Biomedical Engineering Reference
In-Depth Information
current causes amplifier A
1
to charge capacitors
C
1
-
C
3
at a proportional rate. (2) The sam-
ple switch S
1
is then opened, allowing the output voltage of IVC102 to stabilize. After
approximately 5
s, the readout switch S
3
is closed to transfer the charge from
C
1
-
C
3
to the
sample hold capacitor
C
S
. (3) Following successful signal readout, S
3
is reopened and S
2
is
toggled for 10
s to completely discharge
C
1
-
C
3
and allow A
1
to stabilize. (4) A preintegra-
tion hold period
e
is required to ensure that S
2
is fully open before S
1
is closed, thus pre-
venting any input signal loss. Repeating this process provides continuous sampling,
generating a stepwise approximation of the true signal. Provided that the integrator does
not saturate, sampled voltages are proportional to the integrated intensity over the inte-
gration period. By adjusting the timing parameters or feedback capacitance, measurement
sensitivity can be adjusted over a wide range.
The bR photoreceptor has a very high source resistance, which is in the range of
10
10
. The source capacitance value is around 50 pF, which plays a key role in oper-
ation of the switched integrator. This front-end design provides continuous integration of
the input signal. During the short period
b
that S
1
is opened, any signal current produced
by the photoreceptor charges the source capacitance
C
m
and is transferred to
C
1
-
C
3
imme-
diately after S
1
is closed. As a result, no charge produced by the sensor is lost, and the
effective integration time is equivalent to the sampling period (
a
10
12
b
).
S
3
are compatible with standard complementary metal-
oxide-semiconductor (CMOS) or TTL logic signals. A PIC12F675 microcontroller is used to
control the digital timing functions. Special attention must be paid when these logic sig-
nals are routed to their respective input pins on the circuit board, since significant noise
can be introduced by capacitive coupling between the logic signal traces and the sensitive
input pins of IVC102.
The digital timing inputs to S
1
17.3.3
Overall Circuit Architecture
17.3.3.1 Overview of Array Circuit Architecture
In most imaging applications, photosensors are arranged in either 1D or 2D arrays. The
application may require the photosensors to be combined with processing electronics to
achieve the desired functionality. For visible and near infrared detection, the charge cou-
ple device (CCD) imager and the CMOS imager are two of the most common silicon arrays
(70). The general circuit architecture of these arrays can be divided into two stages: the
pixel-level and the array-level processes. At the pixel level, two elementary configurations
include passive pixel sensors (PPS) and active pixel sensors (APS) (71). For PPS, each pixel
contains a photosensor and a sampling switch to synchronize output. To improve the
image quality, more processing components, such as active buffers and preamplifiers, are
incorporated into each pixel, which is thus referred to as APS. An essential task for array-
level design is to develop an efficient way to transfer information from the array elements
to the outside system. This is typically dictated by the system requirements. For arrays
that generate 2D outputs, the common methods of transferring data from arrays include
scanning using shift registers, scanning using decoders, and multichannel readout (72; 73).
Each pixel of a CCD imager is composed of a capacitor that is charged by incident light;
thus, no signal processing occurs. The most widely used method of transferring data from
a CCD is progressively scanning the array using shift registers (74). A row of a sensor array
is selected, followed by the sequential readout of each pixel in the column. The collected
charges are then converted into voltage signals using amplifiers and digitized using ana-
log to digital converters. This method reduces the number of wires required to connect
each pixel. It also maximizes the pixel fill-factor, which reduces measurement noise.
However, CCD imagers tend to be slow and consume significant power.
