Download 7447 circuit history integrated

CONSTRUCTION
Cornea Testing
For Medical Application
MUDIT AGARWAL
EM TESTED
EM TESTED
TED
EM TES
RAJAT JAIN
This is a small portable device for the objective
measurement of the transparency of corneas stored
in preservative medium, for use by eye banks in
evaluation prior to transplantation.
Methods
The optical system consists of a white light, lenses,
and pinholes that collimate the white light beams
and illuminate the cornea in its preservative
medium, and an optical filter (400-700 nm) that
selects the range of the wavelength of interest. A
sensor detects the light that passes through the
cornea, and the average corneal transparency is
Detector
SelectiveFilter (400-700nm)
Electronic
Circuit
Sample (Cornea)
Iris
L2
L1
White Light
Fig. 1 : Optical System for Measurement of Transparency
of the Preserved Cornea
displayed. In order to obtain only the tissue
transparency, an electronic circuit was built to
detect a baseline input of the preservative medium
prior to the measurement of corneal transparency.
Introduction
The cornea consists of three layers the epithelium,
the stroma, and the endothelium. The epithelium
acts as a mechanical protection. The stroma is
responsible for 95 % of its thickness, and is
composed of bundles of collagen fibers arranged in
crossed layers. The regularity of the distribution of
collagen fibers, the absence of blood vessels, and
the low hydration of the tissue are responsible for its
high transparency. Finally, the endothelium is the
internal lining of the cornea, consisting of a single
cell layer with no capacity for regeneration. It is
particularly important in the active preservation of
corneal dehydration. Scars, vascularization, and
edema alter corneal transparency. When the vision
is decreased significantly by one of these
conditions, a corneal transplantation is generally
indicated. The choice of the corneal tissue to be
used considers the medical history and the blood
analysis of the donor to avoid transmittable
diseases, and also the morphological features of
the three layers of the cornea to exclude any
abnormality of the donated tissue. One of the
obvious characteristics considered in the
examination of the cornea is its overall
transparency, which is usually subjectively analyzed
with a slit lamp. In order to obtain an objective
measurement of this parameter, we present here a
device that provides the percentage of light
transmitted by the cornea stored in preservative
medium. The optical system for measuring the
transparency of the preserved cornea is a relatively
simple set up and is shown in fig.1. It provides the
percentage of total corneal transparency, but does
not determine which corneal layer is responsible for
the loss of transparency. The system is very effective
in objectively measuring the average transparency
of the overall cornea. The optical system consists of
a white light source (15 W, 6 V retinoscope lamp)
that passes through a light collimating optical
system comprised of two converging lenses (D =
25.4 mm, F = 50.0 mm, and F = 60.0 mm) and a
3.0 mm pin hole to limit the light passing through
the central portion of the cornea (corneal button), a
CONSTRUCTION
slot for holding the
cornea, a filter (D =
3pf
25.4 mm, F = 45.0
4
mm, Visible
1M
Achromat,
(400-700
8pf
n m ) , a n d a
5
photodiode detector
7.5mv
Vout The light passes
+
through the cornea
OPT101
stored in preservative
3
medium and the
8
sensor detects the
transmitted light. The
Fig. 2 : Pin Diagram of OPT101
OPT101 is a monolithic
photodiode with on-chip transimpedance
amplifier. Pin Diagram of OPT101 is shown if fig.2.
Output voltage increases linearly with light
intensity. The integrated combination of
photodiode and transimpedance amplifier on a
single chip eliminates the problems commonly
encountered in discrete designs, such as leakage
current errors, noise pick-up, and gain peaking due
to stray capacitance. The 0.23 × 0.23 mm
photodiode is operated in the photoconductive
mode for excellent linearity and low dark current.
Output voltage increases linearly with light
intensity. The amplifier is designed for single or dual
power-supply operation, making it ideal for battery
operated equipment. The integrated combination
of photodiode and transimpedance amplifier on a
single chip eliminates the problems commonly
encountered in discrete designs such as leakage
current errors, noise pick-up, and gain peaking due
to stray capacitance. The 0.09 x 0.09 inch
photodiode is operated in the photoconductive
mode for excellent linearity and low dark current.
The system is calibrated by adjusting an external
knob such that 0 % transmittance corresponds to
the absence of light and 100 % corresponds to
transmittance through the preservative medium
without the cornea. The variation of the light
intensity that reaches the detector induces a change
in current. This signal is sent to an analog-to-digital
converter and then transmitted to a 4-1/2-digit
display, which shows the percentage of light
reaching the detector. For calibration of the system,
the electronic circuit stores the 0 % and 100 %
transmittances of the system determined as
described above. The electronics of the prototype
can be separated into three basic blocks:
1. Power supply: Consists of a 1 A transformer
2
V+
1
(primary 110/220 V and secondary -12/+12 V).
The positive output voltages supply the integrated
circuits of the operational amplifier, the A/D circuit,
and the OPT101 photodiode; the negative input
voltages supply the A/D circuit and the operational
amplifiers. The positive output is also a supply for
the "7805" integrated circuit, which provides 5 V
(stabilized) for obtaining constant brightness of the
light.
2. Control and adjusts: This block is responsible for
manipulating the OPT101 voltage output signal
HA17741
Offset
Null
1
Vin(-)
2
Vin(+)
3
VEE
4
8
NC
-
7
VCC
+
6
Vout
5
Offset
Null
Fig. 3 : Pin Diagram of HA17741
and transmitting the processed signal to the A/D
converter in the visualization block. The electronic
circuit uses HA17741 operational amplifiers, pin
diagram is shown in fig.3.
ICL 7135
ICL7135 precision A/D converter, with its
multiplexed BCD output and digit drivers,
combines dualslope conversion reliability with +1
in 20,000 count accuracy and is ideally suited for
the visual display DVM/DPM market. The 2.0000V
full scale capability, auto-zero, and autopolarity
are combined with true ratiometric operation,
almost ideal differential linearity and true
differential input. All necessary active devices are
contained on a single CMOS lC, with the exception
of display drivers, reference, and a clock.
The ICL7135 brings together an unprecedented
combination of high accuracy, versatility, and true
economy. It features auto-zero to less than 10µV,
zero drift of less than 1µV/oC, input bias current of
10pA (Max), and rollover error of less than one
count. The versatility of multiplexed BCD outputs is
increased by the addition of several pins which
allow it to operate in more sophisticated systems.
CONSTRUCTION
+ 5V +
Q6
Q5
Q4
Q3
Q2
+ 5V +
+1
13 16
12
11
10
9
15
14 6 2
9 9 9 9
8
IC5
1
7
5
R10
R11
Q1
R8
Q7
R9
28
27 26
25
24
23
22
21
20
19 18
17 16
15
8
9
10
12
14
IC4
- 5V
1
2
3
4
5
6
7
11
13
+ + 5V
+
C2
R3
R12
+ 5V
C3
+
+ 5V
D5
C4
R2
R7
R6
10K
R13
IC8
R1
R4
C5
C1
8
2
+ 5V +
1
R5
3
4
-
R14
4
C6
7
3 +
IC1
6
IC2
5
2
-
4
PR1
3
Transformer
+5 V DC
D2
D1
1
230V
A.C.
D3
D4
C7
C8
Ic6
2
+
Fig. 7: Complete Circuit Diagram for Cornea Testing
7
IC3
1
8
+
3
C9
CONSTRUCTION
V-
1
28
UNDERRANGE
REFERENCE
2
27
OVERRANGE
Analog Common
3
26
STROBE
INT OUT
4
25
R/H
AZ IN
5
24
DIGITAL GND
23
POL
22
CLOCK IN
BUFF OUT
6
REF CAP-
7
REF CAP+
8
21
BUSY
IN LO
9
20
(LSD) D1
IN HI
10
19
D2
V+
11
18
D3
(MSD) D5
12
17
D4
(LSB) B1
13
16
(MSB) B8
B2
14
15
B4
ICL7135
T h e s e i n c l u d e S T R O B E , OV E R R A N G E ,
UNDERRANGE, RUN/HOLD and BUSY lines,
making it possible to interface the circuit to a
microprocessor or UART. Pin diagram of ICL7135
is shown in fig. 4 and BCD to seven segment
decoder 7447 is shown in fig.5. and LM311 is
shown in fig. 6. Complete Circuit Diagram is shown
in fig 7.
Max Clock Frequency
The maximum conversion rate of most dual-slope
1
16
VCC
A2
2
15
f
LT
3
14
BI/RBO
4
RBI
5
12
A3
6
11
A0
7
10
GND
8
9
Fig. 5: Pin Diagram of 7447
7447
13
Ground
1
8
V+
Vin(+)
2
+
7
Output
Vin(-)
3
-
6
Balance
Strobe
V-
4
5
Balance
Fig. 6: Pin Diagram of LM311
Fig. 4 : Pin Diagram of ICL 7135
A1
LM311
g
a
b
c
d
e
A/D converters is limited by the frequency response
of the comparator. The comparator in this circuit
follows the integrator ramp with a 3µs delay, and at
a clock frequency of 160kHz (6µs period) half of
the first reference integrate clock period is lost in
delay. This means that the meter reading will
change from 0 to 1 with a 50µV input, 1 to 2 with a
150µV input, 2 to 3 with a 250µV input, etc. This
transition at midpoint is considered desirable by
most users however, if the clock frequency is
increased appreciably above 160kHz, the
instrument will flash "1" on noise peaks even when
the input is shorted. For many dedicated
applications where the input signal is always of one
polarity, the delay of the comparator need not be a
limitation. Since the non-linearity and noise do not
increase substantially with frequency, clock rates of
up to app. 1MHz may be used. For a fixed clock
frequency, the extra count or counts caused by
comparator delay will be constant and can be
subtracted out digitally. The clock frequency may be
extended above 160kHz without this error,
however, by using a low value resistor in series with
the integrating capacitor. The effect of the resistor is
to introduce a small pedestal voltage on to the
integrator output at the beginning of the reference
integrate phase. By careful selection of the ratio
between this resistor and the integrating resistor (a
few tens of ohms in the recommended circuit), the
comparator delay can be compensated and the
maximum clock frequency extended by
approximately a factor of 3. At higher frequencies,
ringing and second order breaks will cause
significant nonlinearities in the first few counts of the
instrument. The minimum clock frequency is
established by leakage on the auto-zero and
reference caps. With most devices, measurement
cycles as long as 10s give no measurable leakage
error.
To be continued in the next issue...