Where to start – wow, different board layout, no top side shown, no new cad layout shown, looks like some parts connect to pads with no traces (unless it’s now double-sided?), unused opamp is open (short 6 and 7, and ground 5, as pointed out in previous thread), your schematic improved dramatically in the last thread but is back to contorted version again here… It is difficult to help when progress goes backwards.
Signal gone from gui when disconnected? What does gui signal look like? Is it overdriven? Some soundcard thingy? Does it have level controls? Do you have a scope? Maybe opamp is oscillating. Once again, add that 100pF across the feedback resistor R3 to quiet it down. I realize that you are putting a bunch of time into this, and folks want to help but have limited time – perhaps peruse some resources such as: opamps
Don’t know where the Power is coming from - sounds like you’re using a control board (Arduino, ESP…etc) and did Not pay attention to it’s Requirements for External Powering (generally can’t mix external and internal-USB). They always have different Pins for USB and Vcc External.
CLARIFICATION: I do see you’re bringing Power+/- in via connector - and, your comment about no power/signal when unplugging from Computer suggests to me you’re using a controller/USB/etc. That’s why I mentioned it. Naturally, with correct power hookup, gizmo should run as Stand-Alone…
Hi @teletypeguy, good to hear the board is better now.
I checked on Kicad and some pads that should be connected were disconnected, perhaps that´s why it didn´t work. I´ll try the new layout now and see because I know it worked on a breadboard. The filter capacitor was disconnected so it would not filter giving me a very noisy and unexpected signal. Also some of the power connections were disconnected but I still managed to power it with a battery or an arduino.
What about ground planes how should I use and design them? How do I know if they are needed?
If you don’t have a scope you won’t be able to tell if the opamp is oscillating. It may not be, but it is always good practice to add a cap across (in parallel with) the opamp feedback resistor – this provides a first-order high-freq rolloff. It is a simple calculation: the -3dB point is at 1 / (2 x pi x R x C). I explained this in your other thread so peek at that. If you think you can swap a 100uF in place of a 100pF then you need to learn some electronics fundamentals. Sorry, but this is elementary stuff. Also, your un-connected opamp could be oscillating (though is is likely just sitting at a rail. Dealing with an unused section has been mentioned more than once.
As for the ground plane, it it easy. Just make a rectangle polygon inside your edge cut (I come inside 0.5mm) and define it to be a ground net. Then go to Edit/Fill-All-Zones and badaboom-badabing it is just like magic. You can even be sloppy and make the polygon outside the edge cuts and the outer perimeter will get adjusted by the board margin.
I see, so I´d need to calculate the C based on the f (60Hz) and R3, right?
I just didn´t connect the unused pins as mentioned in the other topic because I have a doubt about ground.
Ground would be BATT-, correct? If yes I can connect it correctly.
When creating a ground plane should I label it BATT- or GND?
I´ll take a look into ground plane tutorials, should it isolate my board better from noises?
I’m also missing a decoupling capacitor over the opamp, and a “decent” passivation for the unused opamp. Just leaving the connections of an opamp open is not a good solution. An opamp can do weird things when it’s not operated within it’s normal operating range.
And I just gotta say, to everyone new to electronics, get a scope. If you don’t have a scope you are working blind on a circuit. Maybe that should be the first question to a new post – if you don’t have a scope to verify power, verify dc bias points, see if the signals look correct… you are wasting your time with any electronic circuit. Spend a few hundred bucks on an entry-level siglent or rigol, or even a cheesy hantek.
You have to not understand what pico, nano and micro means.
If you were understanding you would say only one of those values.
100uF is milion times bigger then 100pF.
If some circuit with 100pF will allow you to see short pulse once per second the same circuit with 100uF will allow you to see milion times longer pulse once per milion seconds.
Not exactly.
Without scope - oscillating opamp takes more current than not oscillating one.
With scope - it is possible that when you connect scope to opamp its state (oscillating/not oscillating) changes.
In 1981/82 having no scope I designed and build chronocomparator. It is the device that when you put (only put) on its table electronic watch it shows its speed with a resolution of about 0.1s per day. My device was about 15x15x30cm in size. I got for it my first serious money I earned on electronics (2.5 times my father month salary).
Then I designed and build the oscilloscope because I really needed to have an oscilloscope and it was not possible for me to buy one (I still have it).
I was so proud of it that I send an article to electronic magazine and they published it.
I doubt that this circuit based on a simple op amp (LM741) can work properly to measure ECG signals.
ECG voltages measured across the body are very small. These low voltages usually need instrumentation amplifiers to obtain a correct signal conditioning.
I know nothing about their level but may be they are not so small as high impedance. CHEST1 input impedance is about 10M. Signal is multiplied around 50 times and then it goes to soundcard which is quite a sensitive input. What is amplified is difference between CHEST1 and CHEST2 (and differential impedance is 10M). Instrumentation amplifier gives high impedance at both inputs. Here we have it only at one input, but may be it is enough. RIGHT_LEG looks like reference GND probably making visible at output noise smaller.
OpAms are powered from +/- voltages against GND. If you have one supply you make ‘virtual’ GND by making a mid point between supplies. Here RIGHT_LEG is 'virtual’GND.
Ground plane will not help you a lot here.
GND plane works against waves with wave length in the range of GND plane size. 1GHz has wave length of 30cm. 30cm GND plane will have very small noise eliminating effect starting from may be 1/100 of 1GHz so from 10MHz and then the higher frequency the higher effect. It can filter out radio frequency disturbances, but your circuit should have so limited bandwidth that it should not see such frequencies at all.
For 1Hz wave length is 7.5 times the length of the earth’s equator.
I’m sure of nothing, but LEGS are used as reference in these measurements. If we have ‘virtual’ GND connected with LEG=reference than may be we don’t have to worry too much about CMRR specially as output signal shape probably needs not to be compared with exact shapes known by doctors. Here we just only wants to have something like signal, I think.
Yes and no. I design eeg circuitry with signals down around 1 uV (ecg is more lax at around 1mV). My signals are digitized and streamed and when I crunch an fft I see my signal ground floor at more than 100 dB down, (which is under a microvolt) and powerline noise is noticeable above that. Yes, much of that is due to common-mode on signal cables and inamp or diff-adc mitigates a lot, but the pcb circuitry is not immune to LF noise pick up. I can definitely see the difference in 60Hz residual as my board revs change with improving ground planes. BIG difference between a rev only having planes on inner layers and a rev that adds flood fill-in on top/bottom as well, with stitching vias. I put ground fill on EVERY board. Yes, wavelengths are of interest at emi freqs, but noise pickup on low-level analog also has capacitive/inductive coupling mechanisms. I am no expert on the physics, but I know that generous ground fill is always beneficial.
Also typical in ecg is right-leg-drive, where the powerline average from signal channels is fed out-of-phase into the leg body ground. It is the same concept as noise-cancelling headphones.
There is also a problem of demodulating RF at any semiconductor even in low frequency circuit. Good GND plane (and shielding) minimizing RF picked up by circuit minimizes effect of those demodulation. But here we have simple DIY circuit and not the problem with signal ground floor but the problem that it worked in prototype and it doesn’t work in next prototype. GND plane has little or no influence on it.
If ecg are 1mV then multiplied by 50 it is 50mV. It should be enough to show by soundcard (I suppose input is about 1Vpp so 1/20 of full scale looks being enough).
