JPH07306069A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To improve an S/N while maintaining saving of power, by intermittently feeding an exciting current, impressing a magnetic field to a measuring fluid, AC coupling and sample holding inter-electrode signals between detection electrodes and switching a reference potential before and after the sample holding. CONSTITUTION:An exciting means 20 intermittently feeds an exciting current to impress a magnetic field to a measuring fluid. At this time, an excitation period is shorter than a non-excitation period. A high pass filter 13 as an AC coupling means AC couples inter-electrode signals output, from detection electrodes 11a, 11b, thereby to obtain an alternate current signal. A first sample holding means 15 sample holds the alternate current signal with a sampling signal having a sampling width including before and after the excitation period, and outputs the signal as a first hold signal. Switching means SW1, SW2 switch to a reference potential before and after the sample holding period. A signal- processing means 7 processes the signal and outputs as a flow rate signal corresponding to a flow rate of the measuring fluid.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter which converts a flow rate of a fluid to be measured into an electric signal and outputs a flow rate signal corresponding to this flow rate through a detection electrode, and more particularly to reduction of energy required for excitation. The present invention relates to an electromagnetic flowmeter improved so as to improve S / N.

[0002]

2. Description of the Related Art As a conventional electromagnetic flow meter for intermittently supplying an exciting current to an exciting coil to reduce exciting energy, there are some examples as described below. First, there is an electromagnetic flowmeter disclosed in Japanese Patent Application Laid-Open No. 54-115163, "Title of Invention: Electromagnetic Flowmeter". This relates to an electromagnetic flow meter of low frequency excitation, and is intended to reduce the average power as a whole by making the excitation period shorter than the non-excitation period.

Second, there is an electromagnetic flowmeter disclosed in Japanese Patent Application Laid-Open No. 55-33685, "Title of Invention: Electromagnetic Flowmeter".
This is because the power is transmitted from the load side DC power source to the transmitter side as a current signal through the two transmission lines, the transmitter side is covered by this power, and the detected flow rate signal is transmitted earlier. Disclosed is a two-wire type electromagnetic flow meter which transmits a current signal to a load side via a wire.

Third, there is an electromagnetic flowmeter disclosed in Japanese Patent Application Laid-Open No. 55-76912, "Title of Invention: Electromagnetic Flowmeter".
This is to monitor the fluctuation of an external signal, for example, the electrode potential, and to excite only when there is a fluctuation so as to reduce the exciting power as a whole.

Fourth, there is an electromagnetic flowmeter disclosed in Japanese Patent Application Laid-Open No. 62-113019, "Title of Invention: Electromagnetic Flowmeter". This is to supply a pulsed positive and negative exciting current to the exciting coil, remove the differential noise by sampling the signal during the time until the differential noise disappears including each excitation period, and remove the voltage between the positive and negative excitation levels. The flow rate signal is obtained from the difference by synchronous rectification.

[0006]

However, in the above-described first conventional electromagnetic flowmeter, the sampling of the signal is executed after the differential noise caused by the rising of the excitation disappears, so that the on-time of the excitation is reduced. τ _ON becomes longer, and if it is attempted to maintain a predetermined repetition period accordingly, the excitation period also becomes longer and the response becomes slower.

The second conventional electromagnetic flowmeter is basically
Although the power consumption is low as a whole as in the first prior art, there is a problem that the power for excitation becomes large and the response becomes slow.

Further, since the third conventional electromagnetic flowmeter samples the signal when the steady value is reached, it takes a long time to reach the steady value as in the first prior art, and the excitation time is increased. There is a drawback that the electric power of becomes large.

Further, the conventional fourth electromagnetic flowmeter cannot uniquely determine the attenuation of the differential noise. For example, if the sampling period is made sufficiently large, the noise during this period increases and the S / N deteriorates, and if the sampling period is short, the influence of differential noise appears. Since the synchronous rectification is performed, signal processing in the low frequency region is required. Further, there is a problem that the hardware and software configurations become complicated because of the requirement of synchronous rectification based on sample values of the excitation period and the non-excitation period and the calculation of the difference therebetween.

[0010]

In order to solve the above-mentioned problems, the present invention converts a flow rate of a measurement fluid into an electric signal and outputs a flow rate signal corresponding to the previous flow rate via a detection electrode. In an electromagnetic flowmeter that outputs, the excitation period is shorter than the non-excitation period, and an excitation current is intermittently applied to apply a magnetic field to the measurement fluid and the inter-electrode signal output from the detection electrode is exchanged. AC coupling means for coupling to obtain an AC signal, and first sample and hold means for sampling and holding the previous AC signal with a sampling signal having a sampling width including before and after the previous excitation period and outputting this as a first hold signal. Switching means for switching to the reference potential before and after the previous sample-hold period, and signal processing means for performing signal processing using the previous hold signal and outputting it as the previous flow rate signal. It is obtained as comprising.

[0011]

[Operation] The excitation means applies a magnetic field to the fluid to be measured by supplying an excitation current intermittently with the excitation period shorter than the non-excitation period.
The AC coupling means AC-couples the inter-electrode signals output from the detection electrodes to obtain an AC signal.

The first sample and hold means samples and holds the previous AC signal with a sampling signal having a sampling width including before and after the previous excitation period, and outputs this as a first hold signal.

The switching means switches to the reference potential before and after the previous sample-hold period. Then, the signal processing means performs signal processing using the above hold signal and outputs it as a flow rate signal corresponding to the flow rate of the measurement fluid.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention. Reference numeral 10 is an insulating conduit through which a conductive measuring fluid Q flows. First, a capacitance-type electromagnetic flowmeter that detects the flow rate of a measurement fluid via capacitance will be described as a base.

The detection electrodes 11a and 11b are insulated from the measurement fluid Q and fixed to the conduit 10, and have _a capacitance C _a ,
It is connected to the measuring fluid Q via C _b . Further, the ground electrode 11c that has come into contact with the liquid is connected to the common potential point COM.

The preamplifier 12 is a buffer amplifier 12a,
12b and a differential amplifier 12, and the detection electrodes 11a and 11b are buffer amplifiers 12 of the preamplifier 12.
a and 12b are connected to the input ends thereof, and these output ends are connected to the input ends of the differential amplifier 12c, respectively.

The output terminal of the differential amplifier 12c is connected to a high-pass filter 13 which functions as an AC coupling means. The high pass filter 13 is selected so as to have a sufficient pass band for the excitation waveform.

Further, the high-pass filter 13 comprises a capacitor 13a and a resistor 13b, and the capacitor 13a
Has one end connected to the output end of the differential amplifier 12c and the other end connected to the common potential point COM via the resistor 13b.

The connection point between the capacitor 13a and the resistor 13b is connected to the input terminal of the buffer amplifier 14 and is also connected to the common potential point COM via the switch SW ₁ whose opening and closing is controlled by the control signal S ₁ . .

The output terminal of the buffer amplifier 14 has a control signal S.
Closing at ₂ is connected to a hold circuit 15 via the switch SW ₂ is controlled. The hold circuit 15 is composed of a resistor 15a and a capacitor 15b.
One end of 5a is connected to the switch SW ₂ and the other end thereof is connected to the common potential point COM through the capacitor 15b.

The connection point between the resistor 15a and the capacitor 15b are connected to a common potential point COM through a switch SW ₃ the opening and closing is controlled by the control signal S ₃ is connected to an input terminal of Baffua amplifier 16 .

The output terminal of the buffer amplifier 16 is connected to the signal processing section 17. The signal processing unit 17 includes an analog / digital converter, a microprocessor, a memory, etc., calculates a flow rate signal, and outputs the flow rate signal to the output end 18.

Reference numeral 19 denotes a timing circuit, which is a switch S.
W ₁ , SW ₂ , SW ₃ , signal processing unit 17, excitation circuit 20
, The control signals _{S 1, S 2, S 3} , S 4, and outputs a S _5, and controls these, and the like of the opening and closing.

In the exciting circuit 20, the switching timing is controlled by the control signal S ₅ , for example, the exciting coil 2
The waveform of the exciting current I _f flowing in 1 is controlled to be a triangular wave, and the repeating cycle of switching is controlled.

The exciting circuit 20 is specifically shown in FIG.
As shown in FIG. ₃ , it is composed of a DC power source 20a, a switch SW ₅ , a diode 20b, etc.
_A pseudo triangular wave is generated by controlling the opening / closing of ₅ with the control signal S ₅ , and the pseudo triangular wave is supplied to the exciting coil 21.

Next, using the waveform charts shown in FIGS.
The operation of the embodiment shown in FIG. 1 will be described. Timing times
The path 19 is connected to the excitation circuit 20 as shown in FIG.
Interval T ₁And de-excitation period T₂Control signal S that repeats_FiveSend out
It

The control signal S ₅ controls the switch SW ₅ shown in FIG. 2 to be turned on during the excitation period T ₁ and turned off during the non-excitation period T ₂ . When the switch SW ₅ is turned on, the exciting coil 21 is connected to the exciting coil 2
The exciting current I _f increases during the exciting period T ₁ as shown in FIG. 4 with a time constant determined by the resistance R _f and the inductance L _f of ₁ .

However, after the excitation period T ₁ , the switch SW ₅ is turned off, so that the supply of energy from the DC power source 20a is stopped and the energy stored in the excitation coil 21 is released through the diode 20b. , Fig. 4
As shown in, the exciting current _If decreases. When the non-excitation period T ₂ has passed, the switch SW ₅ is turned on again, and energy is supplied from the DC power supply 20a to the excitation coil 21.

After that, by repeating this, a pseudo triangular wave can be supplied to the exciting coil 21. In FIG. 3B, the triangular-wave-shaped exciting current I _f thus obtained is shown.

In this way, when a triangular wave-shaped exciting current I _{f is} passed through the exciting coil 21, the measurement fluid Q is correspondingly generated.
Is applied with a magnetic field having a magnetic flux density B having a shape substantially similar to this triangular waveform, and a signal voltage e _s having a similar waveform is generated in the measurement fluid Q.

[0031] During the detection electrodes 11a and 11b, in addition to the signal voltage e _s, the detection electrodes 11a, since 11b preamplifier 12 and a loop magnetic flux density B is interlinked signal line connecting the magnetic flux The differential noise N generated based on the change of the density B is superimposed and appears. The waveform of this differential noise N is as shown by the solid line in FIG.

Thus, as shown in FIG. 1, the capacitance C
_a, in a configuration for detecting a signal voltage at the detection electrodes 11a and 11b via the C _b, the electrode capacity is usually very small as several tens to several thousands pF, the electrode impedance by the eddy current generated in the measuring fluid Q The charge / discharge time constant becomes sufficiently small,
The differential noise N is substantially only the component indicated by the solid line in FIG. 3C, which is substantially proportional to the time derivative of the exciting current.

On the other hand, the waveform shown by the broken line in FIG. 3C shows the case of the conventional liquid contact type electromagnetic flowmeter. In the conventional configuration in which the detection electrode is in contact with the liquid, the electrode capacitance formed by the detection electrode and the measurement fluid is 0.1 to 10
Since there is about μF, and the electrochemical surface state of the detection electrode is unstable, a long tailed waveform as shown by the broken line in FIG. 3 (c) due to the effect of the eddy current flowing in the measurement fluid. Become.

As described above, the detection electrodes 11a and 11
The voltage (e _s + N) generated during b is the preamplifier 12,
It is output to the buffer amplifier 14 via the high-pass filter 13.

The output voltage V shown in FIG. 3 (e) corresponding to the voltage (e _s + N) appearing at the output terminal of the buffer amplifier 14.
_S is sampled and held by being integrated in the hold circuit 15 by turning on the switch SW ₂ for a fixed time T ₃ by the control signal S ₂ for sampling (FIG. 3 (g)). This fixed time T ₃ may have a time width such that the differential noise N shown in FIG. 3C disappears.

In this case, before the switch SW ₂ is turned on and the output voltage V _S is sampled in the hold circuit 15, as shown in FIG. 3D, the switch SW ₁ is turned on by the control signal S _{1 and the} high pass is performed. Filter 13
The output end of is reset and fixed to the reference potential.

By performing such a reset operation, the signal voltage generated corresponding to the excitation period T ₁ can be obtained even if the detection electrodes 11a and 11b, the preamplifier 12 or the like have a change in DC voltage. As shown in FIG. 3 (e), it starts accurately from the reference potential at the common potential point COM.

Therefore, the output voltage V _S shown in FIG. 3 (e) is integrated in the hold circuit 15 for the fixed time T ₃ shown in FIG. 3 (g), so that a voltage accurately proportional to the flow rate is obtained. At the same time, the positive and negative components of the differential noise N shown in FIG. 3C are canceled by each other and become zero, so that the output is not affected.

In the hold circuit 15, the switch SW ₃ is turned on by the control signal S ₃ shown in FIG. 3 (h) to release the electric charge of the capacitor 15b and prepare for the next signal processing.
The sampling voltage thus obtained is output to the signal processing unit 17 via the buffer amplifier 16.

The signal processing unit 17 obtains information regarding the control of the exciting current output from the timing circuit 19 to the exciting circuit 20 through the control signal S _4, and determines the instantaneous flow rate and the integrated flow rate according to the purpose of use of the flow meter. Are calculated and output to the output terminal 18. It is also possible to obtain the value of the exciting current _If from the exciting circuit 20 and obtain the ratio between the signal value and the exciting current value in the flow rate calculation to correct the span.

In the case of the conventional electromagnetic flowmeter which does not perform the reset operation by the switch SW ₁ , the preamplifier 1
3 and the like, the time constant of the high-pass filter 13 changes to the point where the upper and lower portions of the DC potential shown by the diagonal lines in FIG. Amount of residual ε with opposite polarity to the signal in this case
Depends on the magnitude of the signal voltage e _s , so simply sampling the signal during the fixed time T ₃ causes an error.

FIG. 5 is a power supply circuit diagram showing the configuration of a DC power supply including the excitation circuit 20. The power supply circuit 22 in this case is
It is shown as an example of a switching power supply circuit. The DC power source E _b is connected to the common potential point COM via the primary coil n ₁ of the transformer T and the switch SW ₆ . A diode D ₁ and a capacitor C ₁ are connected in series at both ends of this primary coil n ₁ .

Further, the feedback coil n of the transformer T₂Both
Diode D at the end₂And capacitor C ₂And are connected in series
This capacitor C₂Feedback voltage V generated across both ends of_fIs
Reference voltage E at the non-inverting input terminal (+)_rIncreased deviation applied
Width Q₁Is applied to the inverting input terminal (-). That
Deviation amplifier Q₁Output by switch SW₆On /
Turn off.

A diode D ₃ and a capacitor C ₃ are connected in series to the secondary coil n ₃ of the transformer T, and a logic voltage V _L is obtained from both ends of this capacitor C ₃ . This logic voltage V _L is always turned on and is used as the voltage of the power supply that generates each timing signal.

The tertiary coil n ₄ of _the transformer T is _the coil n ₄₁
And n ₄₂ are connected in series, and a diode D ₄ and a switch S ₄ are provided between these connection points and the other end of the coil n _41.
W ₇ and capacitor C ₄ are connected in series,
Positive voltage + V _A is obtained as a voltage for the analog power source from both ends of ₄ .

A diode D ₅ , a switch SW ₈ and a capacitor C ₅ are connected in series between the connection point and the other end of the coil n ₄₂ , and a negative voltage for analog power supply is supplied from both ends of the capacitor C _5. to obtain a voltage -V _a.

A diode D ₆ , a switch SW ₉ and a capacitor C ₆ are connected in series to the quaternary coil n ₅ of the transformer T, and this capacitor C ₆ is connected in parallel with a diode D ₇
Are connected to obtain an excitation voltage V _f from both ends thereof.

The switches SW _{7 to} SW ₉ are turned on only in the vicinity of the intermittent excitation to supply electric power to the loads, respectively, and are turned off otherwise to realize power saving as a whole. A battery or the like can be used as the DC power supply E _{b in} this case.

Then, the deviation amplifier Q ₁ controls the switch SW ₆ to be turned on / off so that the voltage becomes a voltage corresponding to the reference voltage E _r , and a constant voltage is applied from the secondary coil to the quaternary coil according to the load condition. Control to supply.

In the above description, the waveform of the exciting current I _f has been described as an example of the pseudo triangular wave, but it is not limited to this. FIG. 5 shows some examples of other excitation waveforms.

FIG. 6A shows a rectangular wave-shaped excitation waveform as shown in FIG.
(B) has a rectangular wave shape, but positive excitation and negative excitation are alternately inserted, and the signal amount can be doubled by performing synchronous rectification between positive excitation and negative excitation.

Further, FIG. 6 (c) uses an excitation waveform of a triangular wave, and the excitation circuit can be simplified. FIG. 6D shows an example of a trapezoidal wave excitation waveform. When the excitation pulse is shortened, the rectangular wave excitation shown in FIG. 6A actually becomes this trapezoidal wave excitation.

FIG. 7 is a block diagram showing the configuration of another embodiment of the present invention. In this configuration, a reference voltage is taken in at a predetermined cycle, a ratio with a signal voltage is calculated using this reference voltage, and span variation is corrected. Note that FIG.
The same parts as those shown in FIG.

A detailed description will be given below. A reference resistor r is connected in series to the exciting coil 21 in the configuration shown in FIG.
The reference voltage V _r is taken out from both ends of the switch SW ₁₀
Applied to the other switching end of.

Normally, the common end of the switch SW ₁₀ is connected to one of the switching ends by the control signal S ₆ output from the timing circuit 22, and the hold circuit 15 samples the signal voltage e _s .

Further, the switch SW is turned on by the control signal S ₆ at a cycle shorter than the sampling cycle of the signal voltage e _s.
The reference voltage V _r is sampled by the hold circuit 15 by switching to the other of the switching ends of ₁₀ .

This sampling value is the buffer amplifier 16
A memo that is loaded into the signal processing unit 17 via the
Stored in the memory. The signal processing unit 17 uses this signal power.
Pressure e _sAnd calculate the ratio of
Output. This makes it possible to correct span fluctuations.
Wear.

In the embodiment shown in FIG. 1, the signal
Switch SW just before sampling ₁Turn on and reset
Although it has been described as a configuration for switching on,
Switch SW₁Be turned on and before and after the signal excitation period.
It may be configured to be switched off between them.

In the embodiment shown in FIG. 1, two switches SW ₁ and SW ₃ are used. However, as shown in FIG. 8, one switch SW _{10 is used} as a control signal S _7. It may be configured to switch by. In this way, the signal sampling and holding circuit 15
Also serves as a reset.

Specifically, the control signal S ₇ is a control signal obtained by combining the control signals S ₁ and S ₃ and is normally switched to the reference potential on the common potential point COM side. Switching control is performed so that the buffer is connected to the output terminal of the buffer amplifier 14 only at the time of sampling.

In the above description, in order to clarify the difference from the prior art, the capacitance type electromagnetic flowmeter has been described as a base, but a liquid contact type in which the detection electrode is in contact with the measurement fluid is used. If a specific condition is allowed, the electromagnetic flow meter can also be applied in the same manner as the electrostatic capacitance type, as described below.

In the case of the capacitance type, since the electrode capacitance is very small, the time constant of charging / discharging of the electrode impedance due to the eddy current generated in the measured fluid Q becomes sufficiently small, and the rapid response with a short excitation repetition period is obtained. Is also applicable when is required.

On the other hand, in the case of the liquid contact type, since the electrode capacitance formed by the detection electrode and the measurement fluid becomes large,
As shown by the dotted line in FIG. 3 (c), the differential noise N has a trailing shape, and it is necessary to lengthen the excitation period, so a fast response cannot be expected.

However, when it is used for applications where the response may be slow, for example, as a water meter, when the differential noise N disappears, the switch SW ₁ arranged in the subsequent stage of the high-pass filter 13 is set. By resetting at a predetermined timing, the initial value of the integration can be accurately set to zero at the beginning of the integration by the hold circuit 15, so that a good S / N can be obtained.

Specifically, for example, as an electromagnetic flow meter for water supply, a practical level of S / N can be obtained with a power consumption of about 0.1 mW by intermittent excitation about once per second.

[0066]

The present invention has been specifically described with reference to the embodiments, but according to the invention described in each claim,
It has the following effects.

According to the first, third, and fourth aspects of the present invention, the excitation period is shorter than the non-excitation period and the excitation current is intermittently supplied to apply the magnetic field to the measurement fluid. , The inter-electrode signals output from the detection electrodes are AC-coupled to obtain an AC signal, and the sampling signal is used to sample and hold it, and the reference potential is switched before and after the sample-hold period. The flow rate signal can be sampled without error. Further, it is possible to contribute to improvement of S / N while maintaining power saving.

According to the invention described in the second aspect, in addition to the effect of the invention described in the first aspect, since it is configured as a capacitance type, the electrode capacitance is very small and is generated in the measurement fluid. The time constant of charging / discharging the electrode impedance due to the eddy current is sufficiently small, so that the differential noise is attenuated quickly and the signal can be detected while removing the differential noise with a short sampling width. Can be secured.

According to the fifth aspect of the present invention, a high voltage is applied to the exciting coil and the exciting current rising in the exciting coil is turned off at an appropriate timing to generate a triangular wave or a pseudo triangular wave. The power consumed can be reduced.

According to the invention described in the sixth aspect, since the ratio between the sampling signal and the reference voltage is calculated, the span error due to the fluctuation of the exciting current can be easily removed.

According to the invention described in the seventh aspect, since the switching cycle of the sample switch is set to a fraction of the reference voltage side with respect to the AC signal side, the response to the flow rate change is made faster. be able to.

According to the invention described in claim 8, since the power supply voltage can be supplied only during the period corresponding to the sampling width, that is, during the excitation period, it is easy to achieve power saving as a whole. .

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of an excitation circuit shown in FIG.

FIG. 3 is a waveform diagram for explaining the operation of the embodiment shown in FIG.

FIG. 4 is a waveform diagram illustrating an operation of the excitation circuit shown in FIG.

5 is a power supply circuit diagram showing a configuration of a DC power supply including the excitation circuit shown in FIG.

6 is a waveform diagram illustrating another waveform of the exciting current shown in FIG.

FIG. 7 is a block diagram showing a configuration of a modified embodiment with respect to the embodiment shown in FIG.

FIG. 8 is a block diagram showing a configuration of another modified example of the embodiment shown in FIG.

[Explanation of symbols]

 10 Conduit 11a, 11b Detection Electrode 12 Preamplifier 13 High-pass Filter 15 Hold Circuit 17 Signal Processing Section 19 Timing Circuit 20 Excitation Circuit 21 Rage Coil 22 Power Supply Circuit

Claims (8)

[Claims] 1. An electromagnetic flowmeter which converts a flow rate of a measurement fluid into an electric signal and outputs a flow rate signal corresponding to the flow rate through a detection electrode, wherein an exciting period is shorter than a non-exciting period and an exciting current is intermittently supplied. Excitation means for flowing and applying a magnetic field to the measurement fluid, AC coupling means for AC coupling the inter-electrode signals output from the detection electrodes to obtain an AC signal, and sampling including the AC signal before and after the excitation period First sample and hold means for sampling and holding with a sampling signal having a width and outputting it as a first hold signal, switching means for switching to a reference potential before and after the sample and hold period, and signal processing using the hold signal And a signal processing means for outputting the flow rate signal. 2. The electromagnetic flowmeter according to claim 1, wherein the detection electrode detects a flow rate signal via an electrostatic capacitance. 3. The electromagnetic flowmeter according to claim 1, wherein the detection electrode is in contact with a measurement fluid to detect a flow rate signal. 4. The switching means switches between the AC signal and the reference potential by a first switching signal, and the hold signal and the reference potential by a second switching signal at a timing after the sample and hold. Second to switch
The electromagnetic flowmeter according to claim 1, 2 or 3, which comprises switching means. 5. The method according to claim 1, wherein the waveform of the exciting current is a triangular wave or a pseudo triangular wave.
Alternatively, the electromagnetic flowmeter according to claim 3. 6. A second sample-hold means for switching from the AC signal to a reference voltage proportional to the exciting current by a sample switch, sample-holding with the sampling signal, and outputting the sample-holding signal as a second hold signal. The electromagnetic flow meter according to claim 1, further comprising: a ratio calculation unit that calculates a ratio between the reference voltage and the AC signal using a second hold signal. 7. The electromagnetic flowmeter according to claim 6, wherein the switching cycle of the sample switch is set to a fraction of the AC signal side on the reference voltage side. 8. A power supply means for supplying a power supply voltage only to an analog signal processing portion before the signal processing means only during a period substantially corresponding to the sampling width. Alternatively, the electromagnetic flowmeter according to claim 3.