JPH0824670B2 - ECG auto-drift cancellation circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrocardiogram input section of an electrocardiograph or an electrocardiogram analyzer, and more particularly to a cancel circuit for canceling drift due to a low frequency component of the input.

[0002]

2. Description of the Related Art A conventional electrocardiograph is a low-frequency input which is not necessary for an original electrocardiogram measurement, which is generated by body movement of a subject under measurement or a polarization voltage generated between a body surface and an electrocardiogram electrode. The components are also amplified by the preamplifier. Therefore, in order to increase the input dynamic range of the main amplifier in the next stage, the preamplifier and the main amplifier are AC-coupled by a capacitor and a resistor (low-pass cutoff filter) as shown in FIG. The low cutoff frequency of the input part is
In order to satisfy the standard of ECG (low cutoff frequency <0.05Hz), which is determined by the primary filter (partial filter consisting of capacitor and resistor) that constitutes this AC coupling part, the values of the capacitor and resistor are For example, capacitor =
The AC coupling portion was configured by selecting values such as 1 μ or less ± 5% and resistance = 3.3 MΩ ± 1%.

The output of the preamplifier whose low frequency components have been cut by the AC coupling section, which is a low-frequency cutoff filter, is input to the main amplifier and amplified to the effective conversion level of the A / D converter.
The configuration is such that it is input to the A / D converter and is used to form an electrocardiogram. Further, as a means for reducing the amplification factor in an undesired band of the amplifier band, Japanese Patent Publication No. 63-3
There is a vehicle detection device described in Japanese Patent Publication No. 9958.

[0004]

In the above-described conventional electrocardiograph, an AC for cutting unnecessary low frequency components of the input generated by the body movement of the subject, the polarization voltage, etc. by the output of the preamplifier. Since the accuracy of the joint directly affects the ECG waveform,
The capacitors and resistors for forming the AC coupling part require large-capacity / high-precision, high-resistance / high-precision parts, and as a result the parts have to be large in size and expensive. .

In addition, when the input signal abruptly changes and the output of the preamplifier is saturated to the power supply voltage, the waveform of the electrocardiogram is displayed until the capacitor itself is completely charged with the time constant determined by the capacitor and the resistance of the AC coupling section. Since it does not reach the baseline level, it took at least 6 seconds or more for the ECG waveform to enter the conversion range of the A / D converter after the input fluctuation settled down.

Further, the above-described vehicle detection device is intended to remove temperature drift and pulse noise of the circuit, and has two filters, an ultra low pass filter and a high pass filter, which have a fixed cutoff frequency. It is not suitable for electrocardiograph.

In view of the above problems, it is an object of the present invention to provide an automatic drift cancel circuit for an electrocardiograph, which is suitable for an electrocardiograph without requiring a large-sized and expensive component.

[0008]

An aurodrift cancel circuit for an electrocardiograph according to the present invention is an autodrift cancel circuit for an electrocardiograph that produces an electrocardiogram from the output of an A / D converter. A preamplifier for amplifying an electrocardiogram signal inputted through the attached electrocardiogram electrode, and amplifying an output of the preamplifier within an effective conversion range of the A / D converter,
A main amplifier that outputs to the A / D converter, a low-pass filter that extracts a low-frequency component from the output of the main amplifier and that negatively feeds back to the main amplifier, and an output level of the main amplifier have a positive or negative threshold value. The low-pass cutoff frequency of the low-pass filter is switched by the signal output from the window comparator that determines whether or not it has exceeded, and the window-coin comparator that indicates that the output level of the main amplifier has exceeded the positive or negative threshold value. , A low cutoff frequency switching circuit for preventing the output level of the main amplifier from exceeding a positive or negative threshold value.

[0009]

The main amplifier amplifies the output of the preamplifier within the conversion range of the A / D converter. Even if the output of the preamplifier increases rapidly, the window comparator detects it and outputs the detected output. Output. With this detection output, the low pass cutoff frequency switching circuit switches the time constant of the low pass filter to be shorter than usual. By this switching, the output of the main amplifier instantly falls within the conversion range of the A / D converter.

[0010]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the auto drift cancel circuit of the present invention. The output of the differential-type preamplifier 1 for amplifying the electrocardiographic signal input through the electrocardiographic electrodes D1 and D2 worn by the subject is A /
It is further amplified by the main amplifier 2 of the differential type so that the conversion of the D converter 3 has a predetermined precision (5 μV / 1 LSB). The amplified output of the main amplifier 2 is further provided to the window comparator 4 and the low pass filter 5, respectively.

The window comparator 4 determines whether or not the input output of the main amplifier 2 exceeds a positive / negative threshold value set by the voltages Vd and Vs, and outputs a detection output when the threshold value is exceeded. The low-pass filter 5 passes a signal having a frequency equal to or lower than the cutoff frequency set by the cutoff frequency switching circuit 6 among the input outputs of the main amplifier 2 and negatively feeds it back to the main amplifier 2. The cutoff frequency of the low-pass filter 5 is set by changing the switch of the cutoff frequency switching circuit 6. The constants R1 and C of the cutoff frequency switching circuit 6 are determined so that the low cutoff frequency (3 dB or less) of the main amplifier 2 is 0.05 Hz (for example, R ₁
= 3.3 MΩ, C = 1 μF).

The switching control of the switch of the cut-off frequency switching circuit 6 is performed by the detection output of the window comparator 4, that is, when the output of the main amplifier 2 exceeds the threshold value of the window comparator 4 due to an excessive input, The switch of the cut-off frequency switching circuit 6 is turned on by the detection output from the window comparator 4.
The cutoff frequency of the low-pass filter is f ₁ = 1 / 2π
Change from CR ₁ to f ₂ = 1 / 2πCR ₂ (however,
R ₂ << R ₁ , that is, f ₂ >> f ₁ ). Therefore, even if the output signal of the main amplifier 2 exceeds a certain level due to the input signal variation (for example, a variation exceeding the A / D conversion range (± 2.5 V)), the threshold value of the window comparator 4 is set in advance. By setting to ± 2.5V,
It is possible to automatically turn on the switch of the switching circuit 6 and quickly return the output of the main amplifier 2 to the baseline.

[0013]

As described above, according to the present invention, the unnecessary low frequency component of the input generated by the body movement of the subject, the polarization voltage and the like is detected by the low pass filter and is negatively fed back to the main amplifier. As a result, the influence of the precision of the components that form the low-pass filter on the electrocardiogram waveform is reduced, so that there is no need for high-precision components, and it is possible to use small and inexpensive components.

Further, by constantly monitoring the output in the main amplification period by the window comparator, even if the input changes rapidly, the time constant of the low pass filter is switched so as to be shorter than usual, so that the A / D is instantaneously changed. This has the effect of allowing the output of the main amplifier to fall within the conversion range.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an auto drift cancel circuit for an electrocardiograph according to the present invention.

FIG. 2 is a block diagram showing a conventional example.

[Explanation of symbols]

 1 Preamplifier 2 Main amplifier 3 A / D converter 4 Window comparator 5 Low pass filter 6 Cut-off frequency switching circuit

Claims (3)

[Claims] 1. An auto-drift canceling circuit of an electrocardiograph for producing an electrocardiogram from an output of an A / D converter, comprising a preamplifier for amplifying an electrocardiographic signal input through an electrocardiographic electrode worn by a subject. , A main amplifier that amplifies the output of the preamplifier within the effective conversion range of the A / D converter and outputs it to the A / D converter, and extracts a low frequency component from the output of the main amplifier, A low-pass filter that feeds back negatively to the amplifier, a window comparator that determines whether the output level of the main amplifier has exceeded a positive or negative threshold, and a notification that the output level of the main amplifier has exceeded a positive or negative threshold. A signal output from the window comparator to switch the low cutoff frequency of the lowpass filter so that the output level of the main amplifier does not exceed a positive or negative threshold. And a switching circuit, automatic drift cancellation circuit of the electrocardiograph. 2. The auto drift cancel circuit for an electrocardiograph according to claim 1, wherein the low pass filter includes an RC circuit. 3. The electrocardiogram according to claim 2, wherein the low cutoff frequency switching circuit switches the low cutoff frequency of the lowpass filter by changing the resistance value of the RC circuit of the lowpass filter. Meter auto-drift cancellation circuit.