank you in Advance.

/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  ** This notice applies to any and all portions of this file
  * that are not between comment pairs USER CODE BEGIN and
  * USER CODE END. Other portions of this file, whether
  * inserted by the user or by software development tools
  * are owned by their respective copyright owners.
  *
  * COPYRIGHT(c) 2020 STMicroelectronics
  *
  * Redistribution and use in source and binary forms, with or without modification,
  * are permitted provided that the following conditions are met:
  *   1. Redistributions of source code must retain the above copyright notice,
  *      this list of conditions and the following disclaimer.
  *   2. Redistributions in binary form must reproduce the above copyright notice,
  *      this list of conditions and the following disclaimer in the documentation
  *      and/or other materials provided with the distribution.
  *   3. Neither the name of STMicroelectronics nor the names of its contributors
  *      may be used to endorse or promote products derived from this software
  *      without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
  */
/* Includes ------------------------------------------------------------------*/
#define ARM_MATH_CM4
#include "main.h"
#include "stm32f4xx_hal.h"

/* USER CODE BEGIN Includes */
#include "16x2LCD_4bits.h"
#include "mcp3911.h"
#include "dwt_stm32_delay.h"
#include "stm32f4xx_it.h"
#include "arm_math.h"
#include "core_cm4.h"
#include "calculation.h"
#include "arm_const_structs.h "


//#define Buf_Length	300
/* USER CODE END Includes */

/* Private variables ---------------------------------------------------------*/
SPI_HandleTypeDef hspi1;
DMA_HandleTypeDef hdma_spi1_rx;
arm_rfft_fast_instance_f32	fft_handler;
//arm_cfft_radix4_instance_f32 S;    /* ARM CFFT module */
//arm_cfft_instance_f32 fft_handler1;
//const static arm_cfft_instance_f32 *s;
//arm_cfft_instance_f32 varInstCfftF32;
//#define Samples 64
#define Buf_Length		4096	//2^N or fft size
#define Samples (Buf_Length/2)
/* USER CODE BEGIN PV */
/* Private variables ---------------------------------------------------------*/

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_DMA_Init(void);
static void MX_SPI1_Init(void);
void Do_FFT(void);
/* USER CODE BEGIN PFP */
/* Private function prototypes -----------------------------------------------*/

/* USER CODE END PFP */

/* USER CODE BEGIN 0 */
volatile	uint32_t Ch1Result;
volatile	uint32_t Ch2Result;	
volatile float Ch2_Voltage;
uint32_t Temp_Result;
uint32_t Temp_Result1;
uint8_t Power_on_cofig;
uint32_t Temp_Result2;
volatile uint32_t	BCDByte[10];
uint16_t Rx_DMA_Byte_Array[Buf_Length];
float32_t FFT_Input_Array[Buf_Length]={'\0'};
float32_t FFT_Input_Array_1[Buf_Length]={'\0'};
float32_t FFT_Output_Array[Buf_Length];
uint16_t FFT_Result_Array[10]={'\0'};
float32_t FFT_Result_Index[Buf_Length]={'\0'};
float32_t Max_Frequency;
uint16_t Max_Frequency_1;
uint32_t Max_Frequency_Index;
volatile float32_t sampleFreq = 4096/Buf_Length ;
float32_t freq;
float32_t freq_1;
uint16_t	RMSValue;
int j,Result_Flag=0, Result_Update=0;
unsigned char	Print_Array[10]={'\0'};
/* USER CODE END 0 */

/**
  * @brief  The application entry point.
  *
  * @retval None
  */
int main(void)
{
  /* USER CODE BEGIN 1 */

  /* USER CODE END 1 */

  /* MCU Configuration----------------------------------------------------------*/

  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();

  /* USER CODE BEGIN Init */

  /* USER CODE END Init */

  /* Configure the system clock */
  SystemClock_Config();

  /* USER CODE BEGIN SysInit */

  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
	HAL_Delay(10);
	DWT_Delay_Init();
	HAL_Delay(10);
  MX_DMA_Init();	
  MX_SPI1_Init();
  /* USER CODE BEGIN 2 */
//	HAL_DMA_Init(&hdma_spi1_rx);
	HAL_Delay(10);
	LCD_Initialisation();	
//	LCD_Command_Data(0x0c,0);				//display on,curser off
	HAL_Delay(10);
	LCD_String((unsigned char*)"FFT_On",1,0);
	HAL_Delay(1500);
	MCP3911_CS_LOW();
	HAL_Delay(10);	
	HAL_GPIO_WritePin(SPI1_nRESET_GPIO_Port, SPI1_nRESET_Pin, GPIO_PIN_RESET);	//RESET pin of adc low
	HAL_Delay(100);
	HAL_GPIO_WritePin(SPI1_nRESET_GPIO_Port, SPI1_nRESET_Pin, GPIO_PIN_SET);	//RESET pin of adc high
	HAL_Delay(20);
	MCP3911_CS_HIGH();
	HAL_Delay(20);
	Mcp3911Init();
	HAL_Delay(10);
//	HAL_Delay(1000);

//	LCD_Clear_Display();	
//	HAL_Delay(1500);
//	LCD_String((unsigned char*)"DMA_FFT",1,0);
	Temp_Result=Read_data_mcp3911(Mcp3911Gain,1);
//	HAL_Delay(10);
//	HextoBcd(Temp_Result);
//	Temp_Result1=Read_data_mcp3911(Mcp3911StatusCom,2);

//	Temp_Result2=Read_data_mcp3911(Mcp3911Config,2);
//	Ch1Result=Write_Address_Read_data_mcp3911(Mcp3911Ch1,2,2);	
	HAL_SPI_Receive_DMA(&hspi1,(uint8_t*) Rx_DMA_Byte_Array, (Buf_Length*2));
//		varInstCfftF32 = &arm_cfft_sR_f32_len64; 
//	s= &arm_cfft_sR_f32_len512; 
//	HAL_DMA_IRQHandler(&hdma_spi1_rx);
//	Temp_Result1=Write_Address_Read_data_mcp3911(Mcp3911StatusCom,1,2);
  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
	
//	arm_rfft_init_f32(&fft_handler,);

  arm_rfft_fast_init_f32(&fft_handler, Buf_Length);
////		arm_cfft_radix4_init_f32(&S, Buf_Length, 0, 1);
	Ch1Result=Write_Address_Read_data_mcp3911(Mcp3911Ch1,2,2);
//		arm_cfft_init_f32(&varInstCfftF32,Samples);

  while (1)
  {

  /* USER CODE END WHILE */

  /* USER CODE BEGIN 3 */
		
		Ch1Result=Write_Address_Read_data_mcp3911(Mcp3911Ch1,2,2);
		if(Result_Flag==1	&& Result_Update==10 )
		{
			Result_Flag=0,Result_Update=0;
			Float_2_ASCII(freq_1,(char*)Print_Array,4,2,0);
			LCD_String(Print_Array,1,0);
			Float_2_ASCII(Max_Frequency,(char*)Print_Array,6,0,0);
			LCD_String(Print_Array,2,0);
			Hex2Ascii(Max_Frequency_Index,(char*)Print_Array,0,4,0,0);	
			LCD_String(Print_Array,2,7);	//
			Hex2Ascii(Rx_DMA_Byte_Array[1],(char*)Print_Array,0,6,0,0);	
			LCD_String(Print_Array,1,8);	
			Hex2Ascii(j,(char*)Print_Array,0,4,0,0);	
			LCD_String(Print_Array,2,12);	//Max_Frequency
			HAL_Delay(500);
		}
  }
  /* USER CODE END 3 */

}

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{

  RCC_OscInitTypeDef RCC_OscInitStruct;
  RCC_ClkInitTypeDef RCC_ClkInitStruct;

    /**Configure the main internal regulator output voltage
    */
  __HAL_RCC_PWR_CLK_ENABLE();

  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE1);

    /**Initializes the CPU, AHB and APB busses clocks
    */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = 16;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
  RCC_OscInitStruct.PLL.PLLM = 8;
  RCC_OscInitStruct.PLL.PLLN = 132;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
  RCC_OscInitStruct.PLL.PLLQ = 4;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }

    /**Initializes the CPU, AHB and APB busses clocks
    */
  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                              |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV4;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_4) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }

    /**Configure the Systick interrupt time
    */
  HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);

    /**Configure the Systick
    */
  HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);

  /* SysTick_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
}

/* SPI1 init function */
static void MX_SPI1_Init(void)
{

  /* SPI1 parameter configuration*/
  hspi1.Instance = SPI1;
  hspi1.Init.Mode = SPI_MODE_MASTER;
  hspi1.Init.Direction = SPI_DIRECTION_2LINES;
  hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
  hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
  hspi1.Init.CLKPhase = SPI_PHASE_1EDGE;
  hspi1.Init.NSS = SPI_NSS_SOFT;
  hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_4;
  hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
  hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
  hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
  hspi1.Init.CRCPolynomial = 10;
  if (HAL_SPI_Init(&hspi1) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }

}

/**
  * Enable DMA controller clock
  */
static void MX_DMA_Init(void)
{
  /* DMA controller clock enable */
  __HAL_RCC_DMA2_CLK_ENABLE();

  /* DMA interrupt init */
  /* DMA2_Stream0_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(DMA2_Stream0_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(DMA2_Stream0_IRQn);

}

/** Configure pins as
        * Analog
        * Input
        * Output
        * EVENT_OUT
        * EXTI
*/
static void MX_GPIO_Init(void)
{

  GPIO_InitTypeDef GPIO_InitStruct;

  /* GPIO Ports Clock Enable */
  __HAL_RCC_GPIOE_CLK_ENABLE();
  __HAL_RCC_GPIOC_CLK_ENABLE();
  __HAL_RCC_GPIOH_CLK_ENABLE();
  __HAL_RCC_GPIOA_CLK_ENABLE();
  __HAL_RCC_GPIOB_CLK_ENABLE();

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOE, D5_LCD_Pin|D6_LCD_Pin|D7_LCD_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOC, E_LCD_Pin|RS_LCD_Pin|D4_LCD_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(GPIOC, SPI1_nCS_Pin|SPI1_nRESET_Pin, GPIO_PIN_SET);

  /*Configure GPIO pins : D5_LCD_Pin D6_LCD_Pin D7_LCD_Pin */
  GPIO_InitStruct.Pin = D5_LCD_Pin|D6_LCD_Pin|D7_LCD_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_OD;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOE, &GPIO_InitStruct);

  /*Configure GPIO pins : E_LCD_Pin RS_LCD_Pin SPI1_nCS_Pin SPI1_nRESET_Pin */
  GPIO_InitStruct.Pin = E_LCD_Pin|RS_LCD_Pin|SPI1_nCS_Pin|SPI1_nRESET_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_OD;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

  /*Configure GPIO pin : D4_LCD_Pin */
  GPIO_InitStruct.Pin = D4_LCD_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(D4_LCD_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pin : Dr_Input_Pin */
  GPIO_InitStruct.Pin = Dr_Input_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_PULLDOWN;
  HAL_GPIO_Init(Dr_Input_GPIO_Port, &GPIO_InitStruct);

}

/* USER CODE BEGIN 4 */

void HAL_SPI_RxCpltCallback(SPI_HandleTypeDef *hspi)
{
	uint16_t Temp_value;

	MCP3911_CS_HIGH();
	
    for(uint16_t i = 0; i < Buf_Length; i++)
    {
        Temp_value =  Rx_DMA_Byte_Array[i];
        
        FFT_Input_Array[i]=	(1 - cos(2*PI*Temp_value/Buf_Length))/2;            // Apply Windows Tech.
    
    }
    Do_FFT();

	__NOP();
}

void Do_FFT()
{

	arm_rfft_fast_f32(&fft_handler, (float32_t *)FFT_Input_Array,(float32_t *)FFT_Input_Array_1,0);


	arm_cmplx_mag_f32((float32_t *)FFT_Input_Array_1,(float32_t *) FFT_Output_Array,Samples);

	arm_abs_f32((float32_t *)FFT_Output_Array,(float32_t *)FFT_Output_Array,Samples);
	arm_max_f32(&FFT_Output_Array[1],(uint32_t)(Samples),&Max_Frequency,&Max_Frequency_Index);

	Max_Frequency_Index++;

	
/*	j=1;
	freq = FFT_Output_Array[1];
	for(uint16_t i=2;i<Samples;i++)
	{
	//for the maximum valude of spectrum magnitude
		if(FFT_Output_Array[i] > freq)
		{
            j = i;
            freq = FFT_Output_Array[i]; 
//	        Max_Frequency_1=FFT_Output_Array[j];					
		}
	}	
*/
}
/* USER CODE END 4 */

/**
  * @brief  This function is executed in case of error occurrence.
  * @param  file: The file name as string.
  * @param  line: The line in file as a number.
  * @retval None
  */
void _Error_Handler(char *file, int line)
{
  /* USER CODE BEGIN Error_Handler_Debug */
  /* User can add his own implementation to report the HAL error return state */
  while(1)
  {
  }
  /* USER CODE END Error_Handler_Debug */
}

#ifdef  USE_FULL_ASSERT
/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  * @param  file: pointer to the source file name
  * @param  line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t* file, uint32_t line)
{
  /* USER CODE BEGIN 6 */
  /* User can add his own implementation to report the file name and line number,
     tex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
  /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

/**
  * @}
  */

/**
  * @}
  */

/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/