JP2021044690A - A/d converter test device, test method, and semiconductor device - Google Patents

Abstract

To provide a test technology for a new A/D converter.SOLUTION: A test device 100 tests a DUT 10 having an A/D converter 20. A waveform generator 110 supplies the A/D converter 20 with a periodic analog test signal S1. A waveform acquisition unit 120 captures the waveform of an output node of the A/D converter 20 corresponding to the analog test signal S1 or the waveform of an internal node of the A/D converter 20. An evaluation device 130 performs discrete Fourier transform on the waveform acquired by the waveform acquisition unit 120, and estimates the characteristics of the A/D converter 20 on the basis of spectrum information S3.SELECTED DRAWING: Figure 1

Description

The present invention relates to an evaluation technique for an A / D converter.

There is an A / D converter as one of the key devices in a system that performs digital signal processing. Differential nonlinearity error (DNL: Differential Non-Linearity) and integral nonlinearity error (INL: Integral Non-Linearity) are known as indicators of the performance of the A / D converter.

A / D converters or chips and systems incorporating them need to be inspected before shipping. Generally, a sweep waveform across the entire code is input to the input of the A / D converter, and the response of the output to the sweep waveform is measured. Here, the time required for measurement is proportional to the product of the total number of codes and the sampling time.

In the conventional test apparatus, it is necessary to take in and hold a huge number of output codes obtained by digitizing the sweep waveform, so that a huge amount of memory is required.

Some A / D converters have a very slow sampling rate. For example, some A / D converters using a delta-sigma modulator have a sampling rate of about several sps (sample per second). For example, it takes 685 hours to test the entire code of a 24-bit A / D converter with a sampling rate of 6.8 sps, which is not realistic.

The present invention has been made in such a situation, and one of the exemplary purposes of the embodiment is to provide a test technique for a novel A / D converter.

One aspect of the present invention relates to a test apparatus for a semiconductor device having an A / D converter. The test device uses a waveform generator that supplies a periodic analog test signal to the A / D converter, a waveform of the output node of the A / D converter corresponding to the analog test signal, or a waveform of the internal node of the A / D converter. A waveform acquisition unit for capturing and an evaluation device for estimating the characteristics of the A / D converter based on spectral information by performing discrete Fourier conversion on the waveform acquired by the waveform acquisition unit are provided.

According to this aspect, the A / D converter can be tested by a method different from the conventional method. In particular, the number of output codes that should be captured and retained can be significantly reduced.

The evaluation device may calculate the coefficient when the input / output characteristics of the A / D converter are polynomial-approximated based on the spectrum information.

The analog test signal may be a sine wave. In this case, since the analog signal becomes a single tone, the analysis becomes easy. However, the analog test signal may be a rectangular wave or a triangular wave.

The A / D converter may include a delta-sigma modulator and a digital filter. The waveform acquisition unit may capture the waveform of the output node of the ΔΣ modulator. This can shorten the test time.

Another aspect of the present invention is a semiconductor device. This semiconductor device includes an A / D converter including a delta-sigma modulator and a digital filter, and is packaged in one package. The semiconductor device includes a test pin drawn from the connection node of the delta-sigma modulator and the digital filter, and is based on the waveform output from the test pin when a periodic analog test signal is input to the A / D converter. The / D converter can be evaluated.

It should be noted that an arbitrary combination of the above components or a conversion of the expression of the present invention between methods, devices and the like is also effective as an aspect of the present invention.

According to an aspect of the present invention, a novel A / D converter test apparatus and test method can be provided.

It is a block diagram of the test apparatus which concerns on embodiment. It is a figure which shows typically the input / output characteristic of an A / D converter. It is a figure explaining the measurement principle of the test apparatus of FIG. It is a block diagram of the test apparatus which concerns on one Example. It is a block diagram which shows the model of a ΔΣ modulator. It is a figure which shows the simulation result of the input / output characteristic of the model of the ΔΣ modulator of FIG. It is a figure which shows the coefficient when the 6 input / output characteristics of FIG. 6 are polynomial-approximated by the third order. It is a figure which shows the power spectrum obtained by FFTing the waveform of the output of the simulation model of FIG. It is a figure which shows the error between the estimated value of the _{coefficient a 1} and a ₃ obtained from the result of FFT, _{and the theoretical value of the coefficient a 1} and a _{3 shown in FIG.}

(Embodiment)
Hereinafter, the present invention will be described with reference to the drawings based on preferred embodiments. The same or equivalent components, members, and processes shown in the drawings shall be designated by the same reference numerals, and redundant description will be omitted as appropriate. Further, the embodiment is not limited to the invention but is an example, and all the features and combinations thereof described in the embodiment are not necessarily essential to the invention.

In the present specification, the "state in which the member A is connected to the member B" means that the member A and the member B are physically directly connected, or the member A and the member B are electrically connected to each other. It also includes the case of being indirectly connected via other members, which does not substantially affect the connection state or does not impair the functions and effects performed by the combination thereof.

Similarly, "a state in which the member C is provided between the member A and the member B" means that the member A and the member C, or the member B and the member C are directly connected, and their electricity. It also includes the case of being indirectly connected via other members, which does not substantially affect the connection state, or does not impair the functions and effects produced by the combination thereof.

FIG. 1 is a block diagram of the test apparatus 100 according to the embodiment. The test device 100 tests the semiconductor device (DUT: Device Under Test) 10.

The DUT 10 includes an A / D converter 20. The type of DUT 10 is not limited, but may be, for example, a front-end circuit that converts an output for a sensor into a digital signal. Examples of the sensor include a thermocouple, a thermistor, and an RTD (resistance temperature detector).

The waveform generator 110 supplies the A / D converter 20 with a periodic analog test signal S1. In this embodiment, the analog test signal S1 is a sine wave. The waveform generator 110 may include a waveform table that holds the waveform data and a D / A converter that converts the data read from the waveform data into an analog signal. In this case, it is desirable that the resolution of the D / A converter is higher than the resolution of the A / D converter 20 of the DUT 10. For example, when the resolution of the A / D converter 20 is 24 bits, the resolution of the D / A converter of the waveform generator 110 may be about 32 bits.

The waveform acquisition unit 120 captures the waveform of the output node (i) of the A / D converter 20 corresponding to the analog test signal S1 or the waveform of the internal node (ii) of the A / D converter 20. Which node's waveform should be captured may be determined according to the type of the A / D converter 20. The waveform acquisition unit 120 can be composed of an interface circuit that receives the output of the DUT 10 and a memory that stores a data string.

The evaluation device 130 includes a converter 132 that performs a discrete Fourier transform (DFT) on the waveform S2 acquired by the waveform acquisition unit 120, and generates spectrum information S3. The converter 132 can obtain the spectrum information S3 by the algorithm of FFT (Fast Fourier Transform). The analysis unit 134 estimates the characteristics of the A / D converter 20 based on the spectrum information S3.

For example, the analysis unit 134 calculates a coefficient when the input / output characteristics of the A / D converter 20 are polynomial-approximated based on the spectral information obtained by the DFT.

The above is the configuration of the test device 100. Next, the operation will be described. FIG. 2 is a diagram schematically showing the input / output characteristics of the A / D converter 20. When the input of the A / D converter 20 is represented by x and the output is represented by y, the input / output characteristics of the A / D converter 20 are generalized as y = f (x). The function f (x) is represented by a linear polynomial as shown in equation (1).

The input x is represented by the equation (2).

Substituting the equation (2) into the equation (1) and rearranging it, the waveform of the output signal of the A / D converter 20 is a sine wave having a different frequency (f, 2f, 3f ...) As shown in the equation (3). Expressed as a linear sum of.

That is, when a sine wave having a frequency f passes through the A / D converter 20 having a non-linear error, harmonics are generated.

b ₀ , b ₁ , b ₂ , ... b _n correspond to the intensities of the frequency components 0, f, 2f, ... nf of the spectrum obtained by discrete Fourier transforming the signal y. Further, as described _{later, b 0, b 1, b} 2, ... b n has an amplitude A of the analog test signal S1, the coefficient of linear polynomial _{a 0,} a 1, a correlation with ... _{a n.} Therefore, by measuring the spectrum of the signal y, it is possible to obtain information on the input / output characteristics of the A / D converter 20.

FIG. 3 is a diagram illustrating a measurement principle of the test device 100 of FIG. The analog test signal S1 is an ideal sine wave and has only a single frequency f. When this analog test signal S1 is input to the A / D converter 20 having an unknown input / output characteristic y = f (x), waveform distortion occurs, and the spectrum S3 of the waveform S2 of the output signal of the A / D converter 20. The DC component and the harmonic component 2f, 3f, ... Nf appear in. The evaluation device 130 can estimate the input / output characteristic y = f (x) of the A / D converter 20 based on the spectrum information S3 of the output of the A / D converter 20.

The advantages of this test apparatus 100 will be described. In the conventional test method, a sweep waveform across the entire code is given to the A / D converter, and the response of the output to the sweep waveform is measured. Therefore, if the 24-bit A / D converter, it is necessary to hold the 2 ²⁴ enormous number of samples of data.

In the embodiment, the discrete Fourier transform is used, and the number of samples defines the frequency resolution. When a pure sine wave having a frequency f is used as the analog test signal S1, only the intensity of i × f (however, i = 0, 1, ...) Needs to be known, and information on intermediate frequency components is unnecessary. Therefore, in the test method according to the present embodiment, the number of samples can be significantly reduced as compared with the conventional case.

Subsequently, coefficients _{b 0, b 1, b 2} , ... b n and the parameter _A, illustrating the relationship between a 0 ~a _n. When n is odd, cos ⁿ θ is represented by equation (4a), and when n is even, cos ⁿ θ is represented by equation (4b).

式（５）は、ａ_０〜ａ_ｎを未知数とするｎ元連立１次方程式であるから、ｂ_０〜ｂ_ｎおよびＡが与えられると、未知数ａ_０〜ａ_ｎ、すなわちＡ／Ｄコンバータ２０の入出力特性を得ることができる。 When n is an odd number, the coefficients b ₀ , b ₁ , b ₂ , ... b _n are expressed by the equation (5).

Equation (5) _is, a 0 ~a _n since n former simultaneous linear equations and _unknowns, when b 0 ~b _n and A are given, unknown _a 0 ~a _n, i.e. A / D converter 20 Input / output characteristics can be obtained.

When n is an even number, the coefficients b ₀ , b ₁ , b ₂ , ... b _n are represented by the equation (6).

Equation _(6), a 0 ~a _n since n former simultaneous linear equations and _unknowns, when b 0 ~b _n and A are given, unknown _a 0 ~a _n, i.e. A / D converter 20 Input / output characteristics can be obtained.

As an example, from the measured spectrum, the intensity b ₁ and b ₃ of the fundamental wave and third harmonic (third harmonic) is nonzero, the strength of the remaining frequency was zero. In this case, the coefficient of input / output characteristics can be calculated from the following formula.
a ₁ = (b ₁ + 3b ₃ ) / A
a ₃ = 4b ₃ / A ³

From the spectrum measured as another, fundamental wave, the third harmonic (third harmonic), 5 intensity b ₁ of harmonic (fifth _harmonic), b _3, b ₅ is a non-zero, the remaining frequency Assume that the intensity is zero. In this case, the coefficient of input / output characteristics can be calculated from the following formula.
_{a 1 = (b 1 + 3b} 3 -5b 5) / A
a ₃ = (4b ₃ + 20b ₅ ) / A ³
a ₅ = 16b ₅ / A ⁵

FIG. 4 is a block diagram of the test apparatus 100 according to one embodiment. The A / D converter 20 includes a ΔΣ modulator 22 and a digital filter 24. The sampling rate of the A / D converter 20 of this type may be very slow, about several sps. In such a case, the DUT 10 including the A / D converter 20 is provided with the test pin 12. The test pin 12 is connected to the output of the ΔΣ modulator 22, which is an internal node of the A / D converter 20, and can measure a digital signal bypassing the digital filter 24.

The waveform acquisition unit 120 receives the digital signal S2 of the intermediate node 26, which is the output of the ΔΣ modulator 22, and captures it as waveform data. The sampling rate of the output of the ΔΣ modulator 22 is sufficiently higher than the sampling rate of the entire A / D converter 20 passing through the digital filter 24. For example, when the sampling rate of the DUT 10 is several sps (for example, 6.8 sps), the sampling rate of the ΔΣ modulator 22 alone is several kps to several tens kps (for example, 32 kps).

The operating speed of the ΔΣ modulator 22 and 32ksps ⁽⁼ 2 15 sps). It is assumed that a 1 Hz sine wave is input to the input of the ΔΣ modulator 22 and the waveform is captured for 8 seconds by the waveform acquisition unit 120. Sample number this time is 32 ksps × 8s = 262000 pieces ^{(2 18),} and the frequency resolution of the DFT becomes 0.125Hz.

In the conventional testing method, the number of samples is because it is 2 ^24, in this example, are able to reduce the 1/2 ⁶ times.

Also with respect to the measurement time, in the past, because of the need to get ^{2 24} data at a rate of 6.8Sps, and thus requires a 685 hour ⁽²²⁴ /6.8 seconds) time. On the other hand, in the present embodiment, it can be shortened to 8 seconds. If the frequency of the sine wave is further increased, the measurement time will be further shortened.

Subsequently, a logical analysis when the test method according to the present embodiment is applied to a delta-sigma modulation type A / D converter will be described.

FIG. 5 is a block diagram showing a simulation model 30 of the ΔΣ modulator 22. The simulation model 30 of the delta-sigma modulator 22 includes a subtractor 32, an integrator 34, a comparator 36, and a D / A converter 38. In this model, the distortion model 40 of the A / D converter 20 is introduced between the subtractor 32 and the integrator 34. In this model, the following equation holds.

The error signal En (n), which is the output of the subtractor 32, is represented by the equation (7).
E (n) = Vin (n) -Vf (n) ... (7)

Further, the strain model 40 is assumed to include a third-order nonlinear term, and its input / output characteristics are represented by the equation (8). The parameter k _j represents the magnitude of the j-th order nonlinearity (where j is an odd number of 3 or more).
Vm (n) = E (n) + Σ (k _j * E (n) ^j )… (8)

The output Vo of the integrator 34 is represented by the equation (9).
Vo (n) = Vo (n-1) + Vm (n) ... (9)

The output Dout of the comparator 36 and the output Vf of the D / A converter 38 are represented by the equations (10a) and (10b).
(I) When Vo (n)> 1 Dout (n + 1) = 1, Vf (n + 1) = 1 ... (10a)
(Ii) When Vo (n) <1, Dout (n + 1) = 0, Vf (n + 1) = -1 ... (10b)

Next, the simulation results will be described.
First, the simulation results considering only the third-order nonlinearity will be described. That is, k ₃ ≠ 0 and k _{j ≠ 3} = 0.
(I) DC simulation In the DC simulation, the input voltage Vin is swept as a DC signal in the range of −1.0 to +1.0. FIG. 6 is a diagram showing a simulation result of input / output characteristics of the simulation model 30 of the ΔΣ modulator 22 of FIG. The magnitude of the output Vout is the number of bits whose value is 1 in the bitstream of n (= 220) output Vouts. The parameter k has six values of 0.0001, 0.0005, 0.001, 0.005, 0.01, and FIG. 6 shows six input / output characteristics.

FIG. 7 is a diagram showing coefficients when the six input / output characteristics of FIG. 6 are polynomial-approximated in the third order. Distortion model in the simulation model 30 of FIG. 5 40, with respect to input and output characteristics of the ΔΣ modulator 22, it is found to affect _{a 1} and _{a 3.}

(Ii) AC simulation In the AC simulation, the input voltage Vin is given as the cosine wave of the equation (11) with the amplitude A as a parameter.
Vin = Acos (ω ₀ t)… (11)
However, A = 0.1 to 1.0

FIG. 8 is a diagram showing a power spectrum obtained by FFTing the waveform of the output Dout of the simulation model 30 of FIG. The horizontal axis shows _{the frequency normalized by the fundamental wave ω 0} , and the vertical axis shows the electric power. A =.

When considering the third-order nonlinear term, the intensities b ₁ and b ₃ of the fundamental wave and the third harmonic of the power spectrum are expressed by Eq. (12).

Equation (13) is obtained by modifying equation (12). From this equation (13), can be calculated coefficients _{a 1} and _{a 3.}

FIG. 9 is a diagram showing an error between the estimated values of the _{coefficients a 1} and a ₃ obtained from the FFT result (Equation (13)) _{and the theoretical values of the coefficients a 1} and a _{3 shown in FIG.} The amplitude A, by choosing as large as possible within the input range of ΔΣ modulator 22, the error of the coefficients a ₁ and a _3, has been shown to be estimated with high accuracy.

The present invention has been described above based on the embodiments. This embodiment is an example, and it will be understood by those skilled in the art that various modifications are possible for each of these components and combinations of each processing process, and that such modifications are also within the scope of the present invention. is there. Hereinafter, such a modification will be described.

(Modification example 1)
In the embodiment, a sine wave is used as the analog test signal S1, but this is not the case. For example, the analog test signal S1 may be a rectangular wave, a triangular wave, or the like. Periodic signals such as square waves and triangle waves can be Fourier series expanded. When the periodic signal is an even function, it can be expressed by the following equation.
x = Σ _{i = 0: n} c _i · cos (2πft)… (14)

By substituting the equation (14) into the equation (1) and rearranging, the waveform of the output signal of the A / D converter 20 is a sine wave having a different frequency (f, 2f, 3f ...) As shown in the equation (3). Expressed as a linear sum of. As with the case where the analog test signal S1 and a sine wave of a single frequency, and intensity of each frequency component, the input-output characteristic of the A / D converter 20 (a 0 _{~a n)} is from a correlation, spectral information The input / output characteristics can be estimated from.

(Modification 2)
In the embodiment, a single frequency sine wave is used as the analog test signal S1, but a multitone sine wave may be used. In this case, the amplitude A may be different for each frequency. This makes it possible to increase the amount of information that can be obtained in one measurement.

Although the present invention has been described using specific terms and phrases based on the embodiments, the embodiments show only one aspect of the principles and applications of the present invention, and the embodiments are claimed. Many modifications and arrangement changes are permitted within the range not departing from the idea of the present invention defined in the scope.

10 DUT
20 A / D converter 22 ΔΣ modulator 24 Digital filter 26 Intermediate node 100 Test device 110 Waveform generator 120 Waveform acquisition unit 130 Evaluation device S1 Analog test signal S2 Waveform S3 Waveform information

Claims (8)

A test device for a semiconductor device having an A / D converter.
A waveform generator that supplies a periodic analog test signal to the A / D converter,
A waveform acquisition unit that captures the waveform of the output node of the A / D converter corresponding to the analog test signal or the waveform of the internal node of the A / D converter.
An evaluation device that performs discrete Fourier transform on the waveform acquired by the waveform acquisition unit and estimates the characteristics of the A / D converter based on spectral information.
A test device comprising. The test apparatus according to claim 1, wherein the evaluation apparatus calculates a coefficient when the input / output characteristics of the A / D converter are polynomial-approximated based on the spectral information. The test apparatus according to claim 1 or 2, wherein the analog test signal is a sine wave. The A / D converter includes a delta-sigma modulator and a digital filter.
The test apparatus according to any one of claims 1 to 3, wherein the waveform acquisition unit captures a waveform of an output node of the ΔΣ modulator. A one-packaged semiconductor device equipped with an A / D converter including a delta-sigma modulator and a digital filter.
The ΔΣ modulator is provided with a test pin drawn from a connection node of the digital filter, and the waveform output from the test pin when a periodic analog test signal is input to the A / D converter is used. A semiconductor device characterized in that an A / D converter can be evaluated. A test method for a semiconductor device having an A / D converter.
A step of supplying a periodic analog test signal to the A / D converter,
A step of capturing the waveform of the output node of the A / D converter corresponding to the analog test signal or the waveform of the internal node of the A / D converter.
A step of discrete Fourier transforming the acquired waveform and estimating the characteristics of the A / D converter based on the spectral information.
A test method comprising. The test method according to claim 6, wherein the step of estimating the characteristics includes a step of calculating a coefficient when the input / output characteristics of the A / D converter are polynomial-approximated based on the spectral information. .. The test method according to claim 6 or 7, wherein the analog test signal is a sine wave.

Images