JPH0326676Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to switching circuits, especially
This invention relates to a switching circuit that switches analog signals after digital-to-analog conversion in PCM (PuIse Code Modification) related equipment.

A conventional circuit of this type is shown in FIG. In the figure, 1 is a digital-to-analog converter (hereinafter referred to as a DA converter) that converts a predetermined digital signal into an analog signal, and 2 is an analog converter from the DA converter 1 in response to a control signal supplied via a control terminal 3. 4 is a buffer amplifier that receives the output of analog switch 2 as an input; 5 is a low-pass filter (LPF) that interpolates the output signal of buffer amplifier 4 and outputs it as a continuous analog signal. be.

As shown in FIG. 2a, it takes time for the output voltage of the DA converter 1 to stabilize to a correct value corresponding to the digital signal input to the DA converter 1. In order to eliminate the adverse effects of the unstable part of the output voltage a, the analog switch 2 is turned on for a certain period of time (aperture time) _t0 within the sampling period T by the control signal b supplied via the control terminal 3. By doing so,
The output voltage a of DA converter 1 is PAM (PuIse
AmpIitude ModuIation) Convert to wave. Then passed through low pass filter 5 and buffer amplifier 4.
A continuous analog signal can be obtained by interpolating PAM waves.

However, in a switching circuit having such a configuration, since the analog switch 2 is inserted directly into the signal system and cuts the signal in and out, the analog switch 2 generates switching distortion during switching, which worsens the distortion rate of the analog signal. Ta.

The present invention was devised to eliminate the above-mentioned drawbacks of the conventional circuits, and an object of the present invention is to provide a switching circuit that improves the signal distortion rate by compensating for distortion caused by analog switches.

The switching circuit according to the present invention includes a digital-to-analog converter that converts a predetermined digital signal into an analog signal, and a first converter that intermittently passes through and outputs the analog signal by a switching signal that changes high and low levels at a predetermined period. A switching circuit comprising: an analog switch; and an amplifier to which an output signal of the first analog switch is supplied to an input terminal, the switching circuit comprising: a first resistor connected in series with the first analog switch; and switching distortion canceling means connected between the input terminal of the amplifier and a reference potential point, and the switching distortion canceling means is connected in series with a second resistor and a second analog switch controlled on/off by the switching signal. The first and second analog switches have substantially equal characteristics, and the first and second resistors have substantially equal resistance values.

An embodiment of the present invention will be described below with reference to FIG.

In Fig. 3, parts equivalent to those in Fig. 1 are indicated by the same symbols, and a resistor _R1 is connected in series with the analog switch 2, and further between the input terminal of the buffer amplifier 4 and the ground, which is the reference potential point. The analog switch 6 and the resistor _R2 are connected in series. The analog switches 2 and 6 have substantially the same characteristics, ie, resistance changes when turned on, and are controlled on/off by the same control signal (FIG. 2b) supplied via the control terminal 3.
The smaller the aperture time t ₀ of this control signal, the flatter the frequency characteristics.
If the aperture time t ₀ is too small, the S/N
Therefore, it is generally said that it is preferable to select t ₀ =T/4 for the sampling period T. As the buffer amplifier 4, one with high input impedance is used, for example, it is constituted by an operational amplifier with very high input impedance. In addition, resistors R ₁ and R ₂ should have approximately the same resistance value, and the resistance value should be 1/10, preferably 1/100 or less, of the input impedance of the buffer amplifier 4. For example, a value of about 10KΩ has been found in good experiments. Results are being obtained.

Next, the operation of the circuit of the present invention will be explained.

The signal converted into analog by DA converter 1 is switched by analog switches 2 and 6,
The signal is passed through a buffer amplifier 4, interpolated by a low-pass filter 5, and output as a continuous analog signal.

Here, DA converter 1 and buffer amplifier 4
Fig. 4 shows an equivalent circuit when analog switches 2 and 6 are on between analog switches 2 and 6.
The resistance values of and 6 when on are assumed to be Ron ¹ and Ron ² .

However, Ron ₁ and Ron ₂ change depending on the level of the flowing current, so the input voltage is Vi, the output voltage is V ₀ , and the output impedance of DA converter 1, which is assumed to be connected in series with resistor R ₁ , is If r ₀ and the input impedance of the buffer amplifier 4, which is assumed to be connected in parallel to the output voltage V ₀ , are ri, then V ₀ = (R ₂ + Ron ₂ )//ri/ (R ₂ + Ron ₂ )
//ri+(R ₁ +Ron ₁ +r ₀ )Vi...(1) The following formula holds true. Here, the resistance values of resistors R ₁ and R ₂ are almost equal, and the resistance changes when analog switches ₂ and 6 are turned on are almost equal, so R ₁ = R ₂ = R,
ROn ₁ = Ron ₂ = Ron, then V ₀ = (R + Ron) ri / (R + Ron) ri + (
R+Ron+r ₀ )(R+Ron+r) Vi...(2). Further, assuming that the output impedance r of the DA converter 1 is a sufficiently small value with respect to R and Ron, V ₀ =1/2+(R+Ron)/riVi (3). At this time, it can be seen that if the input impedance ri of the buffer amplifier 4 is a sufficiently large value compared to R and Ron, then V ₀ =Vi/2.

On the other hand, the output voltage V ₀ when the analog switches 2 and 6 are turned off can also be expressed as follows: V ₀ = 1/ 2+(R+Roff)/riVi......(3)' Since the off-resistance of switching elements normally used in switching circuits is very high, and Roff is sufficiently large compared to ri,
V ₀ ≒0, and each analog switch operates as a gate. As is clear from the above equation, this is because the distortion generated when analog switch 2 is switched is canceled out by the switching distortion of analog switch 6.

In addition, the characteristics and resistance of analog switches 2 and 6
It is desirable for the resistance values of R ₁ and R ₂ to be completely equal, but it is difficult to obtain completely equal resistance values due to the precision of the parts. Even if there is a deviation, switching distortion can be reduced compared to the conventional one.

As explained above, according to the present invention, the distortion generated when switching the analog switch is configured to be canceled out by the switching distortion of the newly added analog switch, so that the signal distortion rate can be improved and the distortion can be reduced. You can get an analog signal that is not available.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional example, Fig. 2 is a waveform diagram of each part of Fig. 1, Fig. 3 is a circuit diagram showing an embodiment of the present invention, and Fig. 4 is an equivalent of a part of Fig. 3. It is a circuit diagram. Explanation of symbols of main parts 1...DA converter, 2, 6...analog switch, 4...buffer amplifier, 5...low-pass filter.

Claims (1)

[Scope of utility model registration request] a digital-to-analog converter that converts a predetermined digital signal into an analog signal; a first analog switch that intermittently passes through and outputs the analog signal by a switching signal that transitions between high and low levels with a predetermined cycle; a switching circuit comprising: an amplifier whose input terminal is supplied with an output signal of an analog switch; a first resistor connected in series with the first analog switch; and an output terminal of the amplifier and a reference potential point. and a switching distortion canceling means connected between the first and second analog switches, the switching distortion canceling means consisting of a series circuit of a second resistor and a second analog switch controlled on/off by the switching signal, A switching circuit characterized in that the second analog switch has substantially equal characteristics, and the first and second resistors have substantially equal resistance values.