GB2206202A - Infra-red radiation detector - Google Patents

Abstract

A pyroelectric infra-red radiation detector operable in current mode comprises a feedback amplifier 15 whose input is supplied with output signals from one or more pyroelectric elements 10, 11 and between the input and output of which an effective feedback resistance, for example around 100 Gigohms, is provided by means of a switched capacitor circuit 18 (see Fig. 2). The simulated resistance value may be varied by adjustment of the circuit's switching rate so as to vary the detector gain. The amplifier and the switched capacitor circuit conveniently are in the form of one or more integrated circuits and contained with the pyroelectric element in the detector envelope for shielding purposes. <IMAGE>

Description

DESCRIPTION INFRA-RED RADIATION DETECTOR This invention relates to an infra-red radiation detector comprising at least one pyroelectric element for receiving infra-red radiation and amplifier means connected to receive an electrical output from the pyroelectric element.

Such an infra-red radiation detector, commonly referred to a pyroelectric detector, can be used for a variety of purposes, for example in remote switching systems, in intruder detection systems, and in movement sensors generally. The pyroelectric element comprises suitably poled pyroelectric material complete with collection electrodes and can be regarded as a current source in parallel with its own capacitance. Changes in infrared radiation, received by the pyroelectric element caused for example by a person moving through the detector's field of view, produce an electrical output signal from the detector which can be used to activate a switch for alarm or control purposes.

The pyroelectric element has a relatively high impedance by virtue of its high DC resistance and low capacitance and so for most purposes it is necessary to convert the high impedance to a more useful low impedance output. In a known typical pyroelectric detector, the amplifier means consists of a low noise junction FET operating as a high impedance input voltage follower preamplifier.

The FET voltage follower circuit provides a low impedance output and isolation between the pyroelectric element and subsequent circuitry. Pyroelectric detectors of this kind are described for example in British Patent Specification 1,580,403 and 2,046,431B to which reference is invited.

The pyroelectric element, FET, and usually one or more non-linear devices, for example diodes, providing a biasing gate leakage path to protect the gate of the FET from excessive voltage excursions and limit progressively the pyroelectric voltage resulting from large changes in ambient infra-red radiation, are hermetically sealed in a common envelope, such as a TO-5 type canister, with interconnection being etablished via lead wires passing through the envelope wall.

The level of output signal from this kind of detector is very low, being in the order of 1 microvolt, and a high gain amplifier is therefore needed to amplify the detector output further to provide a suitable input signal to a subsequent detection stage.

This high gain amplifier has a number of disadvantages. It requires large coupling capacitors because of the low frequencies involved and adds significantly to the expense of the system incorporating the pyroelectric detector. Moreover, the amplifier can be susceptible to interference from radio transmitters, mains supplies and the like leading to spurious outputs.

An alternative form of pyroelectric detector operates in the so-called current mode. In this detector, the amplifer means comprises a feedback amplifier in the form of an operational amplifier acting as current to voltage inverting preamplifier providing an output voltage at a readily-usable impedance level.

Such a current mode detector offers a high output level, and a similar operational frequency bandwidth compared with voltage follower type detectors. However, the circuit for the current mode detector requires a feedback resistance across the operational amplifier from its output to its inverting input. The modulus of the output voltage from the operational amplifier is equal approximately to the product of the detector current and the feedback resistance value. The detector signal current is typically around 1 to lOpA and therefore in order to obtain an output voltage at a convenient level of around 1 volt, the feedback resistance value needs usually to be around 100 Gigohms (ten to the power eleven ohms). Resistances of this order of magnitude have been obtained in the form of chip resistors but tend to be very expensive.They are considered too costly and difficult to use in the mass-production of detectors and so the current mode detectors heretofore have found only limited use and in particular applications where the advantages of this kind of detector over voltage follower type detectors can be said to justify the high cost.

It is an object of the present invention to provide an infra-red radiation detector operating in current mode which is comparatively cheap to manufacture and which can be mass produced.

According to the present invention, there is provided an infra-red radiation detector comprising at least one pyroelectric element for receiving infra-red radiation and amplifier means connected to receive an electrical output from the pyroelectric element, which is characterised in that the amplifier means comprises a feedback amplifier and a switched capacitor circuit effectively providing in operation a resistance which is connected as a feedback element across the amplifier.

In simulating an effective feedback resistance by means of a switched capacitor circuit, which it has been found can be adapted by suitable selection of operational parameters to provide the very high resistance value necessary, and by employing such a circuit in the detector, the invention avoids the need for, and the problems associated with, the costly, and difficult to produce, discrete resistor as used in conventional current mode type detectors. By appropriate choice of capacitor value and switching rate of the switched capacitor circuit, the switched capacitor circuit can readily be made to simulate the required resistance value, preferably 100 Gigohms approximately. Also, by varying the circuit's switching rate, the effective resistance, and hence the detector's gain, can be adjusted.

For convenience, the feedback amplifier preferably comprises an operational amplifier, although it is envisaged that other suitable forms of feedback amplifier may be employed.

At least some of the switched capacitor circuit components, preferably including at least the switches and the capacitor, may be formed as an integrated circuit, using for example CMOS technology, thereby facilitating mass production and minimising cost.

In a particularly advantageous embodiment, the amplifier and the at least some of the switched capacitor circuit components are formed as a single integrated circuit reducing costs still further and facilitating simplification of the construction of the detector. It is thought that, using mass production techniques, the cost of such an integrated circuit could compare favourably with that of an FET used in the voltage follower type detector.

For shielding purposes, it is preferred that the amplifier and at Least some of the switched capacitor circuit components, whether they in the form of a single integrated circuit or separate integrated circuits, are contained in an hermetically sealed metal envelope together with the pyroelectric element, the envelope being provided with an infra-red transmitting window through which radiation to be detected is received by the pyroelectric element.

Infra-red radiation detectors in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of a detector according to the invention; Figure 2 shows diagrammatically the circuit of a switched capacitor circuit used in the detector circuit of Figure 1 together with associated switching waveforms applied thereto; and Figure 3 is a diagrammatic cross-sectional view through a detector according to the invention.

Referring to figure 1, the infra-red radiation detector operating in current mode includes a pyroelectric element 10, here represented in conventional manner as a capacitor for simplicity.

The element 10 consists of a body of suitably poled pyroelectric material, such as modified lead zirconate titanate, on opposing major surfaces of which are deposited respective, and overlapping, electrodes, for example of nichrome which is substantially transparent to infra-red radiation of a wavelength range for which the detector is intended to be responsive. For the detection of persons, the wavelength range of interest is usually taken to be around 8 to 14 micrometres. For more information regarding the nature of the pyroelectric element 10, reference is invited for example to British Patent Specification No. 1504283.

The pyroelectric element 10 has a very high DC resistance and is therefore essentially capacitive. The capacitance is usually very small, in the region of tens of picofarads, and hence the element has a very high AC impedance.

A further pyroelectric element may be included in the detector as shown in dotted outline at 11 in Figure 1 so as to form a so-called dual pyroelectric element. The two elements 10 and 11 are arranged in a common plane and are formed either from separate bodies or respective portions of a common body of pyroelectric material, the elements each having electrodes on opposite major surfaces in overlapping relationship with the electrodes extending generally normal to the polarisation direction. The two elements 10 and 11 may be connected electrically either in series in which, in the series connection, the directions of polarisation of the pyroelectric material are in oppostion, or, as shown in Figure 1, in parallel, but still with opposite polarities.Such arrangements of dual elements provide some immunity from undesired output signals caused by variations in ambient temperature and background radiation through being connected differentially in this manner.

Uniform changes in radiation incident on both elements produce voltages across the respective elements which cancel one another out, thus producing no net output signal, whereas a change in input radiation in the field of view of one element only, caused for example by a person moving within that field of view, produces a differential output. For further details of an example of the construction of a typical dual pyroelectric element reference is invited to British Patent Specification No. 2143081.

The output from the pyroelectric element 10, or combinations of elements 10 and 11 as the case may be, are supplied to the input of a feedback amplifier, and in this particular embodiment the inverting (negative) input of an operational amplifer 15, whose output is connected to detector output terminal 16. Connected across the inverting input and the output of the amplifier 15 is a switched capacitor circuit, generally designated 18, which in operation acts effectively as a resistive element and provides a feedback resistance across the operational amplifier 15 between its output and inverting input.

In Figure 2, there is shown in greater detail the main components of a form of switched capacitor circuit for use in the detector of Figure 1. Other switched capacitor circuit configurations could be utilised.

Referring to Figure 2, the switched capacitor circuit 18 is shown in greater detail. The circuit comprises a two phase switch, constituted by FETs 20 and 21, and a capacitor 22. The gates of the FETs 20 and 21 are supplied with bi-phase, non-overlapping clock signals, here designated A and B supplied by a simple signal generating logic circuit 19. Typical waveforms for clock signals A and B are represented graphically in Figure 2. It will be appreciated that because the signals A and B are non-overlapping, the respective leading and trailing edges of the A and B signal waveforms, and vice versa, will be separated very slightly in time. The circuit 19 consists of a stable multivibrator circuit and divider chain. The multivibrator circuit defines a basic frequency of oscillation of around a few kilohertz which is divided down by logic circuit to provide the clock signals A and B.Of course, other suitable forms of signal generator may be used instead.

The operation of the circuit can be described as follows.

Initially, with the FETs 20 and 21 turned "on" and "off" respectively, the capacitor charges to the voltage present at terminal 23. When the two phase switch is switched to its other state by the clock signals A and B such that the FETs 20 and 21 and turned "off" and "on" respectively the capacitor 22 discharges to the voltage at terminal 24. The amount of charge flowing from terminal 23 to terminal 24 as the switch changes state is given by the product of the capacitance of capacitor 22 and the difference between the voltages at terminals 24 and 23. The FETs 20 and 21 together can be regarded as a toggle switch.If this switch is toggled back and forth through one complete cycle every T seconds, i.e. the switches 20 and 21 are "on" and "off" for equal, closely approaching T/2, time intervals giving an approximately 50X duty cycle, then the current flowing between terminals 23 and 24 is given approximately by the difference between the voltages at terminals 24 and 23 times the capacitance of capacitor 22 and divided by T. The circuit therefore provides an effective resistance, R, between the terminals 23 and 24 substantially equal to T divided by the capacitance of capacitor 22.

This effective simulated resistance R is used as an equivalent feedback resistance across the operational amplifier whereby the voltage value provided at the terminal 16 in response to a current i (see Figure 1) being produced by the pyroelectric element and supplied to the inverting input of the amplifer 15 is given by the expression -iR.

By appropriate selection of capacitance for capacitor 11 and switching rates of the FETs 20 and 21 a desired feedback resistance value can be readily obtained. In order to obtain an output at terminal 16 of convenient value for further processing, for example by an equipment switching control circuit, and taking into consideration the kind of electrical output typically generated by a pyroelectric element the value of the feedback resistance is preferably chosen to be around 100 Gigohms. Such an equivalent resistance value is easily obtained from the capacitive switching circuit 18 by using, for example, a 0.lpf capacitor and a 100Hz clock rate (T being equal to one hundredth of a second) for the switches. The equivalent resistance value can be adjusted if required simply by varying the clock rate.

Because the clock signals A and B are non-overlapping, the switched capacitor 22 itself does not provide a continuous time path, and so it will not give the continuous time feedback necesary to stabilise the operational amplifier 15. For this reason, a feedback capacitor should be connected in parallel with the effective feedback resistance across the operational amplifier 15.

Such a feedback capacitance is illustrated in dotted outline in Figure 1 at 27 and may typically have a value of around 0.03pF.

However, with an effective feedback resistance of around 100 Gigohms an additional, discrete, feedback capacitor may not be necessary. In practice, stray capacitance may exist anyway to provide the required feedback capacitance thus eliminating the need for any additional component. With a higher effective resistance value, problems may be experienced in view of any existing stray capacitance as a result of the resistance-capacitance product of the feedback circuit.

The switching capacitor circuit 18, including the signal generating logic circuit 19 for producing the signals A and B, is constructed as an integrated circuit using CMOS technology which enables simple and inexpensive fabrication whilst providing sufficent precision to ensure that component value, such as the capacitance of capacitor 22, are within required tolerance ranges, and that the operating characteristics of switches 20 and 21 are near ideal.

The operational amplifier 15 may be fabricated similarly using CMOS technology as a separate integrated circuit. However, in order to minimise both expense and volume occupied, the switching capacitor circuit and operational amplifier, together with the associated logic circuit 19, are preferably formed as a single custom integrated circuit, as indicated by the box referenced 25 in Figure 1. A clock rate adjustment device, for example, a variable resistance, is provided, if necessary, externally of the integrated circuit as shown at 26 in Figure 1 and connected to the logic circuit 19 to enable variations in clock rate, and hence the detector's gain, to be effected. The signal generating circuit 19 may, however, be fabricated separately from the integrated circuit, whilst the FETs 20 and 21 and capacitor 22 are incorporated in the integrated circuit.

The single integrated circuit may be carried in a surface mounting package. Referring now to Figure 3, this package, referenced 30, is contained, together with the pyroelectric element(s) 10 in superimposed relationship, in a metal envelope 32 in the form of a TO-5 type canister. The envelope may be similar to that described in the aforementioned British Patent Specification 2143081. The components 10 and 30 are carried on a header 34 of the canister through which electrical leads 35 pass for external connection. Inside the canister, the leads 35 are connected to contacts provided on the package 30. In an alternative arrangement, the integrated circuit in unencapsulated form may be carried on a hybrid circuit substrate and disposed in similar fashion beneath the pyroelectric element 10.

The envelope 32, which may be evacuated or filled with a relatively inert gas such as dry nitrogen, includes a window 36 transparent to infra-red radiation to be detected by the detector.

This radiation is directed onto the pyroelectric element 10, through the window 36, by means of a suitable optical system (not shown).

In using a switched capacitor circuit in conjunction with an operational amplifier in the above- described manner, a comparatively inexpensive, high gain and voltage output infra-red radiation detector operating in current mode is obtained. The detector requires only a minimum of external components. A power supply for the operational amplifier and switched capacitor circuits is needed, the power being supplied to those circuits via appropriate ones of the leads 35. As the amplifier 15 is contained within the envelope 32, the metal of the envelope acts as a screen to reduce electromagnetic interference effects caused by radio signals or mains supplies.

Where a clock rate adjustment device is provided, this device can be either situated within the envelope, with the clock rate being pre-set during manufacture, or externally and connected through one of the leads 35 to the circuit 19 located within the envelope to facilitate adjustment of clock rate at any time.

AlternativeLy the circuit 19 may be located outside the enclosure and connected to the FETs 20 and 21 in the integrated circuit through respective leads 35.

Although provision may be made for adjustment of the clock rates of signals A and B manually, as described above, in another embodiment of the invention the clock signal generating circuit, whether within or outside the envelope, includes a temperature dependent element, such as a thermistor, which acts automatically to adjust the clock rate, for example to increase the value of the effective feedback resistance to provide rising gain, in accordance with changes in ambient temperature.

Claims (9)

CLAIM(S)

1. An infra-red radiation detector comprising at least one pyroelectric element for receiving infra-red radiation and amplifier means connected to receive an electrical output from the pyroelectric element, characterised in that the amplifier means comprises a feedback amplifier and a switched capacitor circuit effectively providing in operation a resistance which is connected as a feedback element across the amplifier.

2. An infra-red radiation detector according to Claim 1, characterised in that the feedback amplifier comprises an operational amplifier.

3. An infra-red radiation detector according to Claim 1 or Claim 2, characterised in that the effective resistance provided by the switched capacitor circuit is approximately 100 Gigohms.

4. An infra-red radiation detector according to any one of Claims 1 to 3, characterised in that at least some of the switched capacitor circuit components are formed as an integrated circuit.

5. An infra-red radiation detector according to Claim 3, characterised in that the at least some switched capacitor circuit components and the amplifer comprise a single integrated circuit.

6. An infra-red radiation detector according to any one of the preceding claims, characterised in that the feedback amplifier is contained in an hermetically sealed metal envelope together with the pyroelectric element, the envelope being provided with an infra-red transmitting window through which radiation to be detected is received by the pyroelectric element.

7. An infra-red radiation detector according to any one of the preceding claims, characterised in that the switched capacitor circuit includes means for adjusting the switching rate and hence the effective feedback resistance value provided thereby.

8. An infra-red radiation detector according the CLaim 7, characterised in that the adjustment means comprises a temperature dependent element operable to cause the adjustment means to adjust the switching rate in accordance with ambient temperature.

9. An infra-red radiation detector substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.