REFERENCE-VOLTAGE SUPPLY CIRCUIT

Abstract: In an arrangement in which the output (Ua) of a sensor (10) is provided to an evaluation circuit (20) in a pulsed manner, a reference voltage (Uref) from a source (21) within the circuit (20) is supplied to a sampling circuit (15) within the sensor during the gaps between the pulses. To this end, a switch (S1) in the output of the sensor is operated at a common frequency (fo) with a sample and hold circuit (15). The sensor (10) may be a hot film air mass meter (30, Fig. 3 not shown) with a pulsed supply.

Description: Reference-voltage supply circuit The present invention relates to a circuit arrangement for supplying a reference voltage, in particular to a sensor having a voltage output. With sensors which give a voltage output, errors frequently arise in the evaluation of the output when the sensor and the evaluating circuitry, e.g. a computer, are spatially separated. The present invention seeks to overcome or reduce this problem. According to the present invention there is provided a voltage supply circuit for a sensor having a sensor-output receiving circuit with a reference-voltage source, characterised in that a switch is connected between the output (ϋa) of the sensor and the receiving circuit, and that a sampling circuit is connected to a point between the switch and the receiving circuit, the sampling circuit being arranged, when the switch is open, to receive a reference voltage from the source of the receiving circuit. An advantage of this circuit is increased accuracy since sensor and receiving circuit have a common reference voltage. The location of the reference voltage source is not critical, so that it can be placed well away from disturbing influences such as heat sources. Preferably the sampling circuit is a sample and hold circuit which is controlled at the same frequency as the switch and which has a reference voltage output. This involves a time-multiplex operation and ensures that the connection between the sensor and receiving circuit is in permanent use. 8 The circuit may advantageously employed in a sensor which operates in a pulsed manner. Where a comparator receives as a first input the output of the sensor and as a second input a signal determined by a threshold circuit, the upper and lower thresholds may be determined by the reference voltage and can be used to determine the limits of the range of output voltage values. The sensor may be a hot film air mass meter. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig.l is a circuit diagram of a first embodiment of the present invention; Fig.2 shows signal curves associated with the circuit of Fig.l; Fig.3 shows a circuit diagram of a second embodiment of the present invention for use with a hot film air mass meter; and Fig.4 shows signal curves associated with the circuit of Fig.3. Referring-now to Fig.l there is shown a sensor connected by means of a transmission path 11 to an evaluating circuit, e.g. a computer 20. The output ϋa of the sensor is connected via a switch SI and the transmission path 11 to the input ADO of an analogue to digital converter 22 in the evaluation circuit 20. The input ADO is connected via a pull-up-resistor Rp to a source The side of switch SI remote from the sensor output ϋa is connected to a sample and hold circuit 15. Switch SI and circuit 15 are controlled by a common frequency fo. During the period when switch SI is open the reference voltage ϋref is supplied to the sample input of circuit 15; accordingly an output 16 of the sample and hold circuit can be used as a reference voltage source for further operations within the sensor 10. During this sampling phase, the evaluating circuit 20 can ignore the A/D value; alternatively the mark/space ratio of the sampling frequency fo is made so small that the errors introduced by application of the reference frequency can be ignored. The above embodiment has a number of advantages. It has increased accuracy since there is only one reference voltage source. In addition costs are reduced. Moreover, the effects of external conditions such as temperature on the circuit can be reduced because the location of the reference voltage source is not important for the operation of the circuit. Referring now to Fig.3, there is shown a circuit as described in connection with Figs 1 and 2 in combination with a hot film air mass meter 30. The air mass meter is of the pulsed type (such as that disclosed in DE-A- 3803608, the contents of which are hereby incorporated by reference) and the pulse frequency can be used for reading of the reference voltage. This is controlled by the output of a frequency divider 35. The signal from the air mass meter is supplied via a differential amplifier 31 and a low-pass filter 32 to form an output ϋa which is supplied to both switch Si and to the first input of a comparator 41. The mark- space ratio v at the output of the comparator is a function of the air mass, ie. v = v(m). It is this output which is used to control the Operation of a switch S2 for the clock supply to C to the air mass meter. The second input of comparator 41 is determined by an • integrator 42 and a threshold circuit 43. The respective outputs Bl, B2 of the circuits are shown in Fig.4. The upper and lower threshold values of circuit 3 are determined by the reference voltage ϋref obtained from the evaluating or control circuit 20. Taking the upper limit as the product of xl and the reference voltage, and the lower, limit as the product of xO and the reference voltage. Fig. 4 shows the relationship between the various signals. It will be seen that: To Ua - xO'Uref — = v(a) - T (xl - xO) -uref From which it can be derived " that ϋa ■ [xO + v . (xl - x0)] . ϋref Thus ϋa is directly proportional to the reference ϋref of the control device 20. The range of output voltages can only be set as desired by means of the two thresholds. For example, by selecting xO , ϋref - 0,2 V xl . ϋref = 4,5 V and by noting that V always lies between 0 and 1, it will be seen that ϋa also lies in the range 0.2 to 4.5 volts. Although described in connection with a hot film air mass meter, the circuit of the present invention may be used in connection of a wide range of types of sensor.

Claims: 5338 CLAIMS 1. A voltage supply circuit for a sensor (10) having a sensor-output receiving circuit (20) with a reference-voltage source (21), characterised in that a switch (SI) is connected between the output (Da) of the sensor and the receiving circuit, and that a sampling circuit (15) is connected to a point between the switch and the receiving circuit, the sampling circuit being arranged, when the switch is open, to receive a reference voltage from the source (21) of the receiving circuit. 2. A circuit according to claim 1, wherein the sampling circuit (15) is a sample and hold circuit which is controlled at the same frequency (fo) as the switch (SI) and which has a reference voltage output (16). 3. A circuit according to claim 1 or 2, wherein the receiving circuit comprises an analogue-to-digital converter (22) having an input (ADO) connected to the reference voltage source (22) via a resistor (Rp). 4. A circuit according tp any preceding claim, wherein the output (ϋa) of the sensor is supplied to a first input of a comparator (41), the second input of which receives a signal fluctuating between two values determined by the reference voltage (ϋref). 5. A circuit according to claim 4, wherein the second input of the comparator (41) is connected to the output of an integrating circuit (42) which is also connected to a threshold circuit (43), the threshold circuit being connected to the output of the sampling circuit (15) and determining said two values. 6. A circuit according to claim 4 or 5 wherein the supply to the sensor is of the pulsed-type and the output of the comparator (41) has a mark-space ratio (v) and controls a second switch (S2) which supplies in pulsed manner the reference voltage (ϋref) to the sensor. 7. A circuit according to any of claims 4 to 6, wherein the output of the comparator (41) is connected to a frequency divider (35) the output of which controls the first switch (SI) and the sampling circuit (15). 8. A circuit according to any preceding claim, wherein the sensor is hot film air mass meter.

Inventors: KLEINHANS JOSEF, KLEINHANS, JOSEF
Applicants: BOSCH GMBH ROBERT, ROBERT BOSCH GMBH
Application year: 1989
Main classification: G01R1
IPC codes: G01R1