Biomedical Engineering Reference
In-Depth Information
for frequencies under 1 kHz. The output of the AAMI load is ampli
fi
ed with a gain of 10
by instrumentation ampli
fi
er IC1. The RMS value of the ampli
fi
ed signal is then computed
1
2
-digit DVM module. Note
that some modules need to be supplied from a power supply that
by IC2, a true-RMS-to-dc converter IC, and displayed by a 3
floats in relation to the
reference potential of the voltage to be measured. If necessary, use the circuit shown in
the insert of Figure 3.25 to supply
fl
floating power to the DVM module.
R1 must be set to produce a reading of zero with no current
fl
flowing through the AAMI
load. Potentiometers R7 and R9 should be set to produce a reading of 1999 counts on the
DVM for a 1.999-mA dc current through the AAMI load. In this same circuit, notice that
an
RC
low-pass
fl
filter network can be interposed between the AAMI load and the voltage
amplifying circuit. This serves to allow the measurement of the dc component of a patient
auxiliary current.
The circuit presented in Figure 3.26 makes it possible easily to con
fi
gure a measurement
setup to conduct the various leakage and auxiliary current tests required by the safety stan-
dards. In this circuit, switch SW3 selects the connection of the AAMI load to measure either
a patient current, the current on the protective ground pin, or the enclosure leakage current.
Switch SW4 is used to select the source of a patient current among the di
fi
erent possible
combinations of patient connections. Also through this circuit, power line-level voltages
can be applied to the patient connections by way of relay K1. The
ff
gure shows the con-
nection distribution suitable for testing a 12-lead ECG; however, the connections of any
other applied part can be substituted. The connectors used for establishing connection to the
leads of an applied part must be selected carefully so as not to contribute themselves to the
measured leakage or auxiliary currents. A good choice are Ohmic Instruments' 301PB ECG
binding posts, which can accommodate either the standard snap-ons or the pin tips used for
establishing connection to ECG patient electrodes.
The construction of the AAMI load, its power supply, and its switching network
deserves special attention, since stray coupling within the circuit can render measure-
ments useless. This is because wiring and/or PCB traces that are part of the AAMI load
and the voltmeter may be coupled capacitively to ground or the power line. Some fore-
thought in the layout will avoid a lot of aggravation later when you try to calibrate
the instrument. The circuit of Figure 3.27 controls the power supplied through J13 to the
device under test. SPDT relays with contacts rated for 125 V ac at 20 A are used to reverse
the power line polarity at J13 as well as to cause open-ground and open-neutral single-
fault conditions.
Three neon lamps are used to indicate that the measurement instrument is powered as
well as to verify that the ac plug from which power is obtained is wired correctly. Normal
and fault conditions are shown in Table 3.3. In addition, the
fi
figure shows the 115-V ac iso-
lation transformers and the voltage-divider network formed by R17-R19, which are used
to generate 125 V ac for measuring the patient leakage current with applied power line
voltage. Notice that R20 limits the current that can be delivered by this circuit to approxi-
mately 1 mA.
Finally, Figure 3.28 presents the dc power supply section of the measurement instru-
ment. Linear regulators are used to generate the various power levels required by the
microammeter. In addition, R24-R26 are used to derive an ac signal to balance the meas-
urement circuit during application of 125 V ac to the patient connections. When measuring
currents, place the equipment under test on a nonconductive bench, away from grounded
metal surfaces. In addition, make sure that all external parts of an applied part, including
patient cords, are placed on a dielectric insulating stand (e.g., a polystyrene box) approxi-
mately 1 m above a grounded metal surface.
Finally, a word of caution: Be very careful when using this instrument! Remember that
unrestricted power line voltages are used to power the device under test, making the risk
of electrocution or
fi
fi
fire very real. Moreover, single-fault conditions forced during testing
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