# Baby Orangutan B-328 - Question about using AC signal input

So I have a coil that is getting an induced voltage of 1V (peak to peak) and the frequency is 50KHz. Will the inputs on the Baby-O be able to measure the induced voltage from the coil? Or can the inputs only handle DC voltage?

Hello.

1 V is going to be below the digital read-high threshold of the Baby O when Vcc is 5 V, and a 50 kHz signal is going to be too fast to read well with the AVR’s ADC. You could use the ADC to characterize it if you could convert it to something that varies more slowly (the ADC can sample at around 10 kHz). The inputs do not need to be DC voltages, but you should make sure you avoid applying negative voltages or voltages above Vcc+0.5V.

- Ben

Thanks Ben! So I won’t be able to accurately read the voltage from the coil unless its frequency is at or below 10kHz?

It’s not quite as simple as that. Converting an analog voltage to a digital reading on the AVR occurs as a two-step process: sample & hold (~10 us) and successive approximation of that held voltage (~90 us). One thing to consider is that if your signal changes significantly over the 10 us sampling time, your analog reading will not be very accurate and will instead likely be some sort of average voltage experienced during that sampling time. Another thing to consider is how well you need to characterize your input waveform. Are you familiar with the Nyquist frequency? If you are sampling at 10 kHz, the Nyquist frequency is 5 kHz. Trying to sample a signal with frequencies higher than the Nyquist frequency will result in aliasing issues and interfere with reconstruction of the signal.

You should be able to use the ADC to tell that something is going on, but I’m not sure if that will be sufficient or if, rather, you want to reconstruct the waveform to measure things like the frequency and peak-to-peak voltage.

- Ben

Thanks again Ben. That really did help. So if I’m understanding it correctly, the highest frequency I would theoretically be able reconstruct or characterize is 5 kHz. In order to characterize the 50 kHz signal I have, I would need an ADC with a sampling rate of 100 kHz (at minimum) which would put the Nyquist frequency at 50 kHz.

Do you think it would work If I connected the 50 kHz signal to a full wave bridge rectifier and added a smoothing capacitor so that it will be positive non-varying (well not much anyways) DC signal?

–Thanks!

Probably. What exactly are you trying to detect? Are you looking for an “active vs non-active” kind of indicator, or are you looking for something fancier, like detecting subtle changes in peak-to-peak voltage? Depending on what you’re trying to measure, I think the smoothed, rectified signal could be fine, though the smoothing might add some latency.

- Ben