JPH0334628A - Over-sampling a/d converter - Google Patents

Abstract

PURPOSE:To improve the DC conversion characteristic without spoiling a high speed property by bringing a square wave signal from which an offset voltage is eliminated to sampling by a high frequency, and eliminating other component than a signal frequency band to be converted finally. CONSTITUTION:A square wave modulating circuit 6 containing a synchronous ground amplifier outputs a square signal which has a frequency of two folds or more of a band of an input signal and has amplitude being proportional to amplitude of the input signal and does not contain an offset error, to an A/D converter 3. The A/D converter 3 brings this square wave signal to sampling by a higher frequency than a frequency of this square wave signal and outputs it to a digital filter. Subsequently, the digital filter 4 eliminates other component than a signal frequency band to be converted finally from output data of this A/D converter 3. In such a way, the time required for the zero correction becomes unnecessary, and a DC conversion characteristic is improved without spoiling a high speed property.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to oversampling A/D converters, and specifically relates to improvement of DC characteristics.

<Prior art> An oversampling A/D converter is a low-resolution A/D converter.
By making the sample rate of the converter sufficiently higher than the signal band, the distribution of quantization noise is shifted to the high frequency region, and the output data is added to a digital filter to remove noise components outside the signal band, thereby reducing the overall S Figure 6 is a block diagram showing an example of such an oversampling A/D converter. V jrl is applied to a low resolution A/D converter 3 via a preamplifier 2 having an anti-aliasing filter function. The output data of the A/D converter 3 is then output to the outside via a digital filter 4.

By the way, when considering the case where such an oversampling A/D converter is used for converting a DC signal, the influence of the offset voltage of the preamplifier 2 and A/D converter 3 cannot be ignored.

Therefore, as shown in FIG. 7, for example, in the front stage of the preamplifier 2, a contact M that receives the voltage V rl of the input terminal 1 circle and a contact Z that receives the voltage Vz from the ground potential point are alternately turned on.
A changeover switch 5 that is turned off is provided, and these voltages VirL
, VZ are taken in alternately to calculate (VinVz), thereby removing the influence of the offset voltage.

<Problems to be Solved by the Invention> However, according to the configuration shown in FIG. It becomes stable after a certain settling time determined according to the characteristics. FIG. 8(a) shows the switching operation waveform of the contacts of the switch 9, and FIG. 8(b) shows the state of change in the output data of the digital filter 4.

As is clear from FIG. 8, since the output data during the settling time of the digital filter 4 cannot be used for data processing, the output rate of the data from which the influence of the offset voltage has been removed will drop significantly, resulting in oversampling. The advantage of the A/D converter, which is its high speed, is no longer utilized.

The present invention has focused on such points, and an object thereof is to provide an oversampling A/D converter that can improve DC conversion characteristics without impairing high speed.

Means for Solving the Problems> The present invention solves the problems described above, and has a frequency that is more than twice the band of the input signal, an amplitude that is proportional to the amplitude of the input signal, and an offset voltage is removed2. A square wave modulation circuit that outputs a square wave signal, an A/D converter that samples this square wave signal at a frequency higher than the frequency of this square wave signal, and a final output signal from the output data of this A/D converter. A digital filter that removes components other than the signal frequency band to be converted into.

<Operation> A square wave modulation circuit including a synchronous grounded amplifier A/D converts a square wave signal that does not include an offset error and has a frequency that is twice or more the band of the input signal and an amplitude that is proportional to the amplitude of the input signal. output to the device.

The A/D converter samples this square wave signal at a frequency higher than the frequency of this square wave signal and outputs it to a digital filter.

The digital filter removes components other than the signal frequency band to be finally converted from the output data of the A/D converter.

This eliminates the need for time required for zero calibration as in the prior art.

<Example> Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, the same parts as in FIG. It is connected to the modulation circuit 6.

This square wave modulation circuit 6 generates a square wave signal V(llt) which does not include an offset error and has a frequency fM that is more than twice the band of the input signal V irL and an amplitude proportional to the width of the input signal V rl. It is output to the A/D converter 3 via the anti-aliasing analog filter 7.

FIG. 2 is a circuit diagram showing a specific example of FIG. 1.

In the figure, a synchronous grounded amplifier is used as the square wave modulation circuit 6. This synchronous grounded amplifier has a voltage V on the input terminal 1 side.
, a changeover switch 8 in which a contact M that takes in the voltage Vz on the ground potential point side and a contact 2 that takes in the voltage Vz on the ground potential point side are turned on and off alternately at a frequency fM that is twice or more the band of the input signal VjrL, and this changeover switch. 8, a series circuit of a capacitor 10 and a resistor 11 connected in series to the output terminal of the amplifier 9, and a movable contact of a switch 8 that switches the end of the series circuit of the capacitor 10 and resistor 11. The switch 12 selectively connects to the ground potential point in synchronization with the switching connection of the contact point Z to the ground potential point. Note that in FIG. 2, the gain of the amplifier 9 is set to double.

FIG. 3 is a timing chart explaining the operation of FIG. 2.

The movable contact of the changeover switch 8 is alternately connected to the contacts Z and M at a frequency fM and a duty of 50%, as shown in FIG. 9A, and outputs a square wave signal to the amplifier 9. The output signal of this changeover switch 8 is input to an amplifier 9 and is amplified twice as shown in FIG. Here, if the offset voltage of the amplifier 9 is VO6, the L level of the square wave signal output from the amplifier 9 will be 2V (1B), and the H level will be 2 (LrL + OS). The output signal is applied to a switch 12 via a series circuit of a capacitor 10 and a resistor 11, and is applied to the offset voltage VO of the amplifier 9.
S is cut. The switch 12 is turned on and off so as to selectively connect to the ground potential point in synchronization with the switching connection of the movable contact of the changeover switch 8 to the contact Z (ll), so that the switch 12 has a rectangular shape of 12 mm. wave output signal vg
is the offset voltage VO9 of the amplifier 9 as shown in (c).
does not include 2 V; only n.

The square wave modulated output signal vout outputted from the square wave modulation circuit 6 in this manner can be expressed as follows, as shown in FIG. When this is Fourier expanded, it becomes, where ω gate = 2πfM.

-VirL = A Sfn (A)
), M-(J:(k<,-w,'):tco ・(3)
become.

Therefore, if ωbar〉2ωB nax, then 2 of equation (2)
The frequency components after the term are all ω (3nax (trust band)
becomes larger than

As a result, the frequency fM is set to satisfy the condition ωbar〉2ωB IIax, and the square wave modulation output signal is
By passing the D-converted signal through a digital filter for an oversampling A/D converter, Va+t (t) = Vtn (t)
-(4), and as a result, the offset error is removed.

FIG. 5 is an example of the frequency characteristics of each part in FIG. 2. In FIG. It shows the components after the second term of the signal. A shows the frequency characteristics of the digital filter 4, and B shows the frequency characteristics of the anti-aliasing filter. This results in
The quantization noise is reduced to the hatched area C.

With this configuration, the DC component of the input signal VirL can be converted into a digital signal with high precision without being affected by the offset voltage.

In the above embodiment, an example was explained in which the gain of the synchronous grounded amplifier constituting the square wave modulation circuit is doubled, but any gain may be used.

In addition, in the above embodiment, an example is shown in which a synchronous grounded amplifier is used as a square wave modulation circuit, but it has a frequency that is more than twice the band of the input signal and an amplitude that is proportional to the amplitude of the input signal, and the offset voltage The amplifier is not limited to a synchronous grounded amplifier as long as it has the function of outputting a square wave signal from which the noise is removed.

Further, the A/D converter may be a ΔΣ A/D converter.

<Effects of the Invention> As explained above, according to the present invention, it is possible to provide an oversampling A/D converter that can improve DC conversion characteristics without impairing high speed, and in particular, it is possible to provide an oversampling A/D converter that can improve DC conversion characteristics without impairing high speed. /D converter.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the first invention;
The figure is a circuit diagram showing a specific example of Fig. 1, Fig. 3 is a timing chart showing an outline of the operation of Fig. 2, Fig. 4 is a waveform diagram explaining the operation of Fig. 2, and Fig. 5 is a FIG. 6 is a block diagram showing an example of a conventional device,
FIG. 7 is a block diagram showing another conventional device, and FIG. 8 is a timing chart explaining the operation of FIG. 7. DESCRIPTION OF SYMBOLS 1... Input terminal, 3... A/D converter, 4... Digital filter, 6... Square wave modulation circuit, 7... Anti-aliasing filter.

Claims (1)

[Scope of Claims] A square wave modulation circuit that outputs a square wave signal having a frequency twice or more the band of an input signal and an amplitude proportional to the amplitude of the input signal, and from which an offset voltage has been removed; An A/D converter that samples this square wave signal at a frequency higher than the signal frequency, and a digital filter that removes components outside the signal frequency band that is ultimately desired to be converted from the output data of this A/D converter. An oversampling A/D converter comprising: