JPH0793580B2 - Noise shaping DA converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an oversampling D / A converter using a delta-sigma modulation system.

[0002]

2. Description of the Related Art Conventionally, a DA conversion circuit using a delta-sigma modulation system has been known as means for converting digital data into analog data. This D-A conversion circuit uses, for example, 1-bit or low-bit by inputting over-sampling delta-sigma modulation of input digital data, as described in, for example, Radio Technology, July 1989, 39p. Converted to digital data, and this digital data is converted to analog LPF (low pass filter)
To convert to analog data.

FIG. 1 shows a circuit diagram of a first-order delta-sigma modulation circuit. In the figure, a is an adder for inputting a digital input signal, and the output of the adder a is input to a quantizer b composed of a 1-bit or low-resolution comparator and a first latch c, The output of the first latch c is input to the adder a. The output of the quantizer b is input to the second latch d, and the output of the second latch d is sign-inverted and input to the adder a.

Next, the transfer function of the circuit of FIG. 1 is calculated using the z variable. First, assuming that the input to the quantizer (comparator) b is A (z), X (z) −z ⁻¹ · Y (z) + z ⁻¹ · A (z) = A (z) A (z) = (X (z) −z ⁻¹ · Y (z)) / (1−z ⁻¹ )

In the comparator b, since the quantization noise Q is added to the input A (z), the output Y (z) becomes Y (z) = A (z) + Q, and the output A (z) By substituting the contents, the following expression (1) is obtained.

Y (z) = X (z) + (1-z ⁻¹ ) · Q (1) X (z): input Y (z): output Q: quantization noise It can be seen that the digital data X (z) is preserved as it is by sigma modulation and the quantization noise is modulated by (1-z ^-1 ).

Here, from z ^-1 = exp (-jωT), | 1-z ^-1 | ² = 2 (1-cos (ωT)) ω: angular frequency T: oversampling period. Next, the relationship between this | 1-z ^-1 | and frequency is shown in FIG.
Shown in. As can be seen from the figure, the delta-sigma modulated quantization noise is low in the low frequency range and high in the high frequency range. That is, most of the quantization noise included in the delta-sigma modulated digital signal is included in the high frequency component. Therefore, if the delta-sigma modulated digital signal is applied to the analog LPF (low pass filter),
The analog LPF removes most of the quantization noise. As a result, the output of the analog LPF becomes a clean analog waveform with less quantization noise.

Next, the oversampling DA is shown in FIG.
1 shows a block diagram of a transformation system and the accompanying frequency spectrum of the signal. In the oversampling D / A conversion system, the sampling frequency of the input signal is relatively low. Therefore, as shown in FIG. 4A, the input signal is first passed through a digital filter so that the signal has the sampling frequency. The aliasing noise is prevented from occurring (see (b) in the figure), and the output signal from the digital filter is then interpolated through an interpolation filter (see (c) in the figure). ), The sampling rate is increased to the sampling frequency of the oversample. Then, when the signal is delta-sigma modulated, the quantization noise is biased to the high frequency side as shown in FIG. Therefore, by passing this output signal through the analog LPF, the quantization noise is removed and a final analog signal is obtained.

Conventionally, a general delta-sigma type modulation circuit is a radio technology, February 1989 issue 88p- "Staggered delta-sigma type 1-bit ADC / DAC simulation and experiment / measurement / waveform observation". It is known that the system does not operate stably in a system of the third order or higher. This is because the loop gain and the frequency characteristic of the phase of the delta-sigma modulation system of the third order or more satisfy the oscillation condition of the feedback control circuit as described in the same document. Therefore, there is a problem that a system having a transfer function higher than that of the third-order delta-sigma modulation system cannot be easily obtained.

[0010]

SUMMARY OF THE INVENTION According to the present invention, the loop gain and phase frequency characteristics of a delta-sigma modulator do not satisfy the oscillation condition, and the conventional second-order delta-sigma modulation DA conversion is performed. A noise shape with more characteristics than the system
It is an object of the present invention to provide a Ping DA converter with a simple structure.

[0011]

The structure of the noise shaping D / A converter of the present invention integrates a digital input signal, quantizes the integration result with a 1-bit or low-resolution comparator, and further The quantization result is passed through a latch, and a negative feedback is applied to the integrator to perform integration, which results in a noise shape.
In a DA converter that obtains a ping characteristic, a digital input signal is input to a first adder, and the output of the first adder is input to a second adder and a first latch, Output of the second latch is input to a third adder, the output of the second adder is connected to a 1-bit or low resolution quantizer, and the output of the quantizer is input to the second latch. , The output of the second latch is sign-inverted and input to the third adder, and the output of the third adder is input to a first digital filter and a second digital filter, The output of the first digital filter is input to the second adder, and the output of the second digital filter is input to the first adder.

[0012]

According to the present invention, the transfer functions of the first digital filter and the second digital filter can be set freely in the present invention. Therefore, if these transfer functions are appropriately selected, the conventional quadratic function can be obtained. It is possible to easily provide a noise shaping D / A converter that has characteristics more than those of the delta-sigma modulator and operates stably.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIGS.

FIG. 4 shows a noise shape according to an embodiment of the present invention.
2 shows a circuit configuration of a Ping DA converter. In the figure,
Reference numeral 1 is a first adder to which a digital input signal is input, and the output of the first adder 1 is input to the second adder 2 and the first latch 3. The output of the first latch 3 is input to the third adder 4 and the second adder 2
Is connected to the quantizer 5 which is a 1-bit or low-resolution comparator. The output of the quantizer 5 is input to the second latch 6, and the output of the second latch 6 is sign-inverted and input to the third adder 4.

The output of the third adder 4 is input to the first digital filter-7 and the second digital filter-8, and the output of the first digital filter-7 is the second digital filter-7. It is input to the adder 2 and the output of the second digital filter-8 is input to the first adder 1. Next, the transfer function in the z region of the above circuit is obtained. If the output from the adder 4 is A (z), then z ⁻¹ · (A (z) · H 2 (z) + X (z)) − z ⁻¹ Y (z) = A (z) A (z)・ (1-z ^-1・ H 2 (z)) = z ^-1・ (X (z) -Y (z)) A (z) = (z ^-1 / (1-z ^-1・ H 2 (z)) )). (X (z) -Y (z)) (2).

When the output signal is Y (z) and the quantization noise is Q (z), X (z) + A (z) .H2 (z) + A (z) .H1 (z) + Q (z) since = a Y (z) ... (3) , (3) substituting (2) into equation, X (z) + Q ( z) · (1-z -1 · H2 (z)) / (1 + z ⁻¹ · H1 (z)) = Y (z) (4) is obtained. Here, the transfer function H2 (z) = 1, H1
If (z) = 0, the transfer function of the circuit is equal to a first-order delta-sigma modulator, and the transfer function H2 (z) = 2-z ^-1 ,
If H1 (z) = 0, then the transfer function of the circuit is equal to a second order delta-sigma modulator.

Here, the delta-sigma DA modulator is
DSP (digital signal processor) can be configured from input to output all in digital system.
If H1 (z) and H2 (z) are realized by using, for example, the specifications of the transfer functions H1 (z) and H2 (z) can be changed later. In the present invention, the digital filter-H
Since the functions of 1 (z) and H2 (z) can be freely selected independently, a new loop is used instead of the loop gain (1-z ^-1 ) ⁿ of the conventional delta-sigma DA modulation circuit. A delta-sigma DA modulation circuit having a gain can be easily constructed. For example, as a new loop gain, (1-z ⁻¹ ) ² /
If (1 + 1 / 2z ^-1 ) is selected, the circuit configuration is as shown in FIG. In this case, the transfer function H1 of the digital filter -7 and 8 (z), H2 (z ) ^{are, (1-z -1 · H2} (z)) / (1-z -1 · H1 (z)) = ( From 1-z- ¹ ) ² / (1 + 1 / 2.z- ¹ ), H2 (z) = 2-z- ¹ and H1 (z) = 1/2.

FIG. 5 shows the frequency characteristic of the new transfer function and the frequency characteristic of the transfer function so far. As is clear from the figure, the new transfer function has a greater effect of noise shaping in the low frequency region than the conventional transfer functions.
Quantization noise can be kept smaller than in conventional delta-sigma modulation systems.

Since the transfer function of the quantization noise of the conventional n-th order delta-sigma modulation is (1-z ^-1 ) ⁿ ,
The squared amplitude is | (1-z- ¹ ) ⁿ | ² = ²ⁿ (1-cos ([omega] T)) ⁿ (5).

Here, the equation (5) is expressed by ω regardless of the value of n.
It is always 1 at T = π / 3. That is, the quantization noise is ω
= 2π · f = π / 3T, it means that the frequency component higher than f = 1 / 6T increases and the component lower than f = 1 / 6T decreases. Therefore, the cutoff frequency of the analog LPF provided at the output of the delta-sigma modulator is f = 1.
The effect of noise shaping of the delta-sigma modulator cannot be expected sufficiently unless it is set to / 6T or less. However, since the new transfer function of the present invention has a squared amplitude of 1 at a frequency of f = 1 / 6T or more, the cutoff frequency of the analog LPF can be set higher than that of the conventional transfer function.
The new transfer function does not oscillate because the phase moves only from 180 degrees to 0 degrees, as confirmed by the digital simulation result of the circuit shown in FIG.

[0021]

As described above, according to the present invention,
In a delta-sigma modulation noise shaping D / A converter, two digital filters are used as constituent elements, and the transfer functions of these two digital filters are appropriately selected to obtain loop gain and phase frequency characteristics to some extent. It is possible to configure a noise shaping D / A conversion system which can be set freely and has characteristics higher than those of the second-order delta-sigma modulator.

Further, at that time, there is an effect that the cutoff frequency of the analog low-pass filter provided at the final output stage can be set to a high band.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a circuit of a delta-sigma modulator.

FIG. 2 is a diagram showing frequency characteristics of an amplitude value | 1-z ^-1 | of a transfer function of quantization noise of a first-order delta-sigma modulator.

FIG. 3 is a block diagram of an oversampling DA converter and a schematic diagram showing a signal spectrum at each output point.

FIG. 4 is a circuit diagram showing a circuit of a delta-sigma modulator according to the present invention.

[5] second order delta - the quantization noise of the sigma modulator transfer function ^{| (1-z -1) 2} | of the frequency characteristics and the transfer function of the quantization noise in the present invention | (1-z ^-1 ) ² / (1
It is a comparison diagram with the frequency characteristic of + 1/2 · z ⁻¹ ) |.

FIG. 6 shows that the transfer function of quantization noise is (1-z ⁻¹ ) ² /
FIG. 3 is a circuit diagram showing a circuit configuration example in the present invention of a delta-sigma modulator that is (1 + 1/2 · z ⁻¹ ).

FIG. 7 shows various input signal and output signal waveforms as a result of digital simulation of the noise shaping D / A converter according to the present invention.

[Explanation of symbols]

 1 1st adder 2 2nd adder 3 1st latch 4 3rd adder 5 quantizer 6 2nd latch 7 1st digital filter -8 2nd digital filter-

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seikatsu Maruyama 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-2-244910 (JP, A) JP-A-4-244 357717 (JP, A) Japanese Patent Sho 51-20148 (JP, B2)

Claims (1)

[Claims] 1. A digital input signal is integrated, the integration result is quantized by a 1-bit or low-resolution comparator, the quantized result is passed through a latch, and negative feedback is applied to the integrator to perform integration. In the D / A converter which obtains the noise shaping characteristic, the digital input signal is input to the first adder, and the output of the first adder is input to the second adder and the first latch. , The output of the first latch is input to a third adder, the output of the second adder is connected to a 1-bit or low resolution quantizer, and the output of the quantizer is the second Input to the latch, the output of the second latch is sign-inverted and input to the third adder, and the output of the third adder is supplied to the first digital filter and the second digital filter. Is input, and the first digital Filter - output is input to the second adder, said second digital filter - Noizushe output of and wherein the input to the first adder - ping D-A converter.