Current measurement , calibration unstable on tweet a watt

[randykarr]

I see several post about current problems and calibration issues.
I notice on low power devices (25 watt light bulb) the current is unstable and jumpy, making calibration come and go. Current waveform is not a sinewave.

A 100 watt bulb works good and the current waveform looks like a nice sinewave.

Therefore I conclude the current measurement is a very small voltage before the op amp and therefore very prone to noise pickup. Any noise on the power line or even near by electronic devices will add to this noise.

Does anyone have a drawing of the current sensing circuit on the kill a watt ?
I might be able to suggest an improvement - (like using shielded wire for example)

thanks
randy

[adafruit]

yup its not that great. but it averages out OK