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.