JP2758726B2 - A / D conversion circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D converter.
In particular, the present invention relates to an oversampling type A / D conversion circuit configured to suppress fluctuation of the S / N ratio in a band.

[0002]

2. Description of the Related Art Conventionally, in an A / D conversion circuit of an oversampling system such as a ΔΣ modulation system, a phenomenon that a noise component is concentrated in a signal band is one of the major factors that fluctuates an S / N ratio in a conversion output. is there. The concentration of the noise component in the signal band occurs at a specific frequency determined by the input signal level and the input signal frequency.
As a countermeasure, a method of superimposing a dither signal on an input signal has been used. By adding the dither signal to the input signal, noise components are dispersed, the noise level in the signal band is reduced, and the S / N ratio in the signal band is improved.

In this case, a signal used as a dither signal is a rectangular wave having a constant frequency, and the frequency is set outside the signal band. As described above, by setting the dither signal, the A / D conversion circuit of the oversampling method outputs a signal through a low-pass filter when converting to a desired sampling rate. The dither signal set out of band is
It is completely removed outside the signal band.

[0004]

In the conventional oversampling A / D converter described above, the dither signal superimposed on the input signal is a rectangular wave whose frequency is kept constant. However, depending on the frequency of the input signal, the effect of the dither signal is reduced, and the effect of improving the S / N ratio in the output signal is not obtained.
Further, there is a disadvantage that the dynamic range of the A / D conversion circuit itself is narrowed due to the superposition of the dither signal.

[0005]

According to an A / D converter of the present invention, an A / D converter of an over-sampling method has a pseudo pulse width as a dither signal added to an input signal to be A / D converted. configured with a de I the signal generating means for outputting a signal formed by differentiating the random square wave.

[0006]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an oversampling type A / D conversion circuit according to the present invention.
As shown in FIG. 1, this embodiment includes an oversampling type A / D conversion circuit 1 including subtraction circuits 2 and 4, integration circuits 3 and 5, a comparator 6 and a delay circuit 7, and a polynomial counter 9. , A dither generating circuit 8 including a differentiating circuit 10 and a D / A converter 11, and an adding circuit 12.

In FIG. 1, a polynomial counter 9 in a dither generating circuit 8 is constituted by a shift register of about 4 to 8 bits, and one bit is output from any of the shift registers and input to a differential street 10. Is done. In the D / A converter 11, the differential output of the differentiating circuit 10 is D / A converted, a dither signal 103 is output, and the dither signal 103 is input to the adding circuit 12. Note that the clock signal in the polynomial counter 9 and the differential street 10 is
A clock signal having the same frequency as the sampling frequency of the A / D conversion circuit 1 is used. The peak value of the pulse wave forming the dither signal 103 output from the D / A converter 11 is １ ／ to 1/1 of the maximum amplitude level of the input signal 101.
It is set to about 8.

In the adder circuit 12, the input signal 101
And the dither signal 103 are added, and the sampling frequency in the adding circuit 12 is set to a frequency several tens to several hundred times the predetermined sampling frequency determined by the so-called sampling theorem.

The signal obtained by differentiating the dither signal 103 output from the dither generation circuit 8 and having a pulse width of a pseudo-random rectangular wave has a pseudo random pulse width.
Since various signals are included as frequency components and the frequency spectrum is spread, the effect of the dither signal depends on the frequency of the input signal 101 as in the case of using a rectangular wave of a constant frequency as the dither signal. The above-mentioned drawback of reducing the S / N ratio and reducing the effect of improving the S / N ratio is solved.

The clock signal of the differentiating circuit 10 is represented by A
Since the same frequency as that of the clock signal of the / D conversion circuit 1 is set, a signal having a large amplitude obtained by adding the input signal 101 and the dither signal 103 is input to the A / D conversion circuit in the addition circuit 12. The period is only one sampling period, and in the next sampling period, the dither signal 10
The level of 3 is the level of zero or the opposite sign. Therefore, a large-amplitude signal obtained by adding the dither signal and the input signal is not continuously input to the A / D conversion circuit 1, and the deterioration of the dynamic range due to the superposition of the dither signal is avoided.

The subtraction circuits 2 and 4 and the integration circuit 3
And 5, a comparator 6, and a delay circuit 7. The super-sampling A / D conversion circuit 1 outputs an output signal 102 obtained by A / D conversion of the input signal 101. The operation of the circuit 1 is the same as that of the conventional example, and the description is omitted.

[0013]

As described above, according to the present invention, a signal obtained by differentiating a rectangular wave having a pseudo-random pulse width is superimposed on an input signal as a dither signal, thereby reducing the deterioration of the S / N improvement effect due to the dither signal. In addition to this, there is an effect that the reduction of the dynamic range caused by the superposition of the dither signal can be avoided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 A / D conversion circuit 2, 4 Subtraction circuit 3, 5 Integration circuit 6 Comparator 7 Delay circuit 8 Dither generation circuit 9 Polynomial counter 10 Differentiation circuit 11 D / A converter 12 Addition circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03M 3/00 H03M 1/04 H03M 1/12

Claims (1)

(57) [Claims] 1. An oversampling A / D conversion circuit outputs a signal formed by differentiating a pseudo random rectangular wave with a pulse width as a dither signal to be added to an input signal to be A / D converted. a / D converter circuit characterized by comprising a de I the signal generating means for.