Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-10-10
2002-05-07
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG, C356S215000
Reexamination Certificate
active
06384401
ABSTRACT:
BACKGROUND OF THE INVENTION
This application relates in general to measurement and sensing of low power signals. More particularly, the invention relates to the sensing, amplification and measurement of a low power, light-based signal.
FIG. 1
illustrates a circuit
100
of the prior art for amplifying a signal from a photo diode
130
. The circuit of
FIG. 1
includes the photo diode
130
connected across the inputs of an operational amplifier
120
. The positive input of the op amp
120
is tied to ground. A resistive load R
150
is coupled between the negative terminal and the out signal
110
of the op amp
120
.
Notably, the feedback resistor R
150
has inherent thermal noise that can sometimes exceed the actual signal from the photo diode
130
. The output from a resistive feedback amplifier such as circuit
100
is given in equation (1) below:
V
out
=−i R
1
where V
out
is in volts, i is the input signal in amperes from a signal source (such as photo diode
130
) and R is the feedback resistance (such as the resistor R
150
) in ohms.
A component with resistance generates thermal noise with the following RMS values:
V
RMS noise
={square root over (4
kTBR
+L )} 2
I
RMS noise
={square root over (4
kTB/R
+L )} 3
where V
RMSnoise
is in volts and I
RMSnoise
is in amperes and where k=1.38×10
−23
J/°K (Boltzmann's constant), T is the absolute temperature in °K, B is the bandwidth in Hz and R is the resistance in ohms.
Therefore, when an application requires the amplification of a very low signal from a photo diode, the prior art resistive feedback amplifier
100
sometimes proves unuseful due to excessive noise, for example.
FIG. 2
presents a circuit
200
of the art, designed to avoid this thermal noise problem. In
FIG. 2
, the photo diode
130
remains coupled across the inputs of the op amp
120
. In place of the resistive element R
150
, a capacitor
220
, coupled between the negative input and the output
210
of the op amp
120
, serves as the feedback element. The source of a field-effect transistor (FET)
230
is coupled to the output
210
of the op amp
120
while the drain is coupled to the negative input of the op amp
120
. The gate of the FET
230
serves as a Reset signal
240
.
The use of the capacitor
220
as the feedback element eliminates the noise problem of the circuit
100
.
The output from an integrator such as the circuit
200
is given in equation (4) below:
V
out
=−i t/C
4
where i is the input signal from a signal source (such as photo diode
130
) in amperes, t is the time from reset to reading in seconds and C is the feedback capacitance (of capacitor
220
, for example) in farads.
FIG. 3
illustrates the timing of the operation of the circuit
200
of
FIG. 2. A
control circuit (not shown) typically resets the integrator
200
(by means of the Reset signal
240
) at twice the rate of the signal bandwidth. Just prior to each of these resets, the control circuit reads the out signal
210
and extracts the true signal.
The use of the semiconductor switch
230
, however, creates its own problems in the circuit
200
. The charge transfer itself from the Reset signal
240
during the resetting of the integrator
200
induces noise. To avoid this problem, the control circuit reads the out signal
210
right after releasing the reset switch
240
. The control circuit then subtracts this reading from the final reading.
The noise of the photo diode
130
and op amp
120
nonetheless affect the two-reading scheme used with the circuit
200
up to the bandwidth of the system. The system bandwidth has to be much higher than the signal bandwidth in order not to distort the integration curves.
Accordingly, there is a need for a circuit for an improved detector of low levels of light without the thermal noise and other problems described above. These and other goals of the invention will be readily apparent to one of ordinary skill in the art on the reading of the background above and the invention description below.
SUMMARY OF THE INVENTION
Herein is disclosed a method and apparatus for measuring very low power signals such as low power light signals, including integrating a signal from a signal source such as a photo diode, an avalanche photo diode, a photomultiplier tube or the like, digitally sampling the integrator output multiple times during each integration period, fitting a curve to the multiple digitized readings to calculate the integration slope for each integration period and determining the original signal from the calculated integration slope.
According to an aspect of the invention, an apparatus for use in measuring low power signals is provided, the apparatus comprising: an integrator, wherein the integrator receives an original low power signal from a signal source and integrates the signal over multiple integration periods; an analog-to-digital converter having an analog input coupled to an output of the integrator, wherein the converter digitally samples the integrator output more than two times during each integration period to obtain multiple digital samples; and a processor coupled to a digital output of the analog-to-digital converter, wherein the processor determines the original low power signal from the multiple digital samples.
According to another aspect of the invention, an apparatus for use in measuring low power light-based signals in a detection region in a first one of at least two intersecting microchannels is provided, the apparatus comprising: a photo diode located proximal the detection region which detects a low power light-based signal in the detection region and outputs a photo diode signal; an integrator having an input coupled to an output of the photo diode; wherein the integrator receives and integrates the photo diode signal over multiple integration periods; a low pass filter having an input coupled to an output of the integrator, wherein the low pass filter operates to filter out frequencies above a predetermined level in the integrator output signal; an analog-to-digital converter having an analog input coupled to an output of the low pass filter, wherein the converter digitally samples the filtered integrator output signal more than two times during each integration period to obtain multiple digital samples; and a processor coupled to a digital output of the analog-to-digital converter, wherein the processor calculates the integration slope for each integration period using the multiple digital samples, and wherein the processor determines. the original low power signal from the calculated integration slopes.
According to yet another aspect of the invention, a method is provided for measuring low power signals, the method comprising the steps of: receiving an original signal from a signal source; integrating over multiple integration periods the original signal with an integrator to produce an integrator output signal; digitally sampling the integrator output signal more than two times during each integration period with an analog-to-digital converter coupled to the integrator to obtain multiple digital samples; and determining the original signal from the multiple digital samples.
According to a further aspect of the invention, a method is provided for measuring low power light-based signals in a detection region in a first one of at least two intersecting microchannels, the method comprising the steps of: locating a photo diode proximal the detection region, wherein the photo diode detects an original low power light-based signal in the detection region and outputs a photo diode signal; integrating the photo diode signal over multiple integration periods to produce an integrator output signal using an integrator having an input coupled to an output of the photo diode; filtering out frequencies above a predetermined level in the integrator output signal using a low pass filter having an input coupled to an output of the integrator; digitally sampling the filtered integrator output signal
Allen Stephone
Caliper Technologies Corp.
Murphy Matthew B.
LandOfFree
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