JP2000022540A - Ad conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure the digital conversion value which is fast and sequentially converted at a short time interval and regardless of its waiting time by directly inputting the analog signal selected by a multiplexer to an AD conversion part to convert it into a digital signal. SOLUTION: An analog signal selection switch S1 of the 1st channel of a multiplexer 1 is closed and also an analog signal selection switch S2 of the 2nd channel and an analog input selection switch S3 of the 3rd channel of the multiplexer 1 are opened respectively by the control signal of a control arithmetic part 3. At the same time, an AD conversion part 2 samples an analog signal ViC and immediately converts it into a digital signal to secure the digital conversion value. Then the part 2 also samples the signal ViC and converts it into a digital signal to secure the digital conversion value after a time (t). Thus, the analog signal value is calculated as the digital value based on the digital conversion. value obtained at two points set before and after a selected analog signal Vi1, the sampling interval time and a time constant that is inherent in the analog input part of the part 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AD conversion circuit for sequentially switching and selecting an analog signal input by a multiplexer and performing digital conversion by an AD converter.

[0002]

2. Description of the Related Art In a conventional AD converter of this type, a multiplexer has a plurality of signal switching elements.
In addition to increasing the total terminal capacitance of the switch,
Since the wiring for connecting these switching elements becomes longer, the stray capacitance increases, and as a result, the capacitance of the input system from the input terminal of the multiplexer to the AD converter increases.

[0003]

For this reason, from the time when an analog signal is input to the input terminal of the multiplexer until the time when the capacitance of the input system is sufficiently charged by the analog signal, that is, in the AD conversion unit. Wait until it is charged to the extent that conversion accuracy is not affected,
The conversion in the AD converter must be performed, and this waiting time causes a problem that the speed of the sequential conversion is reduced. For example, when an accuracy of 0.1% of the input voltage is required, a waiting time of about 6 hours is required, assuming that the time constant inherent to the input system is Τ.

An object of the present invention is to provide an AD conversion circuit capable of obtaining a digitally converted value which is sequentially converted at high speed at shorter time intervals without depending on such a waiting time at the preceding stage of the AD converter. It is in.

[0005]

Before the analog signal is input to the multiplexer, the voltage of the analog input portion of the AD converter is indefinite, but when the analog signal is input to the multiplexer, the voltage of the AD converter is changed. The voltage at the analog input of the multiplexer approaches the analog signal voltage at the input of the multiplexer as a function of time determined by an inherent time constant τ. If the time constant τ is known, it is not necessary to wait until the voltage of the analog input portion of the AD converter becomes the same as the analog signal voltage of the input terminal of the multiplexer.
The value of the analog signal voltage can be immediately calculated from the digitally converted value obtained by the AD conversion.

The present invention focuses on this point, and has a configuration in which an analog signal selected by a multiplexer is directly input to an AD converter and is simultaneously converted into a digital signal.
The previous digital conversion value converted by the D conversion unit, the current digital conversion value, and the sampling time interval t in the AD conversion unit
And the time constant τ of the analog input part of the AD converter,
The value of the input analog signal is calculated as a digital value by the control operation unit.

That is, in the present invention, the A / D converter immediately performs the A / D conversion without a waiting time in the preceding stage of the A / D converter.
Approximate the value of the input analog signal from the difference between the converted values V1 and V2 at two points before and after, the time interval t (sampling time interval), and the time constant τ of the analog input portion of the AD converter. It is calculated as a typical digital value. Accordingly, by making the sampling time interval t in the AD conversion unit shorter than the conventional waiting time, it is possible to achieve a higher speed than in the past.

The control operation section can be constituted by a CPU, a memory for storing the digital conversion value converted by the AD conversion section, and a timer for performing time control of the switching timing of the multiplexer and the conversion timing of the AD conversion section. .

[0009]

Next, embodiments of the present invention will be described with reference to the drawings.

In FIG. 1, an A / D conversion circuit according to the present invention includes a multiplexer 1 for sequentially switching and selecting one of a plurality of input channels, an A / D converter 2 for digitally converting an analog signal selected and input by the multiplexer, It is composed of a control operation unit 3 that calculates the converted digital conversion value and controls the multiplexer 1 and the AD conversion unit 2.

A plurality of multiplexers 1 (3 in the figure)
) Analog signal selection switches S1 to S3, and the opening and closing timing of these switches S1 to S3 is controlled by the control calculation unit 3. The analog signal selected and input by sequentially turning on the switches S1 to S3 is supplied to the AD converter 2 at a constant sampling time interval t without waiting for a time until the input system capacitance is sufficiently charged as in the prior art. Is converted to digital. The conversion timing is controlled by the control calculation unit 3.

FIG. 3 shows an embodiment showing the internal configuration of the control operation unit 3. The control operation unit 3 includes a CPU 4 and a memory 5 that stores the digital conversion value converted by the AD conversion unit 2.
And a timer 6 for performing time control of the opening / closing timing of the switches S1 to S3 of the multiplexer 1 and the conversion timing of the AD converter 2.

Next, the operation will be described. When the conversion of the analog signal Vi1 is performed, the analog signal selection switch S1 of the first channel of the multiplexer 1 is closed by the control signal of the control operation unit 3, and the analog signal selection switch S2 of the second channel and the analog input selection of the third channel are selected. Open the switch S3. At the same time, the analog signal ViC is sampled by the AD converter 2 and AD conversion is immediately performed to obtain a digital conversion value Vt1.

Next, after a time t, the analog signal ViC is sampled by the A / D converter 2 and subjected to A / D conversion by the control signal of the control calculator 3, thereby obtaining a digital conversion value Vt2.
Here, assuming that the digital conversion values at two points before and after the selected analog signal Vi1 are Vt1 and Vt2, the sampling interval time is t, and the time constant unique to the analog input portion of the AD converter 2 is τ, the analog signal ViC is It is approximately expressed by the following equation.

Vi1 = {Vt2-Vt1.e} (-
t / τ)} / {1-e} (-t / τ)}

In the control calculation unit 3, Vi1
Is calculated to obtain a digital value. Hereinafter, the next analog signal selection switch is selected, and AD conversion is sequentially performed in the same procedure. At this time, the sampling interval time t is
AD conversion can be performed at high speed by making the waiting time shorter than that required by the D conversion circuit.

FIG. 2 is a timing chart for explaining the operation of the present invention. Here, t1 is the time of the first analog signal sampling, t2 is the time of the second analog signal sampling, t is the sampling time interval, and Vt1 is input to the AD converter 2 at the time of the first analog signal sampling. The voltage Vt2 is the voltage input to the AD conversion unit 2 at the time of the second analog signal sampling.

[0018]

According to the present invention, A / D conversion is immediately performed by the A / D conversion unit without a waiting time at the preceding stage of the A / D conversion unit, and the difference between the converted values V1 and V2 at two points before and after the time is calculated. Since the value of the input analog signal is calculated as an approximate digital value from t (sampling time interval) and the time constant τ of the analog input portion of the AD conversion unit, the value is sequentially and quickly increased at a shorter time interval than in the related art. A converted digital value is obtained.

In a conventional AD conversion circuit, a plurality of ADs
The present invention requires only one A / D converter, instead of the configuration supported by the converter, so that the circuit configuration is simplified and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of an example of an AD conversion circuit according to the present invention.

FIG. 2 is a timing chart illustrating the operation of the present invention.

FIG. 3 is a block diagram of an embodiment showing up to an internal configuration of a control operation unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Multiplexer 2 AD conversion part 3 Control operation part S1-S3 Analog signal selection switch

Claims (2)

[Claims] A multiplexer comprising: a multiplexer for sequentially switching and selecting an analog signal input; and an AD converter for digitally converting the analog signal selected by the multiplexer.
In the D conversion circuit, the analog signal selected by the multiplexer is directly input to the A / D conversion unit and is simultaneously converted into a digital signal, and the previous digital conversion value converted by the A / D conversion unit and the current digital conversion A control operation unit for calculating the value of the input analog signal as a digital value from the value, the sampling time interval t in the AD conversion unit, and the time constant τ of the analog input part of the AD conversion unit. AD conversion circuit. The control operation unit includes a CPU, a memory for storing the digital conversion value converted by the AD conversion unit, and a timer for performing time control of the switching timing of the multiplexer and the conversion timing of the AD conversion unit. 2. The AD conversion circuit according to claim 1, comprising: