JPH11136127A - System and method for offset cancellation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To fully utilize the dynamic range of an A/D converter by efficiently canceling an offset. SOLUTION: A system part 108 calculates an offset voltage from an input voltage inputted through an A/D converter 107 for converting the output of a differential amplifier part 104. Then, the processing of inputting a reference voltage corrected by a correction value calculated by a predetermined algorithm based on the calculated offset voltage through a D/A converter 109 to the reference terminal of the differential amplifier part 104 is repeated until the calculated offset voltage falls inside a prescribed allowable range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an offset canceling method and method, and more particularly, to an offset canceling method and method in a biological signal processing device.

[0002]

2. Description of the Related Art Many conventional biological signal processing devices use a biological electrode to capture a biological signal as an electric signal. In order to reduce the contact resistance between the living body and the bioelectrode,
A paste-like electric field solution or the like is applied to the living body electrode and attached to the living body. At this time, depending on the skin of the living body and the state of the electrolytic solution,
Although weak, an electromotive force is generated between the living body and the electrode, and a DC voltage may be applied. This is called a polarization voltage. This polarization voltage becomes a large offset voltage when a large gain is taken by the DC amplifier.
It becomes a very disturbing being. In addition, since the gain is increased, a weak offset unique to the device used in the biological signal input unit circuit also gets on the biological signal as a large offset.

Therefore, for example, an nystagmus inspection apparatus or the like has a variable resistor for offset adjustment for each electrode. After the electrodes are mounted, the variable resistor performs offset adjustment for each electrode, and the polarization voltage is adjusted. And a manual adjustment method for removing the device-specific offset voltage.

FIG. 4 shows one of the biological signal input units of the biological signal processing apparatus in the conventional manual offset adjustment method.
It is a block diagram showing an example. The electrode 102 from the human body 101
The biological signal input through the input / output protection circuit 10
3. Enter the main amplifier 106 through the differential amplifier unit 104, the filter unit 105, and the offset adjustment circuit 401,
The signal is A / D converted by the A / D converter 107 and sent to the system 108 as a digital value.

[0005] In this circuit, the offset is greatly affected by the type of electrode and the mounting method. In addition, since the offset voltage is also affected by a device-specific offset voltage used in the circuit, it is necessary to manually perform offset adjustment for each electrode.

[0006] Japanese Patent Application Laid-Open Publication No. 08-116
As shown in 340, there is also an S / W adjustment method that realizes offset cancellation by S / W-like waveform processing after A / D conversion without performing H / W-like offset adjustment.
But in this case, taking into account the offset,
The dynamic range of the A / D converter must be large.

[0007]

The conventional manual offset adjustment method described above requires manual adjustment of the offset by a variable resistor for each electrode, so that the operability is poor in proportion to the number of electrodes. There was a disadvantage of becoming.

In the S / W adjustment method, offset cancellation is performed by S / W-like waveform processing after A / D conversion, so that the dynamic range of the A / D converter cannot be fully utilized and S / W conversion cannot be performed. There is a problem that processing overhead increases and the processing capacity of the system decreases.

[0009]

According to a first aspect of the present invention, in an offset canceling system, an output of a differential amplifier section is set to A / A.
An offset voltage value is calculated from an input voltage value input via an A / D converter that performs D conversion, and a reference corrected by a correction value calculated by a predetermined algorithm based on the calculated offset voltage value. An internal processing unit that repeats a process of inputting a voltage value to a reference terminal of the differential amplifier unit via a D / A converter until the calculated offset voltage value falls within a predetermined allowable range. I do.

A second invention is characterized in that the initial value of the reference voltage value in the first invention is a value which is a half of a full scale of the D / A converter.

In a third aspect, the algorithm according to the first aspect, wherein the calculated offset voltage value is larger than a half value of a full scale of the D / A converter. The value obtained by subtracting the value of (1/2) ⁿ of the full scale of the D / A converter from the reference voltage value output to the D / A converter last time (where n is the number of repetitions by the internal processing unit) is When the calculated offset voltage value is smaller than a half of the full scale of the D / A converter, the reference voltage value previously output to the D / A converter by the internal processing unit. The full scale (1) of the D / A converter
/ 2) value obtained by adding the value of ⁿ (the n is characterized in that said reference voltage value number of iterations) by the internal processing unit.

A fourth invention is characterized in that the MAX value of the number of repetitions by the internal processing unit in the first invention is determined by the resolution of the D / A converter.

According to a fifth aspect of the present invention, in the offset canceling method, an A / D converter for A / D converting an output of the differential amplifier unit.
An offset voltage value is calculated from an input voltage value input via the converter, and a reference voltage value corrected by a correction value calculated by a predetermined algorithm based on the calculated offset voltage value is converted into a D / A. A process of inputting the signal to a reference terminal of the differential amplifier unit via a converter is repeated until the calculated offset voltage value falls within a predetermined allowable range.

[0014]

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

FIG. 1 is a block diagram of an offset cancel system according to the present invention.

A biological signal input from the human body 101 through the electrode 102 is input to the differential amplifier 104 through the input / output protection circuit 103. Then, the filter unit 10
5. The signal passes through the main amplifier unit 106, and is passed through the A / D converter 107.
A / D-converted and sent to the system 108 as digital values.

The system 108 determines an offset correction value based on the waveform data, and sends digital data to the D / A converter 109. The analog signal that has been D / A converted by the D / A converter 109 passes through the filter unit 110 and the attenuator unit 111 and is input to the reference terminal of the differential amplifier unit 104. The offset can be removed by the voltage applied to the reference terminal. As a result, the DC offset of the signal line from the human body 101 to the A / D converter 107 is removed by changing the reference terminal voltage. Will be.

FIG. 2 shows an operation flow of the biological signal measurement including the present invention. First, the electrodes are mounted (Step 20).
1). Wait for the input waveform to settle (step 20)
2) The offset cancellation of the present invention is performed (step 203), and the polarization voltage at the time of mounting the electrode and the offset unique to the device are removed. The presence or absence of an error display at this time is confirmed (step 204). If there is no error display, the process proceeds to measurement of a biological signal (step 205). Since the biological signal can be measured with almost no offset component, the dynamic range of the A / D converter 107 can be fully utilized. If an error is displayed, since the offset has not been completely removed, maintenance of the measurement environment, reattachment of the electrodes, and the like are performed (step 206), and the process is repeated.

For example, when an eye movement is grasped by a biological signal as in a nystagmus test, an offset such as a polarization voltage at the time when an electrode is attached becomes a hindrance. This is removed by the method shown in FIG. 3 so that the potential of the living body at the time of attaching the electrode is recognized as 0V. Thereafter, the change in the bioelectric potential is due to the movement of the eyes, and the waveform thereof can use the full dynamic range of the A / D converter on both the + and-sides.

FIG. 3 shows an internal processing flow of offset cancellation. After the offset cancel process is started, an internal variable is set to n and the initial value is set to “2” (step 30).
1). The internal variable n is an integer indicating the number of times of the offset cancellation processing loop, and is also a numerical value indicating the accuracy of the offset cancellation. The initial value does not necessarily have to be “2”. The MAX value is determined by the resolution of the D / A converter 109. For example, 12
When a D / A converter of bit is used, the initial value is “2”.
, The D / A converter can repeat the processing until the MAX value becomes “12”. 12bitD / A
In the case of the converter, the MAX value is “12”, which is set because it is easy to understand. If an 8-bit D / A converter is used, the MAX value is "8" for the initial value "2".

The D / A converter 109 supplies 1 / full scale to
2 is output (step 302). Where A / D
The A / D value is read from the converter 107 several times, and the average value is obtained (step 303). It is determined whether the offset value of the data is within a predetermined allowable range (step 304). If it is within the allowable range, the offset cancel processing ends. If it is out of the allowable range, it is determined whether or not the internal variable n is a MAX value (step 305). If n is the MAX value at this time, there is an offset equal to or greater than the offset value that can be removed by this circuit, and an error is displayed (step 306). If the internal variable n is not the MAX value,
It is determined which side, out of the range, is on the side or the side (step 307). According to the result of the determination, is the value obtained by subtracting the value of (1/2) ⁿ of the full scale of the D / A converter from the value previously output to the D / A converter 109 (step 30)?
8), and outputs the result of the addition (step 309) to the D / A converter 109. Here, the variable n is counted up (step 310), and a series of processing is performed again from the A / D value reading step 303.

The dynamic range of the biological signal input section is ±
5V, D / A converter and A / D converter are 8 bit
This process is briefly described using actual numbers. In this case, the analog value of -5V is
The digital value becomes 0, and +5 V becomes 256. 0V
Becomes 128.

First, a half value of the full scale, that is, an analog value of 0 V is output to the D / A converter. At this time, the data is in a state where the offset cancellation has not been performed. The A / D value is read. If the offset of the value is out of the allowable range and is shifted to the + side, a value of 1/4 of the full scale, that is, 64 is output to the D / A converter. As an analog value, offset correction of -2.5 V is performed. The A / D value is read, and if the offset is other than the allowable value and is shifted to the negative side, a value of 1/8 of the full scale, that is, 34, is output to the D / A converter. An offset correction of −1.25 V is performed as an analog value. While this process is performed until n becomes 8, when the offset falls within the allowable range, the offset cancel process ends. In this case, the offset can be canceled up to ± (10V / 256) = ± 0.0391V.

[0024]

As described above, according to the present invention, the problem that the operability is deteriorated in proportion to the number of electrodes is solved, and offset cancellation is more effectively performed in H / W before A / D conversion. Therefore, there is an effect that the dynamic range of the A / D converter can be fully utilized.

[Brief description of the drawings]

FIG. 1 is a block diagram of an offset canceling method according to the present invention.

FIG. 2 is an operation flowchart of a biological signal measurement in the offset canceling method of the present invention.

FIG. 3 is a flowchart of internal processing in the offset canceling method of the present invention.

FIG. 4 is a block diagram of a conventional offset cancellation method.

[Explanation of symbols]

 Reference Signs List 101 human body 102 electrode 103 input / output protection circuit 104 differential amplifier section 105 filter section 106 main amplifier section 107 A / D converter 108 system 109 D / A converter 110 filter section 111 attenuator section 401 offset adjustment circuit

Claims (5)

[Claims] In an offset canceling method,
An offset voltage value is calculated from an input voltage value input via an A / D converter for A / D converting the output of the differential amplifier, and is calculated by a predetermined algorithm based on the calculated offset voltage value. The process of inputting the reference voltage value corrected by the corrected value to the reference terminal of the differential amplifier unit via a D / A converter is repeated until the calculated offset voltage value falls within a predetermined allowable range. An offset canceling method comprising an internal processing unit. 2. An initial value of the reference voltage value is:
2. The offset canceling method according to claim 1, wherein the offset value is a half of a full scale of the D / A converter. 3. The algorithm according to claim 1, wherein the calculated offset voltage value is output to the previous D / A converter by the internal processing unit when the calculated offset voltage value is larger than 1/2 of a full scale of the D / A converter. To the reference voltage value
The value obtained by subtracting the value of (1/2) ⁿ of the full scale of the / A converter (where n is the number of repetitions by the internal processing unit) is used as the reference voltage value. If the value is smaller than 1/2 of the full scale of the A converter, the previous D / A
A value obtained by adding a value of (1/2) ⁿ of the full scale of the D / A converter to the reference voltage value output to the converter (where n is the number of repetitions by the internal processing unit) is defined as the reference voltage value. 2. The offset canceling method according to claim 1, wherein: 4. The offset canceling method according to claim 1, wherein the MAX value of the number of repetitions by said internal processing unit is determined by the resolution of said D / A converter. 5. An offset canceling method, comprising:
An offset voltage value is calculated from an input voltage value input via an A / D converter for A / D converting the output of the differential amplifier, and is calculated by a predetermined algorithm based on the calculated offset voltage value. The process of inputting the reference voltage value corrected by the corrected value to the reference terminal of the differential amplifier unit via a D / A converter is repeated until the calculated offset voltage value falls within a predetermined allowable range. An offset canceling method, characterized in that: