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4-4
Model 340 Telemetry System
Revision A
2006920-001
Theory of Operation: Transmitter Board (No. 2003708-001)
UA
The UA circuitry on the Transmitter Board consists of an
instrumentation amplifier, a secondary amplifier stage, a voltage-
controlled oscillator (VCO), and an active low-pass filter. The
instrumentation amplifier is used both to amplify and to convert the
differential output of a TOCO bridge or IUP device to single ended. This
stage is set for a gain of 100. The second amplifier is used to scale and
filter the signals from the instrumentation amplifier for the VCO input.
This stage has a gain of 13.3 and is offset to accommodate the maximum
usable range of the UA transducers. Feedback components on this stage
are set for a single-pole roll off at 1.6 Hz. The varying voltage levels from
the output of the UA amplifier stages are converted to proportional
changes in frequency in the VCO. This circuit creates a FM subcarrier
with a center frequency of 1.75 kHz and a deviation of ±250 Hz. The
square wave output of the VCO is then filtered by a unity-gain, third-
order, low-pass filter with a breakpoint of 2 kHz. The filter output is used
as part of the composite modulation supplied to the transmitter module.
Power for the TOCO or IUP bridge is provided from a voltage reference
and current amplifier.
ECG
The ECG circuitry is composed of an instrumentation amplifier, a fixed-
gain amplifier, a gain selectable amplifier, a common mode amplifier, a
VCO, and an active low-pass filter. The differential patient signals from
the ECG electrodes are first amplified in the instrumentation stage
which is set to a gain of 10. Additionally, the common mode voltage
present across the gain set resistor on this amplifier is used to create the
right leg drive signal. This common-mode voltage is first buffered in a
unity-gain stage, then inverted in an integrator circuit which amplifies
the common-mode signal. This voltage is fed back to the patient through
the leadwires to help cancel 60 Hz noise. The single-ended output of the
instrumentation stage is then AC coupled to the next amplifier, creating
a single-order, high-pass function with a breakpoint of 6 Hz. This
amplifier operates with a fixed gain of 101 and is rolled off at 80 Hz. The
output of this amplifier is then AC coupled into the finial amplifier stage,
creating an additional high-pass breakpoint at 4 Hz. This is the gain-
selectable amplifier. The gain on this stage is configured by the enabled
mode (FECG or MECG). In the FECG mode, the gain is set at 2.5. In the
MECG mode, the gain is set at 1.25. Feedback components in both modes
set the low-pass roll-off at approximately 42 Hz. The output of this stage
then drives the VCO which has a center frequency of 2.7 kHz and a
deviation of ±250 Hz. This circuit is configured to operate like the VCO
used in the UA channel. The square wave output of the VCO is then
filtered in a unity-gain, second-order, low-pass filter with a breakpoint of
3 kHz. The filter output is used as part of the composite modulation
supplied to the transmitter module.
Содержание Corometrics 340
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