Device for measuring the internal resistance of a linear lambda probe

1. A device for measuring the internal resistance of a linear lambda probe of an internal combustion engine, having a voltage amplifier for amplifying an alternating voltage which drops across the internal resistance and is brought about by an alternating current applied to the first probe terminal, and having a rectifier for rectifying the amplified alternating voltage, wherein the rectifier is a synchronous demodulator whose voltage amplification can be switched between two predefined values with the frequency of the alternating voltage which drops across the internal resistance, and whose output signal is filtered by means of a filter, and

wherein a switch which can be switched with the frequency of the alternating voltage which drops across the internal resistance is arranged between the non-inverting input of the synchronous demodulator and a second probe terminal.

2. The device as claimed in claim 1 , wherein the predefined values of the voltage amplification of the synchronous demodulator of the values “+1” and “−1”.

3. Circuit for measuring the internal resistance of a linear lambda probe comprising:

a voltage amplifier coupled with a resistance which receives a signal having a frequency,

a synchronous demodulator receiving the output signal of the amplifier, wherein the voltage amplification of the demodulator is switched between two predefined values according to the frequency of the signal; and

a filter for filtering the output signal of the demodulator, wherein the filter comprises a resistance and a capacitance coupled in series between the output of the demodulator and a reference potential.

4. The circuit as in claim 3 , wherein the voltage amplifier is a differential amplifier.

5. The circuit as in claim 3 , wherein the demodulator comprises an operational amplifier.

6. The circuit as in claim 5 , wherein the operational amplifier is coupled with the amplifier through a first and second resistance.

7. The circuit as in claim 3 , wherein the predefined values are “1” and “−1”, respectively.

8. The circuit as in claim 3 , further comprising a switch between a noninverting input of the operational amplifier and a bias voltage.

9. The circuit as in claim 8 , wherein the bias voltage is derived from the lambda probe.

10. The circuit as in claim 8 , wherein the bias voltage is a mid voltage.

11. Arrangement for determining the fuel/air mixture comprising:

a lambda probe having at least a first and second terminal;

an amplifier having an input coupled with the first and second terminal and an output;

a signal generator for generating a signal having a frequency being fed to the first input of the lambda probe;

a rectifier having an input coupled with the output of the amplifier and an output, wherein the rectifier comprises a demodulator having a selectable amplification which is switched between a first and second value with a frequency controlled by the frequency of the signal; and

a filter for filtering the output signal of the demodulator, wherein the filter comprises a resistance and a capacitance coupled in series between the output of the demodulator and a reference potential.

12. The arrangement of claim 11 , wherein the demodulator comprises an operational amplifier.

13. The arrangement of claim 11 , wherein the first and second values are “1” and “−1”, respectively.

14. The arrangement of claim 11 , further comprising a switch between a noninverting input of the operational amplifier and the second terminal.