Biomedical Engineering Reference
In-Depth Information
dat
dat
(2 ^ clocknum) * bit ' accumulate from bit 11 to bit 0
Next clocknum ' next bit
Out prinop, power
notcs
mux
bit7 ' deassert *CS without upsetting bit 7
Acquire
dat
' return 12-bit A/D count
End Function
Signal Conditioning
Input signals rarely
fit exactly within the universal sensor interface's input range of 0 to
2.5 V. Signals of smaller amplitude than the full range waste resolution, while signals out-
side the full range will end up being clipped to the limits of the range. Please note that
input signals presented to the A/D multiplexer exceeding the range 0 to 2.5 V may cause
permanent damage to the circuitry of the interface. Measuring a signal that spans 0 V and
a value larger than 2.5 V is easily accomplished. A resistive voltage divider such as that of
Figure 5.5
a
can scale a large unipolar signal to the desired range. For ease of use, the uni-
versal sensor interface has onboard locations reserved for resistive voltage dividers. The
voltage-divider resistor packs are marked R4 and R5. To use them,
fi
first select the appro-
priate 10-pin single-in-line bussed resistor array for R5. Then select either a DIP resistor
pack or individual
fi
1
4
-W precision resistors to be placed on R4.
Logging data from the sensors on a hyperbaric chamber (a pressurized vessel used to
study the therapeutic use of high atmospheric pressures) provides a good example of how
to select components for R4 and R5. Table 5.2 shows typical ranges for sensors that mon-
itor a small hyperbaric chamber. For this application, the range for analog channels 1 to 4
of the universal sensor interface should be 0 to 5 V, the range for channel 5 should be 0 to
10 V, and the range of channels 6 to 8 should remain at 2.5 V. Assume that an impedance
of 10 k
is appropriate for all channels.
For this example, R5 can be selected to be a 10-pin bused 10 -k
Ω
Ω
resistor pack. A suit-
, single-in-line conformal 10-pin bused resistor net-
work (Digi-Key part 770-101-10K-ND) or similar (e.g., Jameco 24643). 1%
able device is a CTS 770-series 10-k
Ω
1
4
-W resistors
for R4 are selected using the following formula:
1
0
k
0
Ω
k
2.5 V
V
max
R4
1
Ω
As such, 10-k
resistors should be placed between pads pairs [1,16], [2,15], [3,14], and
[4,13] of R4 to yield a range of 0 to 5 V for channels 1 to 4. For channel 5, the formula
requires a 30-k
Ω
Ω
resistor for a range of 0 to 10 V. Since 30 k
Ω
is not a standard value, a
30.1-k
1% resistor should be selected and soldered between pads 5 and 12 of R4. Channels
6 to 8 do not require scaling, and thus pad pairs [6,11], [7,10], and [8,9] should be jumpered.
As shown in Figure 5.5
b
, a small unipolar signal requires just an op-amp-based ampli
Ω
fi
er
to take advantage of the full resolution of the A/D. Signals riding on a median di
ff
erent than
the A/D's midpoint (1.25 V) can be of
set appropriately by using the circuit of Figure 5.5
c
.
Alternatively, a “quick and dirty” way of introducing of
ff
set that can sometimes be used to
enable measurement of bipolar signals is to place a 1.5-V battery in series with the signal
source.
Current measurements can be obtained by using a suitable shunt. For example, as
shown in Figure 5.5
d
, the popular 4- to 20-mA current loop used to convey information
from many industrial instruments and sensors can be converted to a voltage by using a
metal-
ff
Ω
1% resistor shunt across the input terminals of the universal sensor
interface. Since the 4- to 20-mA current will be translated into the range 0.496 to 2.48 V,
some measurement resolution will end up being wasted. If the full 12-bit resolution is
desired, you may use a 154
fi
lm 124
Ω
1% resistor instead and use an op-amp to introduce a
0.616-V o
ff
set to the measurement.
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