JPH0191533A - A/d converter - Google Patents

Abstract

PURPOSE:To prevent deterioration of S/N by providing a means generating a 1-bit white noise and a means superimposing an input with a difference at two sampling point of times with consecutive one-bit white noise. CONSTITUTION:A delta modulation circuit A is added with a 1-bit white noise generating means 8, a D flip-flop 10 retarding the output 9 by one sampling time, an inverter 11, and an adder 12. Then the output of the white noise generating means 8 and the output of the inverter 11 are added to form a difference between two consecutive sampling points of time at the output of the white noise generating means 8 is formed as a signal 13 and it is superimposed on the signal from the input terminal 1. Thus, excellent S/N characteristic and output amplitude characteristic even at a small signal are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an A/D converter that encodes a digital signal with a low number of bits at a sampling rate that is considerably faster than the signal band.

[Conventional technology]

Conventionally, when converting a low-frequency analog signal into a digital signal, it has been usual to perform high-resolution A/D conversion at a sampling frequency that is approximately two to three times the maximum frequency of the analog signal to be converted.

At this time, signal components with a frequency of 1/2 or more of the sampling frequency are mixed into the digital signal as noise, so before conversion, the signal with a frequency of 1/2 or more of the sampling frequency is sufficiently attenuated through a high-precision band-limiting filter. A/D conversion has been performed. However, it is extremely difficult to fabricate this band-limiting filter on an integrated circuit with high precision. Therefore, in recent years, attempts have been made to use a simple A/D converter to perform A/D conversion at a sampling frequency sufficiently higher than the signal frequency, and then use a digital filter to extract only the signal within the signal band. Digital filters have good accuracy and very good reproducibility if the clock frequency is stable. A delta modulation method is known as an A/D conversion method for this purpose.

The delta modulator is indicated by the dashed line A in the block diagram shown in FIG.

It is constructed by connecting a predictor 4, means 5 for taking the difference between the input signal from the terminal 1 and the output signal from the predictor, and a comparator 6 for determining whether the output of the difference taking means 5 is positive or negative. An integrator is usually used as a predictor. This integrator may be constituted by a digital circuit, in which case a D/A converter is required between the digital integrator and the means for taking the difference. Since such an A/D converter has a high sampling frequency, it is possible to achieve a high S/N ratio when evaluated in the finally obtained signal band even if the resolution is coarser than in conventional systems. However, it is known that when the input signal is small, the S/N and input/output amplitude characteristics deteriorate due to coarse resolution. Conventionally, this has been achieved by applying a constant DC bias to the input, or by applying a sine wave or square wave with a frequency of 1/2P (P is an integer) and higher than the signal band relative to the sampling rate 1. It is known that deterioration can be prevented.

[Problem that the invention seeks to solve]

The problem with the conventional A/D converter described above is that, according to the method of adding a signal as described above, the signal-to-noise ratio deteriorates significantly at a certain input signal amplitude determined by the amplitude and frequency of the added signal. The problem is that the performance required for an A/D converter cannot be satisfied.

Therefore, an object of the present invention is to provide a means for eliminating such conventionally known deterioration and realizing a delta modulator having good S/N characteristics and input/output amplitude characteristics even for small signals on an integrated circuit with high reproducibility. It's about doing.

[Means for solving problems]

The present invention provides a delta signal having a terminal for inputting an analog signal, means for predicting the input signal from a converted digital output, and means for comparing the input signal and the output of the predicting means to obtain a digital output. A modulation type A/D converter, comprising means for generating 1-bit white noise and means for superimposing the difference between the values of the 1-bit white noise at two consecutive sampling points on the input. It is composed of

〔Example〕

The structure and operation of an embodiment of the present invention will be explained with reference to FIG. The signal input from terminal 1 is sent to predictor 4.
The adder/subtracter 5 subtracts the signal of the output 3 of the input signal 3, and the comparator 6 determines whether the result is positive or negative, that is, the magnitude of the input signal and the predicted signal is determined. If the input signal is larger, a digital code "1" is output to the terminal 2 from which the output signal is obtained to increase the predicted voltage of the predictor. Here, the D-type flip-flop 7 is a circuit that holds an output of one sampling period. The portion described above is the generally well-known delta modulation circuit A.

A 1-bit white noise generating means 8, a D-type flip-flop 10 that delays the output 9 by 1 sampling time, an inverter 11, and an adder 12 are added to this circuit,
By taking the sum of the output of the white noise generation means 8 and the output of the inverter 11, the difference between the outputs of the white noise generation means 8 at two consecutive sampling points is created as a signal 13, and this is added to the signal 13 from the input terminal 1. Superimpose it on the signal. The data in the signal 13 can take three values, 0 and 1.2, but when superimposed on the input signal, O corresponds to -1, 1 corresponds to 0, and 2 corresponds to 1. When superimposing the output 13, it is most desirable to add it to the signal with a magnitude of 1/2 to 178 of the minimum resolution of the predictor 4 for the output code 1.

There are various types of circuits that generate white noise. As an example, FIG. 2 shows a circuit called a scrambler, which is often used in modems. This circuit adds the exclusive OR of the output of the 6th stage and the output of the 7th stage of the 7-stage shift register 100, and the output of the comparator 6 (i.e., the output of terminal 2).
This can be realized by taking the exclusive OR with the signal and returning it to the input side of the shift register 100, and using this as a white noise source (ie, signal 9).

〔Effect of the invention〕

Application of noise according to this method can be predicted by evaluating the power spectrum of this noise. If the effective power of the amplitude of the noise to be applied now is D2, the sampling period is T, and the angular frequency of the signal is ω, then the power spectrum of signal 13 is expressed as D2s in2 (ωT/2), and the low frequency component is very small. It has a characteristic that the spectrum outside the signal band is large. The inventor decided to pass the output of this A/D converter through a digital filter to obtain only the signal band component, so even if this noise component is added, the noise outside the signal band will be about 3 dB compared to the single child noise. The performance of the A/D converter within the signal band does not deteriorate. On the other hand, by adding this noise component, good S/N characteristics can be achieved even when the input signal is small.

Furthermore, it has the advantage that no depression occurs in the S/N characteristic, which is a problem with conventional methods. Therefore, the present invention has the advantage that it is possible to realize an A/D converter that satisfies various standards.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a block diagram showing the configuration of an example of means for generating white noise of the present invention. 4...Predictor, 6...Comparator, 8...White noise generation means, 100...Shift register, A...Delta modulation circuit.

Claims (1)

[Claims] A delta modulation type A having a terminal for inputting an analog signal, means for predicting the input signal from a converted digital output, and means for comparing the input signal and the output of the predicting means to obtain a digital output. /D converter,
A/
D converter.