JP3593691B2 - ΣΔ AD converter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
According to the present invention, a vortex frequency generated in accordance with the flow rate of a measurement fluid is detected by a sensor, and the detection signal is A / D converted and converted into a flow rate. The present invention relates to a ΣΔ AD converter used in a two-wire vortex flowmeter that outputs a current signal, and more particularly to a ΣΔ AD converter in which an insulating means for an input signal is improved to reduce current consumption.
[0002]
[Prior art]
The configuration of a conventional vortex flowmeter will be described with reference to FIG. In FIG. 1, an input signal of a sensor 1 for detecting a vortex generated by a vortex generator is amplified by an amplifier 2 and input to an anti-aliasing filter 3.
[0003]
The signal input to the anti-aliasing filter 3 is output to the ΣΔ A / D converter 4 after removing the high-frequency component.
[0004]
The signal input to the ΣΔA / D converter 4 is converted into a digital signal here and output to the digital filter 5.
[0005]
The signal input to the digital filter 5 is subjected to appropriate filtering here and input to the CPU 6. After performing signal processing such as flow rate conversion on the input signal, the CPU 6 outputs a flow rate signal to the output circuit 7.
[0006]
The output circuit 7 outputs a flow signal of 4 to 20 mA to an external device. In the case of a two-wire vortex flowmeter, the output signal of 4 to 20 mA also serves as the operating power supply of the circuit shown in FIG.
[0007]
In the vortex flowmeter having such a configuration, if the sensor 1 is not insulated from the ground, it is necessary to insulate the input signal from the internal circuit at any point in the circuit in order to obtain a normal flow signal.
[0008]
In a general vortex flowmeter, an insulation circuit is inserted into the ΣΔ A / D converter 4 in order to perform such insulation, thereby performing insulation at a location indicated by a broken line A in FIG.
[0009]
FIG. 5 is a basic configuration diagram of a conventional ΣΔ AD converter, and FIG. 6 is a time chart showing signal waveforms of respective units.
[0010]
5, the signal Ain input from the anti-aliasing filter 3 to the input terminal 61 is input to the integrator 11 via the adder 15, and the output is input to the comparator 12.
[0011]
The output of the comparator 12 is input to the flip-flop 13, and the output is input to the digital filter 5 at the subsequent stage connected via the output terminal 62. The flip-flop 13 is connected to an internal clock signal CLK serving as a sampling signal of the ΣΔ AD converter, and an output of the flip-flop 13 is input to a minus terminal of an adder 15 via a D / A converter 14.
[0012]
In the ΣΔ AD converter having such a configuration, the signal Ain is integrated by the integrator 11, and the integrated signal A11 is compared with a predetermined value of the comparator 12. The comparison output D12 is input to the flip-flop 13 and repeatedly turned on and off at the timing of the clock signal CLK to output the output signal D13.
[0013]
The output signal D13 of the flip-flop 13 is converted into an analog signal A14 by a D / A converter 14, and then added to the signal Ain by an adder 15.
[0014]
By repeating such an operation, the ΣΔ AD converter 4 can output the pulse density signal D13 corresponding to the signal Ain as shown by D13 in FIG.
[0015]
The features of the ΣΔ AD converter described above include that the output is 1 bit (there is a multi-bit output type), that the hardware is small, that power can be easily saved, and that the sampling rate is increased. It has been widely used for vortex flow meters and the like because it can obtain a high resolution without adjustment.
[0016]
FIG. 7 shows an example in which the insulating circuit is inserted into a circuit to insulate an input signal at a location indicated by a broken line A in FIG. 4 by the ΣΔ AD converter having such a structure. The circuit shown in FIG. 4 is provided by providing an insulating circuit 25 between the clock signal CLK and the output of the flip-flop 13 of the ΣΔ AD converter shown in FIG. This is to provide insulation. A circuit having such a configuration is disclosed in US Pat. No. 5,372,046.
[0017]
[Problems to be solved by the invention]
However, in the insulating structure of the conventional ΣΔ AD converter illustrated in FIG. 7, it is necessary to insulate a clock signal that is a high-frequency signal. A large current is required to insulate and transfer high frequency signals.
[0018]
In general, the more an AD converter performs sampling using a high sampling signal, the smaller the quantization noise and the higher the resolution. In particular, the ΣΔ AD converter needs to perform sampling at a frequency several hundred times higher than the signal band.
[0019]
In the case of a vortex flowmeter, the signal band is several kHz, so the sampling frequency needs to be several hundred kHz or more. In order to insulate and transfer such a high-frequency signal, a large current is required.
[0020]
However, in the two-wire type vortex flowmeter, since the flow signal shared with the operating power source is transmitted to an external device as a current signal of 4-20 mA for the measurement range, the total current consumption of the AD converter needs to be operated at 4 mA or less. There is. Therefore, the ΣΔ AD converter of FIG. 7 has a problem that the sampling frequency can be increased (resolution is increased) only within a range in which the isolation can be performed at 4 mA or less due to the limitation of the current consumption of the insulating circuit.
[0021]
An object of the present invention is to solve the above problem and to provide a ΣΔ AD converter provided with an insulating means capable of operating at 4 mA or less and capable of high-speed sampling.
[0022]
[Means for Solving the Problems]
In order to achieve such an object, according to the first aspect of the present invention, an input terminal, an adder, an integrator, a comparator, and a flip-flop are connected in series, and an output signal of the flip-flop is connected to an output terminal.帰 還 Δ AD converter that feeds back its output signal to the adder via a D / A converter and outputs a pulse density signal corresponding to the input signal.
An output of the comparator and an input of the flip-flop;
An insulating means for insulating the output of the flip-flop and the input of the D / A converter is provided.
[0023]
This makes it possible to insulate the sensor-side circuit from the CPU-side circuit without insulating the high-frequency clock signal.
[0024]
According to a second aspect of the present invention, in the first aspect of the invention, a transformer or a photocoupler is used as the insulating means.
[0025]
This makes it possible to configure the insulating means using a general-purpose component instead of a special high-frequency component.
[0026]
According to a third aspect of the present invention, in the first aspect of the present invention, the integrator is configured by connecting a plurality of integrators in series.
[0027]
This makes it possible for the insulating means to correspond to ΣΔ AD converters having various configurations.
[0028]
According to a fourth aspect of the present invention, in the first aspect of the present invention, the D / A converter is configured to receive an output of a flip-flop via a hold circuit. .
[0029]
This makes it possible for the insulating means to correspond to ΣΔ AD converters having various configurations.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram of a ΣΔ AD converter according to the present invention. In the figure, the same operations as those in FIG. 7 for describing the problems of the conventional example are denoted by the same reference numerals, and the description thereof will be omitted.
[0031]
1 differs from FIG. 7 in that an insulating circuit 35 is provided at a connection point between the comparator 12 and the flip-flop 13 and at a connection point between the output terminal of the flip-flop 13 and the D / A converter 14.
[0032]
Since the output D12 of the comparator 12 and the output D13 of the flip-flop 13 are on / off binary signals as shown in FIG. 6, they can be easily insulated.
[0033]
In the circuit of FIG. 1, unlike the circuit of FIG. 7, the high-frequency clock signal CLK does not pass through the insulating circuit 35. The output signal D13 of the flip-flop 13 is in principle less than half the frequency of the clock signal CLK.
[0034]
As described above, in general, the higher the frequency of a signal to be insulated, the higher the current consumption and the higher the price of the insulating circuit. Conversely, the lower the frequency of the signal to be insulated, the lower the current consumption and the lower the price.
[0035]
Therefore, since the signal to be insulated by the insulating circuit 35 is a low-frequency binary signal, a large current is not required for the insulation. Therefore, the insulating circuit 35 can be easily realized by a simple circuit with small current consumption. Further, since the clock signal CLK for determining the resolution of the ΣΔ AD converter does not pass through the insulating circuit 35, the frequency can be easily increased without being limited by this limitation.
[0036]
It should be noted that the foregoing description has been directed to specific preferred embodiments for the purpose of explanation and illustration of the present invention. Therefore, the present invention is not limited to the above-described embodiment, but includes many more changes and modifications without departing from the spirit thereof.
[0037]
For example, the insulating means described in FIG. 1 is also effective in a secondary ΣΔ AD converter in which an adder 45 and an integrator 41 are added as in the circuit shown in FIG. As shown in the figure, the ΣΔ AD converter can reduce the quantization noise and increase the resolution as compared with the primary ΣΔ AD converter by increasing the number of integrators.
[0038]
Further, in the 説明 Δ AD converter in which the hold circuit 56 is inserted between the output terminal of the flip-flop 13 and the A / D converter 14 as in the circuit shown in FIG. 3, the insulating means described in FIG. 1 is effective. . By providing the hold circuit 56 as shown in the figure, the operating speed of the integrator 11 and the comparator 13 can be suppressed lower than the frequency of the clock signal CLK, so that a 電流 Δ AD converter operating with a smaller current can be manufactured at low cost. It is possible to manufacture with.
[0039]
【The invention's effect】
As is apparent from the above description, the present invention has the following effects.
In the invention according to claim 1, in the に お い て Δ AD converter,
An output of the comparator and an input of the flip-flop;
By providing insulating means for insulating the output of the flip-flop and the input of the D / A converter,
The circuit on the sensor side and the circuit on the CPU side can be insulated simply by isolating a relatively low-speed binary signal. Therefore, as compared with the case where a high-frequency clock signal is insulated, it is possible to configure the insulating means with an inexpensive circuit with low current consumption.
[0040]
According to a second aspect of the present invention, in the first aspect of the present invention, since the insulating means is configured using a transformer or a photocoupler, it is manufactured at low cost using general-purpose parts. It becomes possible. Also, the use of general-purpose parts makes it easy to procure parts.
[0041]
According to the third and fourth aspects of the present invention, in the first aspect of the present invention, the insulating means can be applied to the secondary ΣΔ AD converter and a ΣΔ AD converter including a hold circuit. Therefore, it is possible to support ΣΔ AD converters having various configurations.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing one embodiment of a ΣΔ AD converter of a vortex flow meter according to the present invention.
FIG. 2 is a configuration diagram showing an embodiment of a secondary ΣΔ AD converter of the vortex flow meter according to the present invention.
FIG. 3 is a configuration diagram showing another embodiment of the 渦 Δ AD converter of the vortex flowmeter according to the present invention.
FIG. 4 is a configuration diagram showing an example of a conventional vortex flow meter.
FIG. 5 is a configuration diagram showing an example of a ΣΔ AD converter of a conventional vortex flowmeter.
FIG. 6 is a time chart showing signal waveforms of a ΣΔ AD converter of a conventional vortex flow meter.
FIG. 7 is a configuration diagram showing another example of the ΣΔ AD converter of the conventional vortex flow meter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Sensor 2 Amplifier 3 Anti-aliasing filter 4 A / D converter 5 Digital filter 6 CPU
7 Output circuit 11, 41 Integrator 12 Comparator 13 Flip-flop 14 D / A converter 15, 45 Adder 25, 35 Insulation circuit 56 Hold circuit

Claims (4)

An A / D converter used for a two-wire vortex flow meter or the like,
An input terminal, an adder, an integrator, a comparator, and a flip-flop are connected in series, an output signal of the flip-flop is connected to an output terminal, and the output signal is fed back to the adder via a D / A converter. Then, in a ΣΔ AD converter that outputs a pulse density signal corresponding to the input signal,
An output of the comparator and an input of the flip-flop;
A ΣΔ AD converter, comprising insulating means for insulating the output of the flip-flop and the input of the D / A converter. The ΣΔ AD converter according to claim 1, wherein a transformer or a photocoupler is used as the insulating means. The ΣΔ AD converter according to claim 1, wherein the integrator is configured by connecting a plurality of integrators in series. The ΣΔ AD converter according to claim 1, wherein the D / A converter is configured to receive an output of a flip-flop via a hold circuit.

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