Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord
EEE512J2 – Electronic Product Design Page 4 Chapter 2: Fundamentals
Ag-AgCl electrodes are the current standard for use in medical applications related to biophysical
instrumentation and measurements. The gel provides impedance matching at the interface between the
electrode and the surface of the skin, which means that noise effects are reduced, increasing the signal-to-
noise ratio, allowing for a clear signal to be detected. They are non-polarisable, meaning that the differences
in potential that are measured do not depend on current variations in the wires. They are stable, easy to use,
and inexpensive. See reference [W5] for detailed information on the Ag-AgCI electrode.
Figure 2.2b. 3M red dot electrodes cost $14.94 from [W7] Figure 2.2c. 3M red dot resting electrodes cost $9.94 from [W7]
2.3. ECG Amplifier
The heart’s strong pumping action is driven by powerful waves of electrical activity in which the muscle fibres
contract and relax in an orchestrated sequence. These waves cause weak currents to flow in the body,
changing the relative electric potential between different points on the skin by about 1mV. The signals can
change sharply in as little as one fiftieth of a second. So boosting this signal to an easily measured one-volt
level requires an amplifier with a gain of about 1,000 and a frequency response of at least 50 hertz.
At first it appears that an operational amplifier could be used. But two vexing subtleties make most op-amps
unsuitable. First, when two electrodes are placed at widely separated locations on the skin, the epidermis
acts like a crude battery, generating a continuously shifting potential difference that can exceed 2V. The
cardiac signal is small in comparison. Second, the body and the wires in the device make good radio
antennas, which readily pick up the 50Hz hum that emanates from every power cable connected to the
mains supply. This adds a sinusoidal voltage that further swamps the tiny pulse from the heart and because
these oscillations lie so close to the frequency range needed to rack the heart’s action, this unwanted signal
is difficult to filter out.
Both problems generate equal swells of voltage at the amplifier’s two inputs. Unfortunately, op-amps usually
can’t reject these signals. To ensure that this “common-mode” garbage (whose amplitude, can be over 1,000
time greater than the cardiac signal) adds no more than a 1 percent error, a CMRR (Common-Mode
Rejection Ratio) of at least 100,000 to one (100 decibels) is required. This precision eludes most op-amps.
When an application calls for both high gain and a CMRR of 80 dB or greater special devices known as
“instrumentation amplifiers” are required. The AD624AD from Analog Devices (see [W12]) when set to a gain
of 1,000 has a CMRR exceeding 110 dB. It is available from Farnell (order code 102-076) for £22.50. Clearly
at bit expensive, hence another option is the AD620AN available from Farnell (order code 527-567) for
£6.14.
Figure 2.3a shows a simple ECG amplifier using the AD624AD instrumentation amplifier. A gain of 1,000 is
selected by shorting certain pins together as shown. The two-stage RC filter weeds out frequencies higher