JP2543039B2 - Sample-hold circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sample and hold circuit, and more particularly to a sample and hold circuit that handles audio signals, such as a digital audio device such as a digital audio disc player and a demodulation circuit of an FM stereo tuner. Is.

2. Description of the Related Art In digital audio equipment, a D / A converter is used in the part that converts a digital signal into an analog signal. This D / A converter normally generates switching noise (called glitch) in the analog output signal when the digital input data changes. Therefore, it is usual to place a sample hold circuit in the latter stage of the D / A converter, and it is also called a deglitch circuit because it is a circuit for removing glitches.

Fig. 2 is a timing chart showing the operation of a general deglitching circuit. During the period when the output signal of the D / A converter is stable, the sample and hold circuit is set to sample mode and the D / A
During the period in which the output signal of the converter changes and the switching noise is generated, the sample hold circuit is switched to the hold mode to remove the mixing of the switching noise.

FIG. 3 shows an example of a conventional sample-hold circuit generally used in the above-mentioned deglitch circuit.
11, hold capacitor 12, analog switch 14,
And a resistor that determines the gain in sample mode 17,1
It is composed of 8. The output signal of the D / A converter is connected to the signal source terminal 13, and the sample pulse 15 switches between the sample mode and the hold mode. As mentioned above,
During the period when switching noise is applied to the signal source terminal 13, the analog switch 14 is held (ground side in FIG. 3).
It works like switching to. Such a technique is described in the following documents, for example.

"Radio Technology", May 1986, p. 194, Problems to be Solved by the Invention The sample and hold circuit shown in the conventional example of FIG. 3 is widely used as a deglitch circuit for digital audio disc players and the like. However, due to its circuit configuration,
The capacitor 12 connected as the feedback circuit of the operational amplifier 11 becomes a capacitive load when viewed from the operational amplifier, and the operational amplifier generates distortion (harmonic distortion or cross modulation distortion) even in the audio frequency band, especially in a high frequency range. It was easy to do, and it was a major obstacle to improving the sound quality by hearing.

In view of the above-mentioned problems, the sample-and-hold circuit of the present invention is intended to reduce distortion and improve the audible sound quality by equivalently removing the capacitive load of the operational amplifier. is there.

Means for Solving the Problems A sample and hold circuit of the present invention has a bridge circuit composed of a plurality of (four in the embodiments described later) resistors and a second operational amplifier added to a conventional sample and hold circuit. Configured.

Operation According to the above configuration, the second operational amplifier operates so as to maintain the balance of the bridge circuit, and as a result, most of the current for charging and discharging the capacitor is supplied from the second operational amplifier. As a result, the first operational amplifier can equivalently operate at almost no load.

Practical Example FIG. 1 is a circuit configuration diagram showing a sample hold circuit according to an embodiment of the present invention.

The bridge circuit 1 is composed of four impedance circuits, for example, resistors, and a resistor R _{1 having} a resistance value such as 10Ω, 30Ω, 1KΩ, 3KΩ so that the left side and the right side are balanced.
Set to R ₂ , R ₃ and R ₄ . Reference numeral 2 is a first operational amplifier, and 3 is a second operational amplifier.

Of the four connection points of the bridge, the output terminal of the first operational amplifier 2 and the positive input terminal of the second operational amplifier are connected to the first connection point P ₁ , and the second connection point P ₂ is connected. The output terminal of the second operational amplifier 3 is connected. A holding capacitor 4 is connected between the third connection point P ₃ and the negative input terminal of the first operational amplifier 2, and a negative input of the second operational amplifier 3 is provided at the _fourth connection point P _4. The terminals are connected. Then, the positive input terminal of the first operational amplifier 2 is grounded to form a feedback circuit of the first operational amplifier 2 as in the conventional example.

For example, the output signal of the D / A converter is input to the signal source terminal 5, and the first signal is intermittently output by the analog switch 6.
Is connected to the negative input terminal of the operational amplifier 2. The analog switch 6 is switched by the sampling pulse 7,
Switching between two modes: a sample mode (the signal source is connected to the negative input terminal of the first operational amplifier 2) and a hold mode (the signal source is connected to the ground side of the analog switch 6).

The output signal is taken out from the output terminal 8 connected to the third connection point P ₃ of the bridge circuit 1, and the gain of the entire circuit depends on the resistance 9 and the resistance 9 connected between the signal source terminal 5 and the analog switch 6. Connection point of analog switch 6 and output terminal 8
Determined by a resistor 10 connected between.

The operation of the circuit of the above embodiment will be described. When the analog switch 6 is switched to the sample mode,
The output voltage of the first operational amplifier 2 changes and the capacitor 4
However, since the bridge circuit 1 loses its balance at this time, a current is output from the output terminal of the second operational amplifier 3 and the bridge circuit 1 tries to be rebalanced. The fact that the bridge circuit 1 is balanced means that the output current from the first operational amplifier 2 becomes zero. Therefore, in the balanced state, the first operational amplifier 2 is equivalently unloaded (output current Zero) will work. Second
If the slew rate of the operational amplifier 3 is sufficiently high, the bridge can always be balanced and the second operational amplifier 3 charges and discharges the capacitor 4, and the first operational amplifier 2 Equivalently, it can concentrate on performing accurate waveform transmission in the unloaded state.

FIG. 4 is a comparison of the frequency vs. high frequency distortion rate characteristics of the digital audio disc player using the sample and hold circuit of one embodiment of the present invention with the conventional example. It is possible to obtain a sound quality with excellent resolution in terms of hearing.

Although the above-described embodiment assumes that the application is a deglitching circuit of a digital audio device, it goes without saying that the present invention can be applied to an audio device using a sample hold circuit such as a demodulation circuit of an FM stereo tuner. Yes.

EFFECTS OF THE INVENTION As is clear from the above description of the embodiments, in the sample hold circuit of the present invention, the outputs of the two operational amplifiers are connected by the bridge circuit, and the charge / discharge current of the holding capacitor is calculated by the second operation. By supplying from the amplifier, the first operational amplifier can be equivalently operated in a no-load state, and the occurrence of distortion due to a capacitive load can be reduced. It can be applied to equipment, and can improve distortion and sound quality, and has a great practical effect.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit configuration diagram showing an embodiment of a sample hold circuit of the present invention, FIG. 2 is a timing diagram showing the operation of a deglitch circuit, and FIG. 3 is a conventional sample hold circuit. FIG. 4 is a circuit configuration diagram showing an example, and FIG. 4 is a frequency vs. harmonic distortion factor characteristic diagram showing a distortion factor of a digital audio disk player to which the sample hold circuit of the present invention is applied, compared with a conventional example. 1 ... Bridge circuit, 2, 3 ... Operational amplifier, 4 ... Capacitor, 6 ... Analog switch.

Claims (1)

(57) [Claims] 1. A bridge circuit comprising a plurality of impedance circuits and having first and fourth connection points on a first diagonal line and second and third connection points on a second diagonal line; And a second operational amplifier, the output terminal of the first operational amplifier and the positive input terminal of the second operational amplifier are connected to the first connection point of the bridge circuit, and the second input terminal of the bridge circuit is connected. The output terminal of the second operational amplifier is connected to the connection point, and the capacitor is connected between the third connection point of the bridge circuit and the negative input terminal of the first operational amplifier, and the first terminal of the bridge circuit is connected. The negative input terminal of the second operational amplifier is connected to the connection point of No. 4, a switching element is connected between the negative input terminal of the first operational amplifier and the signal source, and the signal source and the bridge circuit are connected. Connect a resistor to the third connection point. Sample-and-hold circuit configured by.