# ADC sampling problem using FPGA

I am trying to sample sinusoid signals using AD7928, which is a 1 MSPS ADC. It is connected to an Altera Cyclone V FPGA, and runs at 20 MHz master clock. Data rate is 1.25 MHz and that's due to the fact that the ADC outputs a new sample every 16 cycles. Using SignalTap Logic Analyser, I monitor the output of the ADC, and it is clocked with the same data rate frequency (1.25 MHz) to meet Nyquist criteria. Input is a 440 Khz sine signal (under Nyquist's frequency 500 KHz), however, it looks distorted in SignalTap, figure below depicts it.

If I apply a higher frequency signal like 1019 KHz, it looks like a regular sinusoid signal.

As I change the frequency, signal dilates and contracts.

What do you think is the reason?

• Is it really distorted ? At 440KHz for 1Msps, you have barely more than 2 samples per period. The beat frequency (larger ondulations) is around 11 samples, so input frequency should be around 1MHz/2 * (10/11)=454kHz. The lower signal is aliased. Commented Jan 1, 2015 at 18:34
• don't you have to sample at least twice the maximum spectral frequency of your siganl to meet Nyquist criteria? Commented Jan 1, 2015 at 18:40
• While it's not the source of your problem (which as has been pointed out is merely one of interpretation), it sounds like you are overclocking your adc by 25% (1.25 MSPS vs 1 MSPS) - it may or may not give good results, and may vary from chip to chip. Commented Jan 1, 2015 at 19:03
• @TEMLIB I didn't get your point about "beat frequency" and what is the 454 KHz signal? would you please elaborate? Commented Jan 1, 2015 at 20:57
• @Siraj : Imagine tuning a guitar, with two strings playing the same note. If the two strings are not perfectly tuned, you will hear a beating modulation whose frequency is the difference between the two strings frequency. Here, we see a similar modulation between the 440kHz sine and the 1Msps/=500kHz sampling frequency. The envelope period is about 11 samples, there is a 1/11 frequency difference. Commented Jan 1, 2015 at 23:31