BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 6

Warning! The materials used to form Ag/AgCl electrodes are relatively dangerous.
Do not breathe dust or mist and do not get in eyes, on skin, or on clothing. When work-
ing with these materials, safety goggles must be worn. Contact lenses are not protective
devices. Appropriate eye and face protection must be worn instead of, or in conjunction
with, contact lenses. Wear disposable protective clothing to prevent exposure. Protective
clothing includes lab coat and apron, flame- and chemical-resistant coveralls, gloves, and

boots to prevent skin contact. Follow good hygiene and housekeeping practices when
working with these materials. Do not eat, drink, or smoke while working with them.
Wash hands before eating, drinking, smoking, or applying cosmetics.

fl

If you don't want to fabricate your own electrodes, you can buy all sorts of very stable
Ag/AgCl electrodes from In Vivo Metric. They make them using a very fine grained homo-

geneous mixture of silver and silver chloride powder, which is then compressed and sin-
tered into various con

fi

gurations. Alternatively, Ag/AgCl electrodes are cheap enough that
you may get a few pregelled disposable electrodes free just by asking at the nurse's station
in the emergency department or cardiology service of your local hospital.
Recording gel is available at medical supply stores (also from In Vivo Metric). However,
if you really want a home brew, heat some sodium alginate (pure seaweed, commonly used
to thicken food) and water with low-sodium salt (e.g., Morton Lite Salt) into a thick soup
that when cooled can be applied between the electrodes and skin. Note that there is no guar-
antee that this concoction will be hypoallergenic! A milder paste can be made by dissolv-
ing 0.9 g of pure NaCl in 100 mL of deionized water. Add 2 g of pharmaceutical-grade
Karaya gum and agitate in a magnetic stirrer for 2 hours. Add 0.09 g of methyl paraben and
0.045 g of propyl paraben as preservatives and keep in a clean capped container.

fi

SINGLE-ENDED BIOPOTENTIAL AMPLIFIERS

Most biopotential ampli er-based circuits. As a refresher, the

voltage present at the output of the operational ampli

fi ers are operational-ampli fi

erential
voltage across its inputs. Thus, the noninverting input produces an in-phase output signal,
while the inverting input produces an output signal that is 180

fi er is proportional to the di ff

out of phase with the input.
In the circuit of Figure 1.4, an input signal V in is presented through resistor R in to the
inverting input of an ideal operational ampli fi er. Resistor R f provides feedback from the

ampli er's output to its inverting input. The noninverting input is grounded, and due to the

fact that in an ideal op-amp the setting conditions at one input will e

fi

ectively set the same
conditions at the other input, point A can be treated as it were also grounded. The power
connections have been deleted for the sake of simplicity.
Ideal op-amps have an in

ff

nite input impedance, which implies that the input current
i in is zero. The inverting input will neither sink nor source any current. According to
Kirchhoff

fi

ff 's current law, the total current at junction A must sum to zero. Hence,

i in i f

But by Ohm's law, the currents are de fi ned by

V
R

i
i

n
n

i in

and

V
R

ou
f

t

i f