JPH0590964A - A/d converter - Google Patents

Abstract

PURPOSE:To easily and a dither signal to an analog input signal by providing an integration circuit including a time constant circuit to an input section of an analog signal of the A/D converter circuit and applying a ramp potential to the integration circuit via a capacitor of the time constant circuit. CONSTITUTION:A capacitor C01 is inserted between an analog signal input terminal 16 and a ground point and the input terminal is connected to an input of an A/D converter 10 via a resistor R11. One terminal of a capacitor C11 is connected to the input of the A/D converter 10 and the other terminal connects to an output of an integration circuit 60 comprising resistors R21, R22 and capacitors C21, C22. Moreover, a time constant circuit comprising a resistor R11 and the capacitor C11 forms an integration circuit 70 with respect to an analog input signal. Then the A/D converter 10 receives an analog input signal via the integration circuit 70 comprising a time constant circuit consisting of the resistor R11 and the capacitor C11. Furthermore, a dither signal is fed to one terminal of the capacitor C11 forming the integration circuit 70 and added to the analog input signal.

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 that converts an analog signal into a digital signal, and more particularly to an A / D converter that provides a highly accurate digital output with a simple structure.

[0002]

2. Description of the Related Art As a technique for converting an analog signal into a digital signal, an integration type and a successive approximation type are conventionally known. The former counts clock signals at predetermined intervals with a counter, integrates the clock signals to generate a gradient potential, and stops counting by the counter at the moment when the gradient potential becomes equal to the level of the input analog signal. The contents of the counter are digital output.

In the case of the successive approximation type, clock signals at predetermined intervals are counted by a counter, and the contents of the counter are D /
It is converted into an analog signal by the A converter, the value is compared with the input analog signal, the counting by the counter is stopped at the moment when both levels become equal, and the contents of the counter at that time are digitally output. The latter successive approximation type is often used as the A / D converter built into recent microcomputers.

The accuracy of the A / D conversion is determined by the number of digits of the counter and the D / A converter. The A / D converter built in the present 4-bit microcomputer has a counter and a D / A converter of 8 bits. There are many things. The resolution in this case is 0.4%
(= 1/256).

On the other hand, as one application example of the A / D converter, consider the case where the sensor output such as temperature or relative humidity is digitally displayed.
At least 0.1% to display with 1% accuracy
A resolution of (1/1000) is required, and the conventional 8-bit A / D converter is insufficient.

A counter and D / A for increasing the resolution
Increasing the number of digits in the converter increases the cost of the device and is not desirable.

Therefore, as an A / D converter having a resolution exceeding the resolution determined by the number of bits of the A / D converter, a gradient potential is added to the analog input and an arithmetic circuit is connected to the output of the A / D converter. Then, an apparatus for giving an average value of A / D conversions of a plurality of times by the arithmetic circuit and giving an A / D conversion output determined by the resolution of the arithmetic circuit is Japanese Patent Application No. 03-18.
Proposed by 1623.

This technique will be described with reference to FIG.

In FIG. 2, 10 is an 8-bit A / D converter, 12 is an adder, 14 is an arithmetic circuit, 16 is an input terminal for the analog signal A _in , 18 is a 10-bit digital output terminal, and 20 is dither. It is an input signal (gradient potential). The dither input signal is a sawtooth wave or a triangular wave, and its amplitude is A /
The minimum resolution value of the D converter 10 (for example, 10 mV) is set.

The analog input signal and the dither signal 20 are added by the adder 12 and then added by the A / D converter 10.
A / D conversion with bit precision is performed. The operation of A / D conversion is performed a plurality of times (N times) within the dither signal inclination time, and the digital output is cumulatively added by the arithmetic circuit 14 each time. After the completion of A / D conversion and cumulative addition a plurality of times, the arithmetic circuit 14 divides the contents by the numerical value N and outputs the resulting quotient to the output terminal 18 with 10-bit precision. A value of 16 is suitable in this embodiment. That is, the arithmetic circuit 14 calculates the average value of the 16 outputs of the A / D converter.

When the instantaneous value of the dither signal is ΔV, A /
The analog input to the D converter 10 is A _in + ΔV, and Δ
The value of V gradually increases (or decreases) according to the slope of the dither signal.

[0012] 8-bit digital output of the m times is equal N-m times to the digital conversion output of the _{A in (A in) A /} D becomes _{(A in) A / D +1} . The value of Nm (since ΔV varies over the range of minimum resolution) varies depending on which side of the width of the minimum resolution A _in − (A _in ) _{A / D} is. For example the minimum resolution and _{10mV, A in - (A in} ) A / D is 7m
If V, ΔV> 3 mV, the digital output increases by 1, and if 3 mV, it does not increase unless ΔV> 7 mV. That is, the number of times N−m when the digital output becomes (A _in ) _{A / D} + 1 is proportional to the value of A _in − (A _in ) _{A / D.} And (N−m) / N is the value of A _in − (A _in ) _{A / D} log ₂
The value is A / D converted with N bits.

Therefore, the analog signal obtained by adding the dither signal to the input analog signal is converted into 16 bits by the 8-bit A / D converter.
It is possible to obtain an A / D converted output with 10-bit precision by performing A / D conversion twice and obtaining a quotient obtained by dividing the cumulative sum of each digital output by 16 with 10-bit precision.

The required number of measurements (1 in the above description)
In 6), when K is the number of digits to be increased, 2 ^K is sufficient,
If K = 2, it will be 4, but it is preferably 2 to 4 times the above value in order to eliminate the influence of the linearity of the gradient potential and the error due to noise.

Further, in order to compensate for the shift of the output level by adding the dither signal to the input analog signal, a predetermined value is subtracted from the output digital signal, or the dither signal is positive and negative with 0 as the center. Must be a sex signal.

[0016]

The adder 12 for injecting the dither signal (gradient potential) is required to satisfy the following conditions. (A) The presence of the adder does not affect the analog input signal. (B) It has an impedance close to infinity with respect to the analog input signal. (C) Do not generate or induce noise. (D) There is no power consumption by the adder. (E) The circuit should be as simple and inexpensive as possible.

Conventionally used adders include a ladder resistance type and a combination of a ladder resistance and an operational amplifier.

The ladder resistance type is a type in which an analog input and a dither input are connected via a resistor, respectively, and both the analog input level and the dither input level are attenuated due to the resistance, which is not preferable.

Further, an adder using an operational amplifier is not preferable because noise, temperature drift, offset and other problems due to the operational amplifier occur, and the circuit becomes complicated and power consumption increases.

Therefore, the object of the present invention is to improve the above-mentioned drawbacks of the prior art and to improve the resolution by the dither signal.
An object of the present invention is to provide an improvement of a dither signal injection circuit in a D / D converter.

[0021]

The features of the present invention are: an A / D converter for giving a digital output of a predetermined number of digits to an input analog signal; and an analog input signal or the A / D conversion. Means for applying a gradient potential having an amplitude substantially equal to the minimum resolution of the A / D converter to the input terminal of the A / D converter, and a number of digits greater than the number of digits of the A / D converter. In an A / D conversion device that has an arithmetic circuit that gives an average value of digital conversion outputs and that gives a digital output with an accuracy determined by the number of digits of the arithmetic circuit, the A / D conversion circuit has a capacitor at an input portion of an analog signal. And an A / D converter having an integrator circuit including a time constant circuit formed of a resistor, and the gradient potential is applied via a capacitor of the time constant circuit.

[0022]

1 shows an A / D converter according to the present invention,
The same reference numbers as in FIG. 2 indicate the same.

In FIG. 1, an analog signal input terminal 16
A capacitor C ₀₁ is inserted between the input terminal and the ground point, and the input terminal is coupled to the input of the A / D converter 10 via the resistor R ₁₁ .
One end of a capacitor C ₁₁ is connected to the input of the A / D converter 10, and the other ends have resistors R ₂₁ , R ₂₂ and a capacitor C 11.
₂₁ and C ₂₂ are connected to the output of the integrating circuit 60. A rectangular wave (a) is applied to the input terminal 50 of the integrating circuit 60.

The time constant circuit of the resistor R ₁₁ and the capacitor C ₁₁ constitutes an integrator circuit 70 for an analog input signal, and this integrator circuit is provided in the input circuit of a commercially available A / D converter for noise absorption. Often In that case, one end of the capacitor C ₁₁ is grounded.

The capacitor C ₀₁ is inserted to prevent the output impedance of the analog signal from changing the time constant of the integrating circuit 70.

The dither signal is input to the integrating circuit 60 at the input terminal 50.
Is applied as a rectangular wave to. The integrating circuit 60 is a double integrating circuit, and the first integrated output has a triangular wave as shown in (b) and the second integrated output has a waveform as shown in (c).

The output of the integrating circuit 60 (waveform of (c)) is
The voltage is applied to one end 80 of the capacitor C ₁₁ which constitutes the integrating circuit 70 as shown in the figure, and the A / D is applied via the capacitor C _11.
Applied to the input of transducer 10. At this time, the circuit 70 acts as an integrating circuit for the analog input signal, but acts as a differentiating circuit for the dither signal from the terminal 80. Therefore, the dither signal at the input of the A / D converter 10 is a triangular wave shown in (d) of the figure, which is the same as the waveform of (b).

Numerical examples of capacitors and resistors are as follows. C ₀₁ = 1 μF R ₁₁ = 100 KΩ R ₂₁ = 510 KΩ R ₂₂ = 1 MΩ C ₁₁ = 0.1 μF C ₂₁ = 0.47 μF C ₂₂ = 0.1 μF

After all, the rectangular dither waveform (a) becomes a triangular wave of the waveform (d) through the double integrating circuit 60 and the differentiating circuit.

If the duty ratio of the dither waveform (a) is 50%, the average value of the dither signal of the triangular wave (d) automatically becomes 0 for the A / D input, so the circuit for the dither signal is No initial setting is required.

Since the time constant circuit 70 is often originally provided in a commercially available A / D converter, the circuit required for injecting the dither signal is only the integrating circuit 60, which is extremely inexpensive and has no power consumption. Moreover, the operation is stable because it is composed of only passive elements. Further, the analog input signal has a noise prevention effect by the integration function, and the circuit for dither injection has a sufficiently high impedance with respect to the analog input signal and does not affect the input signal.

When handling analog inputs of a plurality of channels, only one integrating circuit 60 is provided for all channels, and its output is supplied to one end 80 of the capacitor C ₁₁ of the time constant circuit 70 of each channel. ..

Note that the input dither signal is not a rectangular wave,
When the triangular wave is supplied, the integrating circuit 60 is not a double integrating circuit but a single integrating circuit.

When the present invention is implemented by a microcomputer, the A / D converter 10, the arithmetic circuit 14, the rectangular wave (a), etc. are often mounted on a commercially available microcomputer itself.
Further, the integration circuit 70 is usually required to be inserted to prevent noise.

[0035]

According to the present invention, the dither signal can be added to the analog signal very easily and preferably, and the high precision A / D conversion can be performed by the low precision A / D converter. In particular, since the conversion of 0.1% accuracy, which is in great demand in daily life, can be performed by a commercially available 4-bit microcomputer, the applicable range of the present invention is wide.

[0036]

[Brief description of drawings]

FIG. 1 is an embodiment of an A / D conversion device according to the present invention.

FIG. 2 is a conventional A / D conversion device.

[Explanation of symbols]

 10 A / D converter 12 Adder 14 Arithmetic circuit 16 Analog signal input terminal 18 Digital signal output terminal 50 Dither signal input terminal 60, 70 Integration circuit

Claims (1)

[Claims] 1. An A / D converter that provides a predetermined number of digits of digital output to an input analog signal, and an analog input signal or an A / D converter at an input terminal of the A / D converter.
A means for applying a gradient potential whose amplitude is almost equal to the minimum resolution of the D converter and a digit number larger than the digit number of the A / D converter, and the average value of the digital conversion outputs of the A / D converter at a plurality of times. In an A / D conversion device having an arithmetic circuit for giving a digital output with an accuracy determined by the number of digits of the arithmetic circuit, the A / D conversion circuit includes a time constant circuit composed of a capacitor and a resistor at an analog signal input portion. An A / D converter having an integrating circuit including the gradient electric potential is applied via a capacitor of the time constant circuit.