Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-05-07
2001-12-04
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339130, C250S343000
Reexamination Certificate
active
06326620
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to measurement devices and circuits and, in particular, a circuit for reducing the 1/f noise level of the detection circuit.
In most non-dispersive infrared detection applications (NDIR) using thermal detectors, the radiation source is modulated either because of the AC nature of the detector or to reduce the effects of stray radiation or temperature drifts. The infrared source could also be modulated by some mechanical means, such as a chopper wheel connected to a constant RPM motor to alternately pass and block the infrared radiation. Such an approach used with pyro-electric detectors requires the use of somewhat expensive precision motors, controllers, and chopper wheels precisely synchronized to provide the modulation and demodulation of signals.
In order to overcome the difficulties with such detection systems, it is desirable to use a DC system in which a thermopile is provided to detect the infrared radiation passing through an analyte, however, the use of such a detector requires a DC amplifier for providing a signal level which is representative of the nature of the analyte being detected as well as its concentration. DC amplifiers can be employed for such purpose, however, DC amplifiers inherently have internal noise referred to as 1/f noise which exponentially decreases with increasing frequency. Thus, at very low frequencies, the 1/f noise is significant and for use in NDIR systems with DC emitters, such noise adversely affects the resultant detected signal. The 1/f noise, however, decreases significantly at higher frequencies, such as 1 kHz and is negligible at or above such frequency.
SUMMARY OF THE INVENTION
The system of the present invention, however, provides low noise circuit which modulates input signals at a frequency of about 1 kHz, subsequently demodulates and filters the signals to provide an analog DC output level in which the 1/f noise of the amplifier is effectively eliminated due to the selection of the modulation frequency above the significant level of 1/f noise. Its application in a preferred embodiment is in an NDIR system using a detector having a DC emitter employed with a thermopile detector to provide an analog varying DC low level signal. Such a system, therefore, allows the use of a DC emitter which is more reliable than pulsed emitters and relatively inexpensive circuitry components to provide superior performance for a NDIR detection system used in connection with an analyzer.
These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings.
REFERENCES:
patent: 3793525 (1974-02-01), Burch et al.
patent: 4996431 (1991-02-01), Bonne et al.
patent: 5113143 (1992-05-01), Wei
patent: 5369278 (1994-11-01), Lehto
patent: 6114700 (2000-09-01), Blades
patent: 0745840-A2 (1996-12-01), None
patent: 8-62040-A (1996-03-01), None
Gagliardi Albert
Hannaher Constantine
Leco Corporation
Price Heneveld Cooper DeWitt & Litton
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