Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1999-09-15
2001-05-15
Jeanpierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S155000
Reexamination Certificate
active
06232902
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a sigma-delta analog-to-digital converter for use, for example, with a two wire vortex flowmeter that uses a sensor to detect the vortex frequency occurring according to the flow rate of fluid under measurement, and outputs the measured flow rate to an external device in the form of a current signal that is also used as the operating power source for the external device. More specifically, the invention relates to an improved sigma-delta analog-to-digital converter incorporating means for isolating input signals in order to reduce current consumption.
DESCRIPTION OF THE PRIOR ART
A conventional vortex flowmeter is described in
FIG. 1
, wherein an input signal from a sensor
1
, for detecting vortexes produced by a vortex shredder, is amplified by an amplifier
2
and inputted to an anti-aliasing filter
3
. The signal inputted to the anti-aliasing filter
3
is processed thereby so that the high frequency components thereof are removed, and then the resulting signal is inputted to a sigma-delta analog-to-digital converter
4
. The signal inputted to the sigma-delta analog-to-digital converter
4
is converted thereby into a digital signal and further inputted to a digital filter
5
. The signal inputted to the digital filter
5
is thereby filtered and changed to pulses for inputting to a CPU
6
. The CPU
6
applies processes, such as conversion to a flow rate signal, to the inputted signal, and then the CPU outputs a flow rate signal to an output circuit
7
. The output circuit
7
outputs a 4-20 mA flow rate signal to an external device. In the case of a two wire vortex flowmeter, a 4 to 20 mA signal which is outputted also serves as an operating power source for the circuit of FIG.
1
.
If the vortex flowmeter of
FIG. 1
has a sensor
1
which is not isolated from ground, the input signal thereto must be isolated from the internal circuit at a certain location within the circuit in order to obtain the correct flow rate signal. In a conventional vortex flowmeter, an isolation circuit is added to the sigma-delta analog-to-digital converter
4
, so that such isolation is achieved at the location shown by the dashed line A in FIG.
1
.
FIG. 2
is a schematic view showing a conventional sigma-delta analog-to-digital converter
4
.
FIG. 3
is a timing chart showing waveforms of signals at various points in the circuit of FIG.
2
.
In
FIG. 2
, a signal Ain is inputted, from an anti-aliasing filter
3
, to an input terminal
61
and is fed through an adder
15
to an integrator
11
. The output of integrator
11
is then applied to a comparator
12
. The output signal of comparator
12
is inputted to a flip flow circuit
13
, whose output signal is then fed to a digital filter
5
disposed in a subsequent stage connected to a comparator through an output terminal
62
. An internal clock signal CLK, that serves as a sampling signal of the A/D converter, is supplied to flip flop circuit
13
, whose ouput signal is then fed through a digital-to-analog converter
14
and then to the negative terminal of adder
15
.
In the A/D converter, the signal Ain is integrated by integrator
11
, whose integral signal A
11
is then compared by comparator
12
with a value preset therein. The resulting comparative output D
12
is inputted to flip flop circuit
13
, which repeats high and low outputs at the timing of clock signal CLK to produce an output signal D
13
. The output signal D
13
is converted to an analog signal A
14
by D/A converter
14
, and then the analog signal A
14
is added to the signal Ain by adder
15
.
By repeating the operation, the A/D converter
4
can output a pulse density signal corresponding to signal Ain such as the signal labeled D
13
in FIG.
3
. The A/D converter
4
features a single output (note, some sigma-delta A/D converters have multiple bit outputs), small scale hardware, ease of power saving, and a high resolution that can be attained by increasing the sampling rate and without requiring any adjustment. Accordingly, the converter has been widely used for various instruments, such as the vortex flowmeter.
FIG. 4
shows an example of a sigma-delta A/D converter, wherein an isolation circuit is provided in the converter circuit to isolate the input signals at the part indicated by the dashed line A in FIG.
1
. In
FIG. 4
, an isolation circuit
25
is disposed between the clock signal CLK and the output signal of the flip flop circuit
13
shown in
FIG. 1
, in order to isolate the circuit, including the sensor
1
, on the left side of the dashed line A from the circuit, including the CPU
6
, on the right side of the dashed line A. A circuit configured in such a manner is disclosed in U.S. Pat. No. 5,372,046, for example.
However, the isolation mechanism requires that a high frequency clock signal be properly isolated. This isolation and transferrring a high frequency signal requirees a large amount of current. Normally, a sigma-delta A/D converter provides higher resolutions when an input signal is sampled at a higher frequency, since the quantization noise decreases as the sampling signal becomes faster. Generally, an input signal must be sampled at a frequency which is several hundred times as high as the signal bandwith. Since vortex flowmeters use a signal bandwidth of several hundred kilohertz, the sampling frequency must be at least several hundred kilohertz. Isolating and transferring such a high frequency signal requires a large amount Of current.
A two wire vortex flowmeter uses a current signal of 4 to 20 mA relative to the measurement range, in order to transmit a flow rate signal, which also serves as an operating power source to external equipment. Thus, the A/D converter must be operated so that is total current consumption is less than 4 mA. For this reason, the sigma-delta A/D converter of
FIG. 4
has a problem. As a result of the restriction on the amount of current consumed by the isolation circuit, the sampling frequency, or resolution, can only be increased to the extent at which isolation can be achieved at current levels no higher than 4 mA.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to overcome the aforementioned and other deficiencies and disadvantages of the prior art.
Another object is to provide a sigma-delta A/D converter having an isolation mechanism that functions at current level which are no lower than 4 mA and enables high speed sampling.
REFERENCES:
patent: 3896399 (1975-07-01), McDonald
patent: 5870046 (1999-02-01), Scott et al.
patent: 6107948 (2000-08-01), Scott et al.
Jean-Pierre Peguy
Kojima Moonray
Yokogawa Electric Corporation
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