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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.

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/**
******************************************************************************
* @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
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

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