Changing PCB trace width once signal-to-noise ratio is high
I'm designing a PCB to filter and amplify a differential body signal with amplitude 5 mV up to 5 V. The first part of the circuit consists of a first order highpass filter to remove any DC components from the signal. Then it moves into an instrumentation amplifier for amplification of the differential signal and attenuation of the common mode voltage. The instrumentation amplifier has a common-mode rejection ratio of 120 dB. The first stage of my circuit is seen below.
The signal-to-noise ratio before the instrumentation amplifier is $ \frac{5 \: \text{mV}}{5 \: \text{V}} = -60 \: \text{dB}$. After the amplifier, the SNR is 60 dB.
I want to maintain the signal integrity of my differential signal. To do this, there should be as little voltage drop of my signal across any impedances on the path to the amplifier as possible. A $0.5 \: \text{mV}$ of "lost signal" is not acceptable. For that reason, I chose to route this first stage with a trace width of 1 mm. According to KiCad's calculator tools, a trace of length 5 mm with this thickness has $2.5 \: \text{m}\Omega$ resistance, which I suppose is good enough.
My question is: After the amplification stage, the signal now has a 5 V amplitude, and the SNR is much greater. To what trace width am I allowed to go down to? Routing with 1 mm trace width on the entire board is not doable. Would a trace width of 0.2 mm be appropriate? Is there anything I should be aware of when changing the trace width? The board is entirely analog. Not digital circuitry.
1 answer
Trace width is a silly thing to worry about at such low currents and high impedances.
You've got 1 MΩ resistances in the signal path, and the opamp input is presumably much higher impedance than that. You didn't say what the upper limit of your signal frequency is, but lets say 1 kHz since it's from the body. At 1 kHz, the 1 µF caps in your signal path have an impedance magnitude of 160 Ω. A few mΩ are not relevant.
You should really be focusing on dealing with the common mode signal. That's where your noise will come from. One technique for electrically floating bodies is to drive a spot on the body far from the measurement to try to null out the common mode signal. For heart signals, this is often done by driving the right leg in a feedback loop inversely to the common mode voltage. The reason for the right leg is that it's far away from the heart, so signals induced there will be common mode at the measurement site.
Another possible technique is to float the entire signal front end circuit on the common mode voltage. Then have it pass on digital measurement via opto-couplers to the rest of the device. That might also help with safety certification.
0 comment threads
1 comment thread