AC LEVEL CALIBRATION METHOD AND APPARATUS

Abstract: In an analog test system for testing integrated circuit chips, AC level calibration is achieved by adding a minimal number of new elements and more fully utilizing elements existing in the system. A reference voltage from an existing precision DC power source is fed to an analog switching de­ vice. A control signal is also fed into the analog switching device from an existing digital signal generator. Based on the control signal, the analog switching device converts the refer­ ence voltage into a rectangular waveform signal, which is then digitized. The digital signal is analyzed, and data from the digital signal is compared with reference data, and the AC level error, if any, is determined. AC level calibration is based on this error.

Description: The invention relates to a method of AC level calibration in an analog test system and the system itself according the preamble of claim 10. Such a method and system have been known from the prior art publication according to Electronique Industrielle, No. 78, November 1984, pages 51-56, Paris, France: "Etalonnage d'un calibrateur de haute précision". It describes a microprocessor which generates a control signal which is utilized to interrupt the supply of the DC reference voltage in order to form an AC level calibration signal. This AC level calibration signal is digitized by an analog-to-digital converter (CAN), the digital signal processed by the above-mentioned micro-processor comparing the obtained reference values with predetermined correct values and compensating test data. However, the use of the calibration method in a system for integrated circuits is not mentioned there. Today, with new advanced technology many functions, including analog and digital functions, are internalized in an IC chip. For example, a Coder/Decoder Large Scale Integration (CODEC LSI) device integrates an analog input terminal and a digital signal output terminal. As a result, it is necessary for an analog test system to internalize analog test functions together with digital test functions to test an analog IC component. Some previous analog test systems utilize a DC voltage measuring module to level calibrate, but they can only DC level calibrate with a precision power source and they cannot, for example, calibrate sine waves. Even though some previous analog test systems have the ability to perform AC level calibration, it is necessary to utilize both an AC calibrator (i.e., an AC standard voltage generator) and a digital multimeter which can measure AC voltage. However, AC measuring instruments of high accuracy are costly and rather bulky. Also, they are very sensitive to noise and temperature changes, so that they are difficult and expensive to maintain. In addition, the execution time for calibrating the AC level accuracy of an analog test system is quite lengthy. Fig. 1 is a block diagram of one example of a conventional analog test system 10. A signal generator 12 includes an analog signal generator 13 and a digital signal generator 14, and supplies test signals to a device under test (DUT) 19 in a test station 11. According to this example, a test signal, originally generated digitally at the digital signal generator 14, passes through a D/A converter 15 and a filter 16 in the analog signal generator 13 and is finally supplied in analog form to an analog signal input terminal 17A of the DUT 19. Also, the digital signal generator 14 generates the digital signals "1" and "0", which are supplied to a digital signal input terminal 17B of the DUT 19. An analog signal output terminal 18A and a digital signal output terminal 18B are connected respectively to an analog signal measuring unit 21 and a digital signal measuring unit 22 in a signal measuring unit 20. The analog signal measuring unit 21 is composed of a waveform digitizer 23 and a digital signal processor 24. The waveform digitizer 23 receives analog signals from the DUT 19, converts them to digital data signals, and finally directs those digital data signals to the digital signal processor 24. This processor provides a suitable digital process, (e.g., Fast Fourier Transform (FFT)) to the digital data signals output by the waveform digitizer 23. Consequently, an analog signal from the DUT 19 is analyzed digitally by the signal measuring unit 20. The conventional analog test system 10 of Fig. 1 also includes a power source 25 for supplying power to the DUT 19 and a precision power source 26. The precision power source 26 is utilized for a DC level calibration of the power source 25, the analog signal measuring unit 21, and other units in the analog test system 10. However, since AC measuring instruments are expensive and complicated to use, as has been described, the conventional analog test system has disadvantages in AC level calibration. SUMMARY OF THE INVENTION An object of this invention is to add a minimum number of components to an analog test system in order to develop highly accurate AC level calibration. It is another object of this invention to provide a method for AC level calibration in an analog test system for integrated circuits. According to this invention it is important to supply the output signal from the waveform digitizer to a digital signal processor, where it is transformed to Fourier spectra. The Fourier transformed value is then compared to a predetermined correct value, and thus the AC level error is obtained. These values are stored in a calibration data file. When a DUT is tested, test data generated therefrom is compensated for the AC level error, that is, the AC level error is added to or subtracted from the values obtained from testing the DUT 19. Furthermore, the above-mentioned object is achieved by the analog test system according to the features of claim 10. These together with other objects and advantages of the invention will become more apparent from the following description, reference being had to the accompanying drawings wherein like reference numerals designate the same or similar part throughout. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a prior art analog test system; Fig. 2 is a block diagram of an embodiment of the present invention; Fig. 3 is also a block diagram showing details of the analog switching device 27 of Fig. 2; Fig. 4 is a waveform chart explaining the output of the switch circuit 39 of Fig. 3; Fig. 5 is a waveform chart of the AC level calibration signal for the waveform digitizer 23 in the analog measuring device 21 at point A; and Fig. 6 is a spectral map after Fourier transformation of the AC level calibration signal illustrated in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 2 is a block diagram of an embodiment of the present invention. Elements which correspond to elements in Fig. 1 are indicated by the same symbols and their descriptions are omitted. According to the present invention, an analog test system is designed so that it can calibrate both DC and AC levels. An analog switching device 27 is coupled between a precision power source 26 and an analog signal measuring unit 21 as a component of the AC level calibration circuit of the analog test system 10' of the present invention. The analog switching device 27 is controlled by a digital signal generator 14 during AC level calibration, so that an AC level calibration signal is supplied to a waveform digitizer 23 in the analog signal measuring unit 21. An output from the precision power source 26 is supplied to the digitizer 23 through a DC level calibration switch S₁, and is also supplied to the analog switching device 27, from which it passes through an AC level calibration switch S₂ to the digitizer 23. Further, an output from an analog signal generator 13 is supplied to the digitizer 23 through a signal generator calibration switch S₃. All of these switches (S₁-S₃), and a test switch S₄ between an analog signal output terminal 18A in a DUT 19 and the digitizer 23, are controlled by the signal from the digital signal generator 14. For testing the DUT 19, all the test switches must be open except the test switch S₄, so that the analog signal outputs from the DUT 19 pass directly to the digitizer 23. In contrast, for DC level calibration, all the test switches except the DC calibration switch S₁ are open so that DC voltage V₁ of the precision power source 26 is supplied to the digitizer 23. In addition, by closing the signal generator calibration switch S₃, the signals from the analog signal generator 13 are directed to the digitizer 23, which has completed its DC level calibration. Since the waveform digitizer 23 has finished its calibration, a DC level calibration of the analog signal generator 13 will then be performed. According to the present invention, an AC level calibration is completed by closing only the AC calibration switch S₂ and controlling the analog switching device 27, so that AC level calibration signals are directed to the waveform digitizer 23. Fig. 3 is a more detailed block diagram of the analog switching device 27 and its connection through the AC level calibration switch S₂ to the digitizer 23 of the present invention. A high quality direct current power source is utilized for the precision power source 26, and its output voltage V₁ is supplied to an input terminal 31 in the digitizer 23 through a first analog switch 27A and a coaxial cable 29 via the AC level calibration switch S₂. The signal supplied to the input terminal 31 then passes through a resistor 34 which is connected to an analog test system's ground. The coaxial cable 29 is also connected to a second analog switch 27B which is controlled inversely from the first analog switch 27A and which is connected to ground. Therefore, with this second analog switch 27B, the input terminal 31 can be connected to ground via the coaxial cable 29. An outer conductor 38 of the coaxial cable 29 is also connected to ground and the input terminal 31. Both the first analog switch 27A and the second analog switch 27B are controlled by a control signal from the digital signal generator 14, which alternately outputs signals "0" and "1" during the AC level calibration process. Fig. 4 illustrates a waveform of the control signal. The digital signal generator 14, for example, outputs a signal "1" for a time period t o and outputs a "0" for the next time period t o . Consequently, this control signal repeatedly outputs "1" then "0" (Fig. 4a). Therefore, the digital signal generator 14 outputs a rectangular wave signal with a duty ratio of 50 percent. This control signal is provided to drive a switch circuit 39, where first and second on/off signals (Figs. 4b and 4c) are generated. The first and second on/off signals, whose polarities are opposite to each other, are supplied to the first and second switches 27A and 27B, respectively. In this example, first and second analog switches 27A and 27B are open when their respective on/off signals are at H-level, and closed when their respective signals are at L-level; that is, they are controlled oppositely to each other. For example, when first analog switch 27A is closed and second analog switch 27B is open for a time period t₀, DC voltage V₁ is supplied to the input terminal 31 of the digitizer 23. During the next time period t o , both analog switches are oppositely controlled; the first analog switch 27A is open and the second analog switch 27B is closed, so that a ground potential is supplied to the input terminal 31 of the digitizer 23. Then, DC voltage V₁ is supplied during the next time period t o . Therefore in this embodiment, as an AC level calibration signal, a precise rectangular waveform is supplied to the digitizer 23. In Fig. 3, a capacitor C is set to exclude spikes from the first analog switch 27A. According to the present invention, the output from an A/D converter which forms the digitizer 23 is first Fourier transformed in a digital signal processor 24, using either a Discrete Fourier Transform (DFT), or a Fast Fourier Transform (FFT). Then a fundamental wave level included in the AC level calibration signal measured. Finally, the system is AC calibrated based on this fundamental wave level. The rectangular wave shown in Fig. 5 is Fourier transformed to produce: f(t)=(4V₁/π)·(sin(ω)t+(1/3)sin3ωt+(1/5)sin5ωt+ (1) As shown in equation (1), the rectangular waveform is represented by the sum of the fundamental frequency component and the harmonic frequency components. Therefore, if there is no AC level error in the digitizer 23, the magnitude of the fundamental frequency component obtained by signal processor 24 should be 4V₁/π. Fig. 6 is a spectral map corresponding to the rectangular waveform of Fig. 5 after Fourier transformation. The magnitude of the fundamental component 4V₁/π, in Fig. 6 can be predetermined correctly from the DC voltage V₁, and the duty ratio of the rectangular waveform in Fig. 5. Thus, by comparing the predetermined value with the Fourier transformed value, the AC level error is obtained, and the AC level error is stored in a calibration data file 41. This data, together with previously collected data for DC level error, is used for calibrating by adding or subtracting the obtained error from the test data. In operation, the complete procedure for AC level calibration in the analog test system of the present invention is as follows: 1. The DC offset voltage and DC voltage gain in the digitizer 23 is calibrated while a DC reference voltage from the precision power source 26 is supplied. The calibration data is stored in the calibration data file 41. 2. An AC level calibration signal is generated by switching back and forth between the reference voltage V₁ from the precision power source 26 and ground. This signal is provided to the digitizer 23. The output signal from the digitizer 23 is then supplied to the digital signal processor 24, where a Fourier transform is performed on the signal. Finally, the waveform digitizer 23 is AC level calibrated by compensating for the difference between the measured fundamental component level and the calculated fundamental component level. This data is also stored in the calibration data file 41. 3. The signal which tests the DUT 19 is generated by the analog signal generator 13 and is supplied to the digitizer 23 which has completed calibrating steps 1 and 2 above. All the data collected through above steps 1 and 2 are used for calibrating test data, so that test data are compensated for any improper calibration in the analog test system. According to the above embodiment, DC level calibration and AC level calibration are performed using separate components, namely by passing their calibrating signals through a calibration switch S₁ and a calibration switch S₂ respectively. However, in an alternate embodiment, instead of controlling a DC level calibration switch S₁ for DC level calibration in step 1 above, the first analog switch 27A can be closed so that a voltage V₁ from the precision power source 26 is directed to the digitizer 23 before the on/off switching of the first and second analog switches 27A, 27B begins for the AC level calibration. Also in the above embodiment, a rectangular waveform having a 50 percent duty ratio is used in AC level calibration as a calibration signal. However, the particular duty ratio of 50% is not essential for the calibration, i.e., other duty ratios may be employed. In the preferred embodiment, the precision power source 26 is a part of the system itself, but supplying power may be performed by an element outside the analog test system as well. Also the driving circuit 39 which controls the first and second analog switches 27A, 27B can be placed outside the analog test system. Finally, analog signal measuring unit 21 may be composed of a frequency selection level meter (a band pass filter and RMS meter) as well as composed of the digitizer 23 and the digital processor 24. A high precision analog switch control signal is easily generated with a precision power source and a digital signal generator driven by an accurate standard clock. This precision power source is an accurate input signal supply for a DC level calibration of an analog test system. Therefore, highly accurate AC level calibration is available by simple modification of conventional analog test systems. The present invention has been described referring to one main embodiment, however, the present invention permits various modifications within the scope of the subject matter of the present invention. Since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the appended claims.

Claims: A method of AC level calibration in an analog test system (10') for integrated circuits, said method comprising the steps of: generating a rectangular control signal; generating a DC reference voltage; forming an AC level calibration signal by utilizing the control signal to repeatedly interrupt the supply of the DC reference voltage; digitizing the AC level calibration signal to form a digital signal representative of the AC level calibration signal; performing a Fourier transform on the digital signal to obtain the magnitude of the fundamental frequency component of the digital signal; comparing this magnitude with the predetermined correct value in order to find an AC level error; and compensating test data for the AC level error. A method of AC level calibration in an analog test system (10') for integrated circuits, as recited in claim 1, wherein said forming step is performed by utilizing the control signal to alternately open and close first and second switches (27A, 27B), the first switch (27A) being connected to the DC reference voltage, and the second switch (27B) connected to ground. A method of AC level calibration in an analog test system (10') for integrated circuits, as recited in claim 2, wherein said forming step comprises forming a rectangular waveform signal as the AC level calibration signal. A method of AC level calibration in an analog test system for integrated circuits, as recited in claim 1, wherein a fast Fourier transform (FFT)is performed on the digital signal. A method of AC level calibration in an analog test system (10') for integrated circuits, as recited in claim 4, wherein said forming step is performed by utilizing the control signal to alternately open and close first and second switches (27A, 27B), the first switch (27A) being connected to the DC reference voltage, and the second switch (27B) being connected to ground. A method of AC level calibration in an analog test system (10') for integrated circuits, as recited in claim 5, wherein said forming step comprises forming a rectangular waveform signal as the AC leve calibration signal. A method of AC level calibration in an analog test system (10') for integrated circuits, as recited in claim 1, wherein a discrete Fourier transform (DFT) is performed on the digital signal. A method of AC level calibration in an analog test system (10') for integrated circuits, as recited in claim 7, wherein said forming step is performed by utilizing the control signal to alternately open and close first and second switches (27A, 27B), the first switch (27A) being connected to the DC reference voltage, and the second switch (27B) being connected to ground. A method of AC level calibration in an analog test system(10') for integrated circuits, as recited in claim 8, wherein said forming step comprises forming a rectangular waveform signal as the AC level calibration signal. An analog test system (10'), including a DC power source for generating a DC reference voltage, and a digital signal generator (14) for generating an alternating control signal which is a first level signal for a first time period, and a second level signal for a second time period, an analog switching device (27), coupled to an output of said DC power source (26) and an output of said digital signal generator (14), for interrupting the reference voltage based on the alternating control signal generated by said digital signal generator (14) and for thereby changing the reference voltage into an AC level calibration signal waveform; a waveform digitizer (23) for converting the AC level calibration signal into a digital data signal; and compensating means for calibrating for an AC level error, characterized in that said waveform digitizer (23) is coupled to said analog switching device (27); and that said analog test system (10') comprises digital signal processing means (24) which is coupled to the waveform digitizer (23) and performs a Fourier transform on the digital data signal for obtaining the magnitude of the fundamental frequency component of the digital data signal; and that said analog test system (10') further comprises comparing means for comparing said magnitude with a predetermined correct value and thereby obtaining said AC level error. An analog test system (10') according to claim 10, wherein said analog switching device (27) further comprises: a first analog switch (27A) coupled between said DC power source (26) and said waveform digitizer (23); a second analog switch (27B) coupled between ground and said waveform digitizer (23); and a switch circuit (39) connected to said digital signal generator (14) for controlling said first and second switches (27A, 27B) based on the alternating control signal from said digital signal generator (14), wherein upon receiving the first level signal; said switch circuit (39) closes said first analog switch (27A), connecting said power source (26) to said waveform digitizer (23) and allowing the reference voltage to flow to said digitizer (23), and wherein upon receiving the second level signal, said switch circuit (39) opens said first analog switch (27A) and closes said second analog switch (27B) connecting said waveform digitizer (23) to ground. An analog test system according to claim 11, wherein said drive switch circuit (39) further comprises a capacitor (C) coupled between said power source (26) and said first analog switch (27A), for excluding spikes from said first analog switch (27A). An analog test system according to claim 10, wherein said digital signal processing means (24) comprises means for performing a fast Fourier transform (FFT) on the digital data signal. An analog test system according to claim 10, wherein said digital signal processing means (24) comprises means for performing a discrete Fourier transform (DFT) on the digital data signal. An analog test system according to claim 10, characterized by a first analog switch (27A) coupled to the power source and the waveform digitizer (23); a second analog switch (27B) coupled to the waveform digitizer (23) and the ground; and a switch circuit (39) connected for controlling said first and second analog switches (27A, 27B), said switch circuit (39) alternately opening and closing said first and second analog switches (27A, 27B) based on the control signal from the digital signal generator (14), so that an AC level calibration signal is sent to the waveform digitizer (23). A circuit for AC level calibrating in an analog test system (10') according to claim 15, wherein the AC level calibration signal is a rectangular waveform.

Inventors: KANNARI, SHIGERU
Applicants: ADVANTEST CORPORATION
Application year: 1987
Main classification: G01R35
IPC codes: G01R35, G01R31