Abstract:
A low noise amplifier circuit 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.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical circuit diagram in block form of the system of the present invention; and 
     FIG. 2 is an electrical circuit diagram, partially in block and schematic form, of the major components of the circuits shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, there is shown a non-dispersive infrared (NDIR) detection system  10  for an analyzer (not shown). The system comprises a DC infrared emitter  11  which is mounted in a temperature-controlled cell  14  through which a carrier gas, such as helium, flows and into which flow stream an analyte is introduced. The infrared cell construction, together with the mounting of a thermopile detector  12  in the cell  14 , is conventional. The analyte supplied to the cell  14  will include fluids and/or gases which absorb infrared radiation including, for example, NO, CO 2 , H 2 O, and the like as examples only. 
     The infrared emitter  11  emits infrared radiation  13  which passes through the analyte flowing between the emitter  11  and detector  12 , which detector provides an analog DC varying output represented by waveform  15 . which may, for a helium carrier, have an output of, for example, 40 millivolts and dip, as indicated by waveform section  16 , to a level of, for example, 20 millivolts as an analyte passes through the cell  14 . This low level time-varying DC analog signal is applied to the circuit  20  of the present invention to provide a noise-free amplified output signal at output terminal A. Circuit  20  includes a modulator  21  coupled to an oscillator  22  for modulating the time-varying signal  15  at a frequency which is above the significant 1/f noise level of a typical DC amplifier. In the preferred embodiment of the invention, oscillator  22  had a frequency selected at 1 kHz, although frequencies slightly below and significantly above could also be employed. 
     Modulator  21  chops the analog DC signal and selectively applies the positive and negative terminals of the detector to the input of an amplifier  24  which is an operational amplifier, as described in greater detail below in connection with FIG. 2, to provide an alternate polarity square-wave output signal  18 . This output signal from amplifier  24  is applied to a demodulating amplifier  26  also coupled to the oscillator  22  for receiving signals therefrom to invert one half of the square-wave signal  18  to provide a unipolar output signal  19  therefrom which is applied to a low pass filter  28  passing frequencies substantially at 3 Hz or below. The DC level output from filter  28 , in turn, is coupled to an A-to-D converter  30  having an output at terminal B comprising a binary number representing the detected signal level of an analyte to be analyzed. The signal from A-to-D converter  30  is conventionally applied to a microprocessor associated with an analyzer to provide the operator with a readable output which is representative of not only the analyte detected but the level of analyte in a given specimen. The analyzer infrared cell and the combustion furnace associated with such an analyzer can be conventional components and do not form part of the present invention other than the environment in which the circuit of the present invention is employed. Having briefly described the overall system of the present invention, a more detailed description of circuit  20  and its operation is now presented in connection with FIG.  2 . 
     Referring now to FIG. 2, the thermopile detector  12  provides a polarized varying DC signal of, for example, 20 millivolts which drops, as indicated by waveform  15  in FIG. 1, 20 millivolts as an example when an analyte passes through the cell. The modulator  21  coupled to oscillator  22  comprises FET analog switches, such as Analog Device&#39;s Model ADG433 which is controlled by a signal from oscillator  22  to switch the polarity of the detector output signals on conductors  11 ′ and  13 ′ alternately to ground through conductors  17  or to the positive input  23  of amplifier  24 . Thus, modulator  21 , as shown schematically, constitutes, in effect, a two-pole, double-throw switch driven at 1 kHz to provide reverse-polarity signals to the positive input of amplifier  24 . Amplifier  24  has an output  31  coupled to its negative input  32  by a 200 kOhm resistor  33 . Negative input  32  is coupled to ground through a 1 kOhm resistor  34  with resistors  33  and  34  controlling the gain of amplifier  24  to approximately 200. 
     The switched polarity square-wave signal  18  at output  31  of amplifier  24  is essentially a 1 kHz square-wave having an amplitude of plus or minus 4 volts for the waveform  15  shown in FIG. 1 at the 40 millivolt level and plus or minus 2 volts when an analyte passes through infrared cell  14 , as shown by section  16  of the waveform  15 . It is noted that cell  14  has a controlled temperature environment such that temperature stability and drift is not a factor in connection with the signal detection and the analyzer&#39;s microprocessor discriminates between the steady state carrier gas flowing through infrared cell  14  and the signal change resultant from the presence of an analyte. 
     The output  31  of amplifier  24  is coupled to a demodulator  26  comprising a second analog switch  40  coupled to oscillator  22  and comprising a single-pole, double-throw switch selectively coupling the output terminal  31  of amplifier  24  to the positive input terminal of an operational amplifier  44  having its negative input terminal  42  coupled to output terminal  31  of amplifier  24  by means of a 10 kOhm resistor  45 . Thus, the square-wave signal  18  at terminal  31  is continuously applied to the inverting input terminal  42  of amplifier  44 , and oscillator  22  in connection with the analog switch  40  applies the positive half of square-wave  18  to the positive or non-inverting input  43  of amplifier  44 . Amplifier  44  has unitary gain selected by resistors  45  and a feedback 10K resistor  46 . From the time t 0  to t 1  comprising the positive first half of square-wave  18 , therefore, the amplifier acts as a follower. During the second half of the square-wave cycle (t 1  to t 2 ), switch  40  moves from the position shown to ground so that only the t 1  to t 2  negative half of the square-wave  18  is applied to the inverting input  42  of amplifier  44 . Amplifier  14  inverts the waveform to a positive form shown as waveform  48  at output terminal  47  of amplifier  44 . 
     Waveform  19  comprises a varying DC level of composite square-waves, which have a transition noise at ti of a switching frequency of about 1 kHz and harmonics thereof. This switching noise is eliminated by the low pass filter  28  comprising a 100 kOhm resistor  27  and a 2.2 microfarad capacitor  29  coupled in a low pass filter configuration as shown in FIG.  2 . The junction of resistor  27  and capacitor  29  is output terminal A which, as shown in FIG. 1, is coupled to the input of A-to-D converter  30 . The terminal of capacitor  29  remote from such junction is coupled to ground. The signal output at terminal A of FIG. 2, therefore, is a filtered DC signal having a level representative of either the steady state carrier gas level, such as 4 volts, or a lower level such as 2 VDC reflective of the analyte detected and its quantity. The signal is essentially free of any 1/f noise which is internally generated by amplifier  24  in view of the utilization of a modulating and demodulating frequency essentially above the frequency of significant 1/f noise generated by the DC amplifier. 
     The conventional A-to-D converter  30  is a 24 bit A-to-D converter which has an eight-pole low pass filter selected at 2.6 kHz, such that in combination with filter  28 , eliminates the switching transients and provides an output signal free of 1/f components. The resultant circuit allows use of a DC infrared emitter, a DC detector such as a thermopile, and DC amplifiers to provide a relatively low cost and yet stable NDIR sensing system having superior signal-to-noise characteristics, and one which is relatively compact and reliable in operation. 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.