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Old 14th November 2007, 11:01 AM   #31
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I'll certainly investigate the low pass filter - will it work without the notch filter?
Yes.

The low pass filter will get rid of signals which have a frequency greater than the filter's cut off frequency. The notch filter will try and get rid of signals with a particular frequency, 50Hz in this case.

50Hz noise is very common in instrumentation due to all the mains power cable everywhere.
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Old 14th November 2007, 12:08 PM   #32
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Yep. And looking at your trace, 50Hz noise is your big issue atm
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Old 14th November 2007, 1:13 PM   #33
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Yep. And looking at your trace, 50Hz noise is your big issue atm
Fantastic. Well i'll give it a go. I assume i connect the LHS of your circuit diagram to the ecg, and the RHS to the pc? Also, does software exist that will remove that noise for me so i can see if it cleans up the trace in advance? I remember that an old oscope i had years ago would do that - but i don't have one anymore.
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Old 14th November 2007, 1:14 PM   #34
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Yes.

The low pass filter will get rid of signals which have a frequency greater than the filter's cut off frequency. The notch filter will try and get rid of signals with a particular frequency, 50Hz in this case.

50Hz noise is very common in instrumentation due to all the mains power cable everywhere.
Great. I'll go with the 4opamp IC and build both at the same time, pending the results of software mitigation.
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Old 14th November 2007, 1:18 PM   #35
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Test them one at a time though, problems with one will affect the other.
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Old 14th November 2007, 5:04 PM   #36
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Test them one at a time though, problems with one will affect the other.
Thanks. I'll buy the stuff tomorrow and update.
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Old 16th November 2007, 1:32 PM   #37
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Ok, These are the components (DSE cat numbers) that i'll go and buy. Hopefully this will work
Questions:
Does the ground get connected to normal case ground? And also, i assume that the input comes from the leg 7 'tip' output of opamp 3 of the ECG, and goes in next to C1 of the notch filter. The output of the notch filter then goes in next to R1 of the low pass filter. Then, the output of the pass filter goes to the tip of the audio cable.

Quantity/Component/Can be made from

2 X 330n 6 X R2400 (.01uf) and 6 x R2360 (.1uf)
2X 193k 2 X R0628 (150k) and 2 X R0615 (43k)
2X 9.65k 2 X R0602 (15k) and 2 X R0608 (27k)
2X 10k 2 X R0598
1 X TL074CP 1 X Z6034 and IC Holder
1 X Board

1 X 2.79k R0583 (2k4 res) and R0564 (390R)
1 X 18.1k R0601 (13k) and R0591 (5k1 res)
1 X 68n R3023 (68n)
1 X 160n R2629 (150n) and R2055 (10n)
1 X 2.57k R0585 (3k0) and R0604 (18k)
1 X 16.5k 2 X R0612 (33k)
1 X 43n R2055 (10n) and and R2325 (33n)
1 X 240n 2 X R3026 (120n)
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Old 16th November 2007, 1:41 PM   #38
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Do DSE still sell components, or even have stock of them?
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Old 16th November 2007, 1:47 PM   #39
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Do DSE still sell components, or even have stock of them?
My local does - it's very much a self service thing though. The staff don't know what the components are, but i seem to be the only one who purchases them - hence i know where they all are
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Old 16th November 2007, 7:18 PM   #40
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My local does - it's very much a self service thing though. The staff don't know what the components are, but i seem to be the only one who purchases them - hence i know where they all are
I don't think DSE stores will be restocking components after they run out as its not in line with their plan to become a Hardly Norman type store. Additionally the staff won't know what component has run out let alone what to re-order.
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Old 16th November 2007, 7:28 PM   #41
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The capacitor types are very important. You're using 3x 3kv caps in parallel with 3xceramics, this will not work out for the best, and will take up a huge amount of board space. DSE sells a 330n as R3031, and if they're out of that you can use an R3027 in parallel with an R3028. Stick to the MKTs where possible - monos would be better, but hard to get any decent range of values. Use the R3027 in parallel with the R3012 for the 160n. R3019 in parallel with R3012 for 43n. Otherwise looks okay, assuming those resistors are all 1/2W 1% tolerance.
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Old 16th November 2007, 7:53 PM   #42
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I've bought all polyfilm except for the 160n, where i bought R2629 and R2055 - before seeing your reply.
Is this likely to stuff up the circuit?

EDIT: On further testing, the R2629 tests out at 162nf - it should be ok just to use it by itself right?

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Old 16th November 2007, 8:03 PM   #43
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Can't hurt, just might not work
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Old 17th November 2007, 2:32 PM   #44
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Right, well i've built it.
I get no output from the low pass filter, but if i draw an output from the notch filter, i get more - not less - noise than before.
I've checked & rechecked the circuit, and am out of ideas. I can't see any dry solder joints, incorrect placements or anything else

Any ideas for troubleshooting?
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Old 18th November 2007, 9:14 PM   #45
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Can you give us a photo of the board front and back? It might also be worthwhile measuring the exact values of the components you used, though you'd have to take them off the board.
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