GB2349523A

GB2349523A - Testing an analogue component

Info

Publication number: GB2349523A
Application number: GB9910116A
Authority: GB
Inventors: Stephen Firth
Original assignee: STMicroelectronics Ltd Great Britain; SGS Thomson Microelectronics Ltd
Current assignee: STMicroelectronics Ltd Great Britain
Priority date: 1999-04-30
Filing date: 1999-04-30
Publication date: 2000-11-01
Also published as: GB9910116D0

Abstract

A method of testing at least one characteristic of an analogue component such as as an analogue to digital converter or operational amplifier, includes, the steps of applying white noise to the input of the analogue component 42 and analysing the output of the analogue component to determine the characteristic.

Description

A Method and Apparatus for Testing An Analogue Component The present invention relates to a method and apparatus for testing an analogue component. The analogue component may, for example, be an analogue to digital converter.

Reference is made to Figure 1 which illustrates how analogue to digital converters are currently tested. A sine wave 2 is applied to the input 4 of an analogue to digital converter 6. The output 8 of the analogue to digital converter is checked to determine whether or not the analogue to digital converter is performing correctly. The output 8 of the analogue to digital converter 6 is checked for its similarity to the sine wave 2 input to the analogue to digital converter 6. However, it is very difficult if not impossible to generate a noise free pure sinusoidal wave for input to the converter 6. This is shown in Figure 2 which shows the sinusoidal wave in the frequency domain. Spike B represents the output of a perfect analogue to digital converter 6 which receives a perfect sinusoidal wave. However, the response referenced A shows the more typical output of an analogue to digital converter 6. The response A has a main frequency peak 12 and a number of harmonic peaks 14 to 22 respectively. As can also be seen from Figure 2 there is random noise superimposed on the signal. This is represented by the response A never touching the horizontal axis but being spaced therefrom by a distance 24.

In practice, these short comings are dealt with in current arrangements by adding a bandpass filter to remove the unwanted harmonics. This is shown diagrammatically in Figure 3. The unwanted signal 26 removed by the bandpass filter has been shaded. However, whilst this bandpass filter improves the situation, there are still some problems. Firstly, some of the harmonic components will remain and will not be completely filtered out by the bandpass filter. This is because the bandpass filter cannot be made too narrow because the location of the main peak 12 will not be known exactly. Furthermore, the bandpass filter does not deal with the problem of the residual noise. This noise causes the magnitude of the signals to be larger than expected. As a result, the resolution and accuracy of the test is limited.

Another problem of the known method is that a signal at each of the different frequencies to be tested must be input separately to the analogue to digital converter 6. This makes the task of testing an analogue to digital converter time consuming.

It is therefore an aim of embodiments of the present invention to provide a method of testing which addresses at least some of the problems of the prior art.

According to one aspect of the present invention, there is provided a method of testing at least one characteristic of an analogue component, said method comprising the steps of applying white noise to the input of the analogue component; and analysing the output of the analogue component to determine the characteristic.

The characteristic may be the frequency response of the component. The analysing step may be carried out using a fast Fourier transform technique to determine the frequency response thereof. The analysing step may comprise transforming the output of the analogue component to the frequency domain. The analysing step may therefore produce a wave form which, in the frequency domain, has a constant amplitude in the region where the response of the component is substantially independent of frequency. This method provides a particularly simple way of determining the frequency response for the component. It should be appreciated that as white noise contains a number of different frequencies, the frequency response can be ascertained in a single measurement. This contrast with the prior art where each individual frequency had to be applied separately to the component under test.

In an alternative embodiment of-the invention, the frequency response may be determined based on the ratio of the total power of the white noise applied to the input of the component to the total power of the output of the component. If the frequency response is perfect, then the input and output powers will be substantially the same.

The characteristic may be the linearity of the component. In preferred embodiments of the present invention, the frequency response and the linearity of the component will be determined.

However, it should be appreciated that in other embodiments of the present invention, one or other of those characteristics may be determined.

To determine the linearity, the number of occurrences of each possible output of the component may be determined. When the component is linear, each possible output of the component occurs substantially the same number of times.

Preferably, the white noise contains a plurality of different frequencies, at least some of those frequencies being in the range over which the component is to be tested.

The analogue component may be one of the following components: analogue to digital converter; operational amplifier; and an amplifier. Embodiments of the present invention are particularly advantageous when applied to analogue to digital converters.

The output analysed in the analysing step may be a digital output.

According to a second aspect of the present invention, there is provided apparatus for testing a characteristic of an analogue component, said apparatus comprising a noise source for generating a source of white noise to be applied to an analogue component under test; and analysis means for receiving the output of the component under test and analysing said output.

The analysis means may be arranged to determine the frequency response of the component. The analysis means may comprise Fourier transform means. Alternatively, the analysis means may be arranged to compare the total power of the white noise input to the component with the total power of the output of the component to thereby determine the frequency response of the component.

The analysis means may be arranged to determine the linearity of the component. The analysis means may comprise means for determining the number of times each possible output of the component occurs to determine the linearity of the component.

A calibration component may be provided for calibration of the component under test, the calibration component being the same type of component as the component under test. The calibration component may be in parallel with the component under test.

An integrated circuit may comprise apparatus as described hereinbefore and the analogue component under test. This provides a test system on chip.

For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made to the accompanying drawings in which: Figure 1 schematically shows the testing of an analogue to digital converter in the prior art; Figure 2 shows the wave form output by the analogue to digital converter of Figure 1 in the frequency domain; Figure 3 shows the effect of a bandpass filter connected to the output of the analogue to digital converter of Figure 1, in the frequency domain; Figure 4 shows a first embodiment of the present invention; Figure 5 shows the input to the analogue to digital converter of Figure 4; Figure 6 shows the output of the fast Fourier transform unit of Figure 4; Figure 7 shows the output of the weighting analysis block of Figure 4, if the analogue to digital converter is linear ; Figure 8 shows an output of the weighting analysis block of Figure 4 if the analogue to digital converter is not linear; Figure 9 shows one example of apparatus for testing an analogue to digital converter; Figure 10 shows a known test on chip system for an analogue to digital converter; and Figure 11 shows a test on chip system for testing an analogue to digital converter, embodying the present invention.

Reference is now made to Figure, 4 which shows an embodiment of the present invention. A noise source 40 generates a white noise signal. A white noise signal is a signal in which there is an equal distribution of a range of frequencies. The noise source is arranged to generate the white noise signal over the entire range of frequencies over which the analogue to digital converter is to be tested. Figure 5 shows, in the frequency domain, the white noise signal to be input to a analogue to digital converter 42 to be tested.

As can be seen from Figure 5, the different frequencies have equal weight in the white noise signal output by the noise source 40.

The output of the analogue to digital converter 42 is connected to the input of a Fourier transform unit 44 and a weighting analysis unit 46.

The fast Fourier transform unit 44 performs a fast Fourier transform on the output of the analogue to digital converter 42.

A typical output from the fast Fourier transform unit 44 is shown in Figure 6. As can be seen, the response is divided into two sections 48 and 50. The first section 48 represents the range of frequencies over which the analogue to digital converter will work correctly. The second section 50 shows the roll off of the analogue to digital converter. Typically, it is ensured that during normal operation of the analogue to digital converter, it will operate in the range of frequencies represented by the first section 48. The first section 48 is thus one where the magnitude of the signals in the frequency range covered by that section of the response are equal. The second section 50 represents the frequencies for which the analogue to digital converter provides outputs of lower amplitudes than for those frequencies in the first section 48. The second section 50 therefore represents the frequencies which fall outside the normal frequency range of the analogue to digital converter.

By considering the graph shown in Figure 6, the frequency response of an analogue to digital converter can be simply and easily determined. There is no need to input each of the different frequencies separately.

The weighting analysis unit 46 is arranged to determine the linearity of the analogue to digital converter 42. The output of a perfect analogue to digital converter should contain all of the possible codes, in equal weight, for a white noise input. For example, a four bit converter should return an equal number of codes for all possible values, i. e. 0, 1, 2,3,4,5,6,7,8, 9,10,11,12,13,14 and 15. Figure 7 shows a perfect output for 11 of those values. The number of occurrences of each of the codes is the same. Therefore, the analogue to digital converter is linear.

Figure 8 shows the output for a non-linear analogue to digital converter. As can be seen, not all of the codes occur an equal number of times. Thus, it can be deduced that the analogue to digital converter is not linear.

This analysis is much easier to perform with a white noise input than with the sinusoidal input of the prior art. For a sinusoidal wave, a number of occurrences of each code would be different for a linear analogue to digital converter. This is because the rate of change of value of the sinusoidal wave varies from region to region of the wave. This makes it much more difficult to analyse whether or not the output of an analogue to digital converter is linear.

In an alternative embodiment of the present invention, the fast Fourier transform unit 44 is omitted and the analysis provided by the weighting analysis unit 46 is used to determine the frequency response of the analogue to digital converter. With this alternative method, the ratio of the total input power (represented by the area under the waveform of Figure 5) to the integral of the height of the code densities (the total output power of the analogue to digital converter, calculated from the graph shown in Figure 7) is determined. This calculation can be performed successfully only, if the roll off rate of the analogue to digital converter is known. The roll off rate is represented by the gradient of the second section 50 of the graph shown in Figure 6.

Reference is made to Figure 9 which shows how embodiments of the present invention can be implemented in practice. A white noise signal is generated by a noise generator 60. The output of the noise generator is input to a first analogue to digital converter 62 which is under test and a second analogue to digital converter 64 which acts as a calibrator. The output of the analogue to digital converter 62 under test is input to an analysis block 66 which incorporates the fast Fourier transform unit and the weighting unit shown in Figure 4. The analysis block 66 also receives the output of the analogue to digital converter 64 used for calibration. The analogue to digital converter for calibration can be a slow oversample type so that the impact on test manufacture is minimal.

Figure 10 is a prior art test on chip system which is included for comparative purposes. A sinusoidal wave is input to a chip 70. The chip includes an analogue to digital converter 72 which is to be tested. The output of the analogue to digital converter 72 is input to a microprocessor 74 which is effectively transparent. The output of the microprocessor 74 is input to a digital to analogue converter 76 which attempts to reconstruct the sinusoidal wave. An external analysis unit receives from the digital to analogue converter 76 the reconstructed sinusoidal wave. From this, a measure of the effectiveness of the analogue to digital converter 72 and the digital to analogue converter 76 can be obtained. It should be appreciated that the analogue to digital converter 72, the digital to analogue converter 76 and the microprocessor 74 are all on the chip 70.

Figure 11 shows a on chip testing arrangement embodying the present invention. This arrangement is particularly advantageous in built in self test arrangements. The chip 80 has an analogue to digital converter 82 the output of which is connected to a microprocessor analysis unit 84. The analogue to digital converter is used conventionally in a normal mode and is tested in a test mode. The chip 80 also has a linear feedback shift register 86 which generates a white noise signal in the test mode. The output of the register 86 is connected to a digital to analogue converter 88. The output of the digital to analogue converter 88 is input to the input of the analogue to digital converter 82. This connection may be done externally of the chip or internally of the chip. The linear feedback shift register 86 and the digital to analogue converter 82 effectively provide the white noise source.

The white noise signal input to the digital to analogue converter 88 provides an analogue white noise signal. This is input to the analogue to digital converter 82 which converts it back to digital form. A microprocessor analysis unit 84 carries out the fast Fourier transform and weighting analysis discussed in relation to Figure 4.

It should be noted that in embodiments of the present invention, apparatus has been described for determining the linearity and the frequency response of the analogue to digital converter. It should be appreciated that in some embodiments of the present invention, a check for linearity or a check for frequency response may be performed, and not both of these checks.

Embodiments of the present invention have been described in the context of the testing of an analogue to digital converter.

However, it should be appreciated that embodiments of the present invention be used for the testing of other components.

Embodiments of the present invention are particularly applicable to the testing of analogue devices. The output of those analogue devices may be in digital form. Other examples of components with which embodiments of the present invention can be used are amplifiers, for example operational amplifiers.

Claims

CLAIMS : 1. A method of testing at least one characteristic of an analogue component, said method comprising the steps of: applying white noise to the input of the analogue component ; and analysing the output of the analogue component to determine the characteristic.
2. A method as claimed in claim 1, wherein said characteristic is the frequency response of the component.
3. A method as claimed in claim 2, wherein said analysing step is carried out using a fast Fourier transform technique to determine the frequency response thereof.
4. A method as claimed in claim 2 or 3, wherein said analysing step comprises transforming the output of the analogue component to the frequency domain.
5. A method as claimed in claim 4, wherein said analysing step produces a waveform which, in the frequency domain, has a constant amplitude in the region where the response of the component is substantially independent of the frequency.
6. A method as claimed in claim 2, wherein in said analysing step, said frequency response is determined based on the ratio of the total power of the white noise applied to the input of the component to the total power of the output of said component.
7. A method as claimed in any preceding claim, wherein said characteristic is linearity of the component.
8. A method as claimed in claim 7, wherein said analysing step, to determine the linearity, comprises determining the number of occurrences of each possible output of the component.
9. A method as claimed in claim 8, wherein the component is determined to be linear if each possible output of the component occurs substantially the same number of times.
10. A method as claimed in any preceding claim, wherein said white noise contains a plurality of different frequencies, at least some of said frequencies being in the range over which the component is to be tested.
11. A method as claimed in any one of the preceding claims, wherein said analogue component is one of the following components: analogue to digital converter ; operational amplifier; and amplifier.
12. A method as claimed in any one of the preceding claims, wherein the output analysed in said analysing step is a digital output.
13. Apparatus for testing a characteristic of an analogue component, said apparatus comprising: a noise source for generating white noise to be applied to an analogue component under test; and analysis means for receiving the output of the component under test and analysing said output.
14. Apparatus as claimed in claim 13, wherein said analysis means is arranged to determine the frequency response of the component.
15. Apparatus as claimed in claim 14, wherein said analysis means comprises Fourier transform means.
16. Apparatus as claim in claim 13 wherein said analysis means is arranged to compare the total power of the white noise input to the component with the total power of the output of said component, to thereby determine the frequency response of the component.
17. Apparatus as claimed in any of claims 13,14,15 or 16, wherein said analysis means is arranged to determine the linearity of said component.
18. Apparatus as claimed in claim 17, wherein said analysis means comprises means for determining the number of times each possible output of said component occurs, to determine the linearity of said component.
19. Apparatus as claimed in any of claims 13 to 18, wherein a calibration component is provided for calibration of the component under test, the calibration component being the same type of component as the component under test.
20. Apparatus as claimed in claim 19, wherein said calibration component is in parallel with the component under test.
21. An integrated circuit comprising apparatus as claimed in any of claims 13 to 18 and the analogue component under test.
22. An integrated circuit comprising an analogue component, a white noise source, white noise being applied to analogue component in a test mode, and an analyser connected to the output of the analogue component for analysing the output thereof in a test mode.