Gas station without pumps

2014 January 31

Wire loop vs. twisted pair try 1

Filed under: Circuits course — gasstationwithoutpumps @ 21:54
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I was thinking about things we might do in the first lab day for the Applied Circuits course next quarter, now that I have 2 lab sessions a week.  The first, obvious thing is to unpack the lab kits and identify the different components, labeling the capacitor zip locks with values of the contained capacitors, and learning to use the wire strippers, the power supplies, and the voltmeters.

I’m thinking that I should have the students make a 3′ red-black twisted pair and pack it with their parts, so that they don’t try getting new power wires every week.

One experiment I was considering having them do is to look at the AC signal from a large loop of wire and from twisted pair, to see the difference in noise pickup.  I tried doing this at home today, with rather disappointing results.  The large loop did not pick up more 60Hz noise than the twisted pair when viewed on either my Kikusui oscilloscope or by Bitscope USB oscilloscope.  In fact I couldn’t see any noise with either one.

I tried adding my 1500-gain EKG amplifier as a front end, and then I could see about 60μV noise, but that did not change whether I used a long loop, a twisted pair, or a very short loop between the instrumentation amp inputs.

I first tried looking at how much noise there was from the Bitscope with the DP01 active probe using AC coupling:

Looking at shorted inputs  for the AC-coupled DP01 active probe in its high gain mode.  This is 2mV/division, 100µs/division, "macro" mode.  The noise is a couple of mV.

Looking at shorted inputs for the AC-coupled DP01 active probe in its high gain mode. This is 2mV/division, 100µs/division, “macro” mode. The noise is a couple of mV, about the same as the noise level without the DP01, but the resolution is much better with the DP01.  The high-speed trace here is to show the component of the noise that is around 30.8kHz, which is about as big as the 60Hz component.  The 30.8kHz noise is most likely from the USB power supply into the Bitscope.  The Bitscope does not appear to have as good power-supply noise rejection as one might want.

I then hooked up the EKG amplifier board (which should have a gain around 1508) using a separate power supply, and looked at the noise on the Vref signal (which is just buffered from a voltage divider on the power supply).

At 2mV per division and 10ms per division, we can see a little 60Hz noise added to the high-frequency noise of the Bitscope, but the noise is still only 2–3mV, which would be an input-referenced noise of about 2µV given the amplifier gain around 1500.

At 2mV per division and 10ms per division, we can see a little 60Hz noise added to the high-frequency noise of the Bitscope, but the noise is still only 2–3mV, which would be an input-referenced noise of about 2µV given the amplifier gain around 1500.

The noise on Vref is not much more than noise inherent to the Bitscope and is very similar to the noise I see looking just at the output of the power supply without the EKG attached.
Next I looked at the output of the EKG, with both inputs shorted to Vref.

This is now 40mv/division and 10ms/division looking at the output of the EKG amplifer with both inputs shorted to Vref.  The output noise is around 90mV, so the input-referenced noise is around 60µV.

This is now 40mv/division and 10ms/division looking at the output of the EKG amplifer with both inputs shorted to Vref. The output noise is around 90mV, so the input-referenced noise is around 60µV.

Now we see a signal that is not just Bitscope or power supply noise, and have a noise floor for looking at signals at the input of the EKG amplifier.

Unfortunately, replacing the short with a large loop of wire does not appreciably change the signal, but if I connect the two EKG inputs to Vref via separate 2.2MΩ resistors, I get a large output:

200mV/division, 10msec/division.  Inputs of EKG amplifier separately connected to Vref via 2.2MΩ resistors.  This signal is about 1.04V peak-to-peak (so the input is about 690µV.

200mV/division, 10msec/division. Inputs of EKG amplifier separately connected to Vref via 2.2MΩ resistors. This signal is about 1.04V peak-to-peak (so the input is about 690µV).

The output can be changed substantially by putting my hand near the resistors—the 60Hz noise appears to be capacitively coupled into the amplifier. I can reduce the peak-to-peak voltage to about 500mV  (that is around 300µV at the input) and make it have a larger 120Hz that is almost 10dB larger than the 60Hz component, , just by moving my finger around near the resistors. I can also raise the signal until the EKG amplifier is swinging rail-to-rail (at least 3mV at the input).

So I don’t have a demonstration circuit for electromagnetic pickup here, but I do have one for capacitive coupling.  To detect currents induced in a loop, I probably need a transimpedance amplifier to detect small currents, rather than an instrumentation amplifier to detect small voltages .  I’ll leave that for a separate post.

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