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ECG1 • 8

call the differential change a normal mode. A well designed differential

amplifier has a large common mode rejection level (CMRR), or the ratio of

response for the normal mode to the response of the common mode signal of

the same level.

The differential circuit is used extensively in modern electronics circuits. There

is a great amount of common noise present on most long runs of wire

(telephone transmission, theater audio, computer networking, etc.), and the

differential amplifier can be used to recover the original signal provided that

the common mode signals are not too large.

Take a look at the circuit schematic for the ECG1 kit and follow the two inputs.

Notice first that the input voltages are capacitively coupled to the circuit, with

extremely high resistances following the capacitors. [Note that at no time

should you EVER attempt to run a current through the skin in the vicinity of

your heart]. Notice that they are both tied to differential inputs of the LM324 op

amp. Resistor R5 acts as a gain control for the differential voltage with a range

of about 100 to almost 10,000, ideally suited for the bioelectric potentials we

are attempting to measure.

Since the action of the nerves in the heart muscle are relatively slow (on the

order of milliseconds), we can incorporate several “low pass” filters to

eliminate any high frequency noise that may be present on our signal as well.

You’ll notice a few R-C combinations (R8 and C7, R9 and C8, R10 and C9)

that effectively filter off any high frequency noise that is present on the signal

leaving our “thump-thump” heartbeat signal intact for analysis.

Our output signal is coupled to the indicator light (led D2) that is triggered

whenever transistor Q1 is turned on by a positive peak. We have also

included an audio amplifier (U2) to provide gain for a small speaker or

headphone output. The monitor output provides a convenient connection for

an oscilloscope to view the EKG pattern directly.

Our circuit runs directly using an alkaline 9V battery, keeping us away from

any nasty ground loops or potential differences that could affect the

performance of the kit. This also limits the possibility of bodily harm through

the improper use of an external power supply.

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Ramsey Electronics MR6 Bedienerhandbuch Seite 8