JPH0795688B2 - DA conversion circuit for audio - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio DA converter circuit.

[0002]

2. Description of the Related Art As shown in FIG. 5, a conventional audio DA converter circuit is an I / V converter circuit (current-voltage converter) in which an analog signal output 1a of a current output DA converter is directly formed by an active element such as an operational amplifier 3. The conversion circuit) was connected, and the low-pass filter 2 was connected to the output of this I / V conversion circuit.

However, it has been found by the inventor's research that such a structure has the following problems. Figure 6
Analog signal output 1a of DA converter as shown in
Since the current of (1) changes stepwise according to the input digital data, the output current contains an unnecessary high frequency component having a large rate of change per time.

The current change rate per hour of the output signal of the DA converter is usually as large as 200 mA / μs or more. For example, in order to obtain a standard output voltage amplitude of the DA conversion circuit, the operational amplifier for the I / V conversion circuit is required. When 1 kΩ is used for the feedback resistor 5, the slew rate of the operational amplifier is 200 V / μs (= 200 mA /
(μs × 1 kΩ) or more, if the output current of the DA converter changes, the output voltage of the operational amplifier 3c
Can not follow accurately.

However, the slew rate of a low-distortion, low-noise operational amplifier generally used for audio is about 50 V / μs at most, and the conventional audio DA conversion circuit has an operational amplifier lacking such a slew rate. Inevitably, it had to be used for an I / V conversion circuit.

[0006]

Therefore, an unnecessary high frequency component contained in the output signal of the DA converter causes a transiently large differential input voltage (3a-3b) at the input of the operational amplifier, which causes a difference between the input and output of the operational amplifier. Of the TIM, which is a cause of generation of distortion represented by TIM distortion. In terms of hearing, there is a drawback that reproducibility of reverberant sound is deteriorated and timbre transparency is lost, and reproducibility of minute level signals is remarkably deteriorated.

The problem to be solved by the present invention is to prevent the occurrence of distortion in the operational amplifier due to unnecessary high frequency components, and at the same time, to secure the flatness of the frequency characteristic in the voice band, and to obtain a DA converter circuit of good sound quality. To provide.

[0008]

According to the present invention, the output of a DA converter is connected to a secondary or higher-order low-pass filter composed only of passive elements such as capacitors, coils and resistors, and the output of the low-pass filter is connected to an operational amplifier. Connect to the input of. Then, the output of the operational amplifier is used as the output of the DA conversion circuit.

[0009]

[Function] Since the unnecessary high-frequency component contained in the output signal of the DA converter is attenuated by the low-pass filter and then input to the operational amplifier, the slew rate of the operational amplifier does not have to be large, and distortion and audible reproducibility are not required. Deterioration can be prevented, and at the same time, since the low-pass filter is second-order or higher, it is possible to achieve both flatness of frequency characteristics within the voice band by adopting Butterworth characteristics or the like.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention, in which 1 is a DA converter and 2 is a low-pass filter composed of only passive elements, which is of the fifth order. Reference numeral 3 is an operational amplifier, 4 is a resistor, 5 is a feedback resistor, and 6 is an output of the DA conversion circuit.

With such a configuration, as shown in FIG. 2, the unnecessary high frequency component contained in the analog signal output 1a of the DA converter is sufficiently attenuated in advance by the fifth-order low-pass filter 2 and then the non-inversion of the operational amplifier is performed. Since it is input to the input 3a, the output voltage 3c of the operational amplifier (the same as the output 6 of the DA conversion circuit) can sufficiently follow the change of the input signal even if the slew rate of the operational amplifier is not so large. Large transient input differential voltage (3a-3b)
It is possible to prevent distortion from occurring in the output of the operational amplifier due to an unnecessary high frequency component and insufficient slew rate of the operational amplifier.

Further, since it is a second or higher order low-pass filter, the cut-off characteristic for a large high-frequency component and the flatness of the frequency characteristic in the voice band can be made compatible by appropriately selecting the characteristics of the low-pass filter. Furthermore, because it is a low-pass filter that is composed of only passive elements, unlike active-type low-pass filters that are composed of active elements such as operational amplifiers, the attenuation characteristics of unnecessary high-frequency components due to insufficient loop gain and insufficient slew rate in the high frequency range Problems such as deterioration and TIM distortion do not occur.

In the case of the first embodiment, the operating conditions of the DA converter are as follows: output current amplitude ± 2 mA, sampling frequency fs
= 44.1 KHz, 8 times oversampling, the low pass filter is a Butterworth type with a cut-off frequency of 40 KHz, and the output voltage amplitude of the DA conversion circuit is ± 2 V, the frequency characteristic in the voice band at the output 6 of the DA conversion circuit is It is sufficiently flat within +0, -0.01 dB at DC to 20 KHz, and it is sufficient to secure the attenuation characteristic of the unnecessary high frequency component at -85 dB or more at 7fs = 308.7 KHz. Further, the slew rate required for the operational amplifier at this time is about 0.5 V / μs, which is considerably smaller than that in the case of the conventional technique, which is a range that can be sufficiently covered by a general audio operational amplifier.

FIG. 3 shows an example in which the order of the low-pass filter is changed to the third order in the embodiment according to the present invention. FIG. 4 shows an example in which the inverting input of an operational amplifier is used by using a fourth-order low-pass filter.

In the above embodiments, the amplifier is described as an operational amplifier, but it may be replaced with an amplifier composed of a set of discrete discrete components including active elements such as transistors. In addition to the general Butterworth characteristic, various characteristics such as Bessel characteristic, Chebyshev characteristic, and simultaneous Chebyshev characteristic can be considered for the characteristics of the low-pass filter.

[0016]

As described above, since the output of the DA converter is connected to the second or higher order low-pass filter composed of only passive elements, unnecessary high frequency components output from the DA converter together with the audio signal are attenuated. After that, it can be input to the amplifier, and the active amplifiers such as operational amplifiers will not be distorted by unnecessary high frequency components, and at the same time, the flatness of the frequency characteristic in the voice band can be sufficiently secured, and for audio with good sound quality. It can be a DA conversion circuit.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a DA converter circuit according to the present invention.

FIG. 2 is a waveform diagram of the first embodiment.

FIG. 3 is a block diagram showing a second embodiment of the DA converter circuit according to the present invention.

FIG. 4 is a block diagram showing a third embodiment of the DA converter circuit according to the present invention.

FIG. 5 is a block diagram of a conventional DA conversion circuit.

FIG. 6 is a waveform diagram of a conventional DA converter circuit.

[Explanation of symbols]

 1 DA converter 1a DA converter analog signal output 2 Low-pass filter 3 Operational amplifier 3a Operational amplifier non-inverting input 3b Operational amplifier inverting input 3c Operational amplifier output 4 Resistor 5 Feedback resistor 6 DA converter output

Claims (1)

[Claims] 1. An analog signal output of a current output type DA converter is connected to a low-pass filter of a secondary or higher order composed of only passive elements, and the output of the low-pass filter is a separate component including an operational amplifier or a transistor. An audio DA conversion circuit characterized by connecting an amplifier constituted by a set.