Biomedical Engineering Reference
In-Depth Information
Bipolar input signals, present at J1-3, and referenced to isolated ground at J1-2, are
introduced into the isolated signal input of IC1. IC1's gain can be set in the range 1 to 10
by changing the value of a resistor connected between JP1-1 and JP1-2 according to
1
00
k
R i (k
gain
1
10.7k
)
To preserve high CMR, this resistor and all connections to it must be guarded with a
shield connected to JP1-3. Best performance is achieved by placing a shorting jumper
between JP1-1 and JP1-2 and operating the circuit at a gain of 10. Leaving JP1 open
results in unity gain.
The bandwidth supported by the 284J is dc to 1 kHz (small signal), dc to 700 Hz (full
power G
1), and dc to 200 Hz (full power G
10). IC1's output is bu
ff
ered by IC2, a
unity-gain bu
er, in order to drive low-impedance loads connected between the module's
output at J3-5 and nonisolated ground at J3-6. Trimmer R3 is used to zero the output o
ff
ff
set
voltage over the gain range. IC1's output is low-pass
fi
filtered to roll off
ff
noise and output
ripple. Cutoff
ff
(
3 dB) of the low-pass
fi
filter is given by
1
2
f 3dB (Hz)
C4(F)
1000
Use of a 1-
frequency of approximately 160 Hz.
The input voltage range for this module is
F capacitor results in a cutoff
ff
5 V di
ff
erential at unity gain. However, this
module can also be used for the direct low-level ampli
fi
cation of biopotential signals with
a low input noise 10
V p-p , medium input impedance of 10 8
, and high CMR (110-dB
inputs to output, 78-dB inputs to guard). Di
erential measurement of biopotential signals
is achieved between J1-3 (noninverting input) and J1-2 (inverting input), while CMR is
optimized by connecting J1-1 to a distant reference electrode. If the module is used as a
biopotential ampli
ff
er, the leads to the electrodes should be low-loss low-capacitance coax-
ial cables, whose shields are connected to J1-1. This module should be operated using a
symmetric
fi
15 V regulated power supply (J3-1
15 V, J3-2
nonisolated ground,
J3-3
15 V). Dual
8.5 V dc at 5 mA of isolated power are provided at J2. These lines
may be used to power
fl
floating input circuitry such as biopotential signal bu
ff
er preampli
fi
ers
or instrumentation ampli
fi
ers.
THREE-PORT ISOLATION
Most isolation ampli
er ICs on the market that contain an internal dc/dc converter to
power the isolated side of the ampli
fi
fi
er (as well as support circuitry) are labeled as input
or output isolation ampli
ers. This refers to the direction in which power is sent across the
isolation barrier. An input isolation ampli
fi
fi
er thus powers the isolated input side of the
ampli
er through an internal isolating dc/dc converter while operating its output side from
the same source that powers the dc/dc. Conversely, an output isolation ampli
fi
fi
er uses the
power directly to supply the input side of the ampli
er and the dc/dc converter, while the
output of the dc/dc is used to power the isolated output stage of the ampli
fi
fi
er. Since most
isolation ampli
fi
ers with internal dc/dc converters have additional power capacity, input
isolation ampli
ers typically make extra power available on the isolated input side for driv-
ing external signal conditioners or preampli
fi
fi
ers, while an output isolation ampli
fi
er makes
extra power available on the isolated output side for driving external loads.
Although most applications in medical devices require input isolation ampli
fi
ers, there
are cases in which a signal generated by the device is required to be sent to a
floating applied
part. For example, a stimulus waveform may be generated using a D/A on a nonisolated side
of the device and delivered to a stimulation circuit using an output isolation ampli
fl
fi
er. An
interesting isolation ampli
fi
er that can ful
fi
ll both roles is Analog Devices' AD210. This
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