This ultra-short blog post is just a quick follow-up to the measurement amplifier project Viewing Noise with a Measurement Amplifier  which basically describes a X100 or X200 gain amplifier for working with sensors or measuring noise on power rails.

The amplifier is fully described in the earlier blog post, but at that time I had not finished assembling it into the enclosure. The video here shows the completed amplifier in use:



The oscilloscope capture in the photo below shows the results from amplifying a Honeywell SS94A2SS94A2 sensor taped onto the back of an old Swatch Watch (these contain a tiny electromagnet).



The sensor actually works from the front face of the watch too, but I got a larger signal from the back of the watch. Close-up of the sensor:



There is a Texas Instruments Flux-Gate Magnetometer which is more sensitive incidentally. There's a blog post and comments there where jw0752 and I had some fun experimenting with it in various set-ups: Building a Fluxgate Magnetometer based Current Probe - the comments section there goes into a bit more detail/speculation about the magnetic field with a different watch.


The Honeywell sensor operates to around 100 kHz (I have not tested this), whereas the TI sensor has a lower bandwidth (approx. 20 kHz). The TI sensor also contains a reference output, which allows for easier DC measurements. With the Honeywell sensor, there is a 4V offset which the measurement amplifier cannot remove (the offset adjustment on the measurement amplifier is only for fine trimming). Perhaps a second larger offset removal control should be a feature for the next version of the measurement amplifier one day... or even better, some sort of auto-offset removal? I don't know how to design that, but if it were possible then it would be very useful.


With DC offset removal capability, it would be possible to see the remaining magnetic field after the electromagnet has switched off. It is possible to do that with the DRV425 sensor today with the measurement amplifier, by using the reference voltage as the ground signal, and then switching the measurement amplifier to DC mode.


Thanks for reading!