Auto zeroing, chopping or other extreme measures aimed at ultra-low DC offset may not be required here.0.5V (half a volt) or 0.5mV (half of a thousandth of a volt)?
Amplifying 0.5V is no big deal - you could do it with a jellybean op-amp like the LM324 or TL071. Amplifying 0.5mV on the other hand will be tricky, although not impossible. You'll probably need an auto-zeroing low noise op-amp. And if you want any kind of bandwidth (more than 1kHz or so), you'll need a high speed one. These are expensive.
Because the physician who did the initial development work on it was Dr. Emil du Bois-Reymond, who was German, where it is spelled "Elektrokardiogramm". I can only surmise that the abbreviation EKG is used in the English language as a nod to the German physician who originated electrophysiology. [eta] It's more likely because the first machines were made by a German company in Munich; see below.Many medical words about the heart include "cardio", not kardio.
Then why do Europeans call ECG things EKG?
Yes, the 741 opamp circuits would leave a lot to be desired. The 324 opamp circuits might work somewhat, but the CMRR (common mode rejection ratio) would be terrible due to resistor tolerances.There are some very old ECG circuits done by school kids using old 741 or LM324 opamps on the internet. I doubt if any of them worked properly.
Yes, exactly. For the sake of our new hobbyists, that's why a high CMRR is important; you want to reject as much of the signal that's the same on both inputs as possible; and this is very difficult to do with "home-made" instrumentation amplifiers as documented in the Application Note that I referenced earlier.Sometimes the common-mode signal is very high. The DC offset voltage is amplified a lot and can also be high.
This is just what is shown in the example application schematics in the INA118/INA128 datasheets on the bottom of page 11.Many ECG circuits feed negative feedback along the shield to the patient's body to cancel DC offset and common-mode AC and DC.
DC offset can be a problem, as others have pointed out. It's just that since we don't need a zero-frequency response, the effect of DC offset can normally be suppressed by feedback or other filtering methods.Ah, did not realise DC offset was not a problem. In which case an auto-zeroing op-amp is not necessary.
Your idea to use old opamps was tried 40 years ago. I don't think anybody was able to make a circuit that worked.Hi audioguru,
It is for ECG.
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by Jake Hertz