Part Number: ADS1115EVM-PDK
Other Parts Discussed in Thread: ADS1115
Hello,
I purchased the ADS1115 EVM with the PAMB baseboard. I altered the ADS1115 AIN0 input to be a short to the input terminal (J5). I created a breadboard with a 33Hz LPF Salley-key filter. I then feed the breadboard with a 3Hz signal, which then feed the ADS1115 EVM. OK, that's my setup.
Summary of issue:
1) I first set the GUI to take 128 samples at 128SPS. This should give me three periods of the input (3Hz) signal. This is OK. So time domain is verified.
2) I then change the sampling to 65536 (a power-of-2 sample width, so there's no wraparound with the FFT). However, the GUI FFT shows a peak at 23Hz. How can this be possible? At 128 SPS sampling and a 3 Hz input signal, the images are at 3Hz, 125Hz, etc. And since my input LPF is like 60 dB down at 125Hz, I don't understand how I can be seeing an image frequency.
3) I think increase the sampling rate to 860 sampling rate, and then I do indeed see the 3 Hz tone in the GUI FFT tab.
Screenshots (sorry they are so small -- this editor shrinks the size -- quite "unhandy," frankly). I can re-send the screenshots via email though. They are actually a readable size.
A) 3Hz in, 65536 samples, 128 SPS:
B) 3 Hz in, 65536 samples, 860 SPS:
So in summary, why does my 128 SPS capture incorrectly show an image at 23Hz?
Regards,
Mark
