METHOD FOR TESTING AN ANALOG-TO-DIGITAL CONVERTER UNIT HAVING DELTA-SIGMA MODULATION

Abstract

A method for testing an analog-to-digital converter unit, which is equipped to convert an analog input signal into a digital output signal with the aid of delta-sigma modulation. The method includes: generating an analog input signal; applying a predefined interference signal to the analog input signal and storing the resulting digital output signal as test result; determining that a fault is present if a transfer function of the analog-to-digital converter unit, which is ascertained from the test result and the input signal, has a deviation from a predefined target transfer function that is greater than a predefined reference value, a fault signal being output if a fault is determined.

Claims

1-10. (canceled)

11. A method for testing an analog-to-digital converter unit, which is equipped to convert an analog input signal into a digital output signal using delta-sigma modulation, the method comprising the following steps: generating an analog input signal; applying a predefined interference signal to the analog input signal and storing a resulting digital output signal as a test result; and determining that a fault is present based on a transfer function of the analog-to-digital converter unit, which is ascertained from the test result and the input signal, having a deviation from a predefined target transfer function that is greater than a predefined reference value, a fault signal being output when a fault is determined.

12. The method as recited in claim 11, wherein the analog-to-digital converter unit has a loop filter and a quantizer as parts of the delta-sigma modulation, the predefined interference signal being applied between the loop filter and the quantizer.

13. The method as recited in claim 11, wherein the interference signal is an offset signal and/or a sinusoidal signal and/or a pseudorandom noise.

14. The method as recited in claim 11, wherein after the predefined interference signal is applied, a negated predefined interference signal is applied to the analog input signal and a resulting digital output signal is stored as additional test result, a presence of the fault being determined based on a difference between the test result and additional test result being greater than a predefined reference value.

15. The method as recited in claim 14, wherein an identical input signal is provided for ascertaining the test result and the additional test result.

16. An analog-to-digital converter unit, comprising: a delta-sigma modulator equipped to convert an analog input signal into a digital output signal using delta-sigma modulation; and a self-test module equipped to: generate an analog input signal, apply a predefined interference signal to the analog input signal and store a resulting digital output signal as a test result, and determine that a fault is present if a transfer function of the analog-to-digital converter unit, which is ascertained from the test result and the input signal, has a deviation from a predefined target transfer function which is greater than a predefined reference value, the self-test module being configured to output a fault signal if a fault is determined.

17. The analog-to-digital converter unit as recited in claim 16, wherein the delta-sigma modulator has a loop filter and a quantizer, the predefined interference signal being applied via a coupling point between the loop filter and the quantizer.

18. The analog-to-digital converter unit as recited in claim 16, wherein the self-test module is configured to supply, as the interference signal, an offset signal and/or a sinusoidal signal and/or a pseudorandom noise.

19. The analog-to-digital converter unit as recited in claim 16, wherein the self-test module is configured to, after the predefined interference signal has been applied, apply a negated predefined interference signal to the analog input signal and store a resulting digital output signal as an additional test result, a presence of the fault being determined if a difference between the test result and additional test result is greater than a predefined reference value.

20. The analog-to-digital converter unit as recited in claim 19, wherein the self-test module is configured to generate an identical input signal during ascertainment of the test result and the additional test result.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the following, exemplary embodiments of the present invention are described in detail with reference to the figures.

[0019] FIG. 1 shows a schematic view of an analog-to-digital converter unit according to one exemplary embodiment of the present invention.

[0020] FIG. 2 shows a schematic view of an ideal as well as a real transfer function of the analog-to-digital converter unit according to the exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0021] FIG. 1 shows schematically an analog-to-digital converter unit 1 according to one exemplary embodiment of the present invention. Analog-to-digital converter unit 1 has a delta-sigma modulator 10 as well as a self-test module 11. Delta-sigma modulator 10 is equipped to convert an analog input signal 100 into a digital output signal 200 with the aid of the delta-sigma modulation. In addition, analog-to-digital converter unit 1 may advantageously have a low-pass filter 7 as well as a sampling-rate converter 8, which act upon output signal 200. However, the last two components are not relevant for the exemplary embodiment according to the present invention, and are therefore not described more precisely hereinafter.

[0022] Delta-sigma modulator 10 has a loop filter 2 and a quantizer 3, so that after passing through loop filter 2, input signal 100 is converted by quantizer 3 into a digital signal. For example, quantizer 3 may be a comparator which compares the input signal to a predefined reference signal 500. A closed loop is implemented by a feedback loop 9, which is used to correct the quantization error of quantizer 3. To that end, analog input signal 100 is superimposed with the fed-back signal at a feedback point 6, the fed-back signal being converted by a digital-to-analog converter 5 into an analog signal. Digital-to-analog converter 5 utilizes output signal 200 output by quantizer 3 and converts it again into an analog signal, this being accomplished advantageously only by 1/bit conversion, so that a reference voltage V.sub.Ref either with positive sign or with negative sign is superimposed on analog signal 100.

[0023] In order to detect a fault in delta-sigma modulator 10, loop filter 2 is examined in particular. This loop filter 2 usually has at least one integrator, by which the quantization errors are integrated in order to be able to correct them. If a gain of this integrator is too small, then a controller gain of the closed loop of delta-sigma modulator 10 is not sufficient. A controller gain is too small particularly in the case of a defect of delta-sigma modulator 10. This is able to be determined with the aid of self-test module 11.

[0024] Self-test module 11 is used first of all to generate an input signal 100 which is intended to be used for test purposes. In addition, self-test module 11 is used to generate an interference signal 300. Interference signal 300 is applied at a coupling point 4 between loop filter 2 and quantizer 3. Thus, interference signal 300 acts directly on quantizer 3, thereby avoiding an influencing of other components of delta-sigma modulator 10. In the case of a functioning closed loop, delta-sigma modulator 10 corrects this interference signal 300, so that no or virtually no change at all is detectable at output signal 200. However, if a controller gain is not sufficient, then a correction cannot be carried out.

[0025] Therefore, self-test module 11 is equipped to first of all apply predefined interference signal 300, in order to store resulting output signal 200 as test result. In addition, self-test module 11 is designed to apply negated predefined interference signal 300, in order to store resulting output signal 200 as additional test result. During the ascertainment of the test result and the additional test result, generated input signal 100 remains identical. The only difference between the test result and the additional test result is therefore that different interference signals 300 were present while acquiring these results.

[0026] As described before, in the ideal case, it is expected that no change of output signal 200 at all is present in response to interference signal 300. Therefore, a difference between the test result and the additional test result is formed by self-test module 11. If this difference is greater than a predefined reference value, this may then be attributed to a defect of the control and therefore a defect of delta-sigma modulator 10. In this case, self-test module 11 is designed to output a fault signal. Thus, a particularly simple special case of the general principle is described here, in which a transfer function is deduced based on the response characteristic of analog-to-digital converter unit 1, this transfer function being compared to a predefined target transfer function. If a deviation is greater than a predefined reference value, then said defect is determined.

[0027] In the simplest case, interference signal 300 is an offset signal that is generated by DC voltage. Likewise, interference signal 300 may also be a sinusoidal signal and/or a pseudorandom noise. Depending on information available about loop filter 2, various interference signals 300 may be used to permit optimal testing.

[0028] In an alternative development, the applying of negated interference signal 300 may be omitted. This is especially the case when interference signal 300 is not a simple offset signal.

[0029] FIG. 2 shows a transfer function 400 of analog-to-digital converter unit 1, that is, a relationship between input signal 100 and output signal 200. If an ideal transfer function 400a were present, then each input signal 100 could be assigned unequivocally to one output signal 200, that is, each analog input signal 100 would be converted into a unique digital output signal 200. However, transfer function 400 may have a plateau 401, so that a dead band is produced. In this dead band, a clear-cut assignment of input signal 100 and output signal 200 cannot be ensured. This plateau 401 may be detected reliably with the aid of the method steps described above, which are carried out by the above-described self-test module 11. In this case, the test result and/or the additional test result and/or the difference between the test result and additional test result represent a measure for said plateau 401. In particular, a local shift 402 of transfer function 400 represented by plateau 401 may be determined based on these parameters. The following relationship prevails in particular: If interference signal 300 is described as Q.sub.os, then the following influence on output signal 200, which is described hereinafter as D.sub.out, may be expected:

D.sub.out=kQQ.sub.os/H(f=0)+x

[0030] Herein kQ represents a gain of quantizer 3 and x represents digital output signal 200, which would be converted from input signal 100 without interference signal 300. Thus, a complete correction of interference signals 300 is to be expected only when the gain of the loop filter is adequately large. If this is not the case, then a correction cannot be carried out and plateau 401 remains, whose location may be determined on the basis of parameter kQ Q.sub.os/H(f=0). As described above, if a difference is formed from the test result and the additional test result, then the result is exactly this parameter, since the difference D.sub.out, 1−D.sub.out, 2 leads to an elimination of value x identical in the case of identical input signal 100.

[0031] Besides location 402 of plateau 401, its width 403 may also be determined. An output of plateau 401 is implemented advantageously as the above-described fault signal in the event of a fault of delta-sigma modulator 10, that is, of analog-to-digital converter unit 1. Thus, a reliable and precise test of analog-to-digital converter unit 1 may be carried out in an easy manner. Namely, a fault is able to be detected quickly and reliably. In this case, self-test module 11 dependably outputs a corresponding fault signal.