Self-calibrating AD converter

Abstract

The present invention comprises: a first reference voltage unit 10 that generates a temperature compensated reference voltage; a second reference voltage unit 11 that generates a second reference voltage calibrated by the reference voltage; an integration unit 32 that, at the time of calibration, generates an integrated voltage obtained by integrating a unit voltage with one of the reference voltage, the second reference voltage, and a ground voltage as an initial value; a comparator 34 that compares the integrated voltage with a threshold value voltage 34 and outputs a determination signal; a calibration control unit 21 that, at the time of calibration, measures an integration time required for the integrated voltage to exceed the threshold value voltage from the initial value, and calibrates the unit voltage and an offset voltage of the comparator 34; and a conversion control unit 22 that, at the time of conversion, converts an input voltage to a digital value using the second reference voltage and a conversion integration time that is the integration time for a case in which the input voltage is the initial value.

Description

[Technical Field]

[0001] The present invention relates to an AD converter with a self-calibration function that does not require a measuring device for calibration. [Background technology]

[0002] As is well known, an AD converter is composed of a DA converter and a comparator that outputs a known voltage, and the output value of the DA converter is sequentially changed to set the minimum DA converter output value at which the comparator output changes from a low output voltage to a high output voltage, and the digital value used is the conversion value of the AD converter (Non-Patent Document 1). Fluctuations due to changes in the offset and linearity of the DA converter over time lead to changes in the AD converter over time.

[0003] Typical DA converters include an R-2R ladder circuit, a resistor string circuit (Non-Patent Document 2), and a PWM circuit (Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] [Retrieved June 10, 2021], Internet (URL: http: / / memes.sakura.ne.jp / memes / ?page_id=1120) [Non-patent document 2] [Retrieved June 10, 2021], Internet (URL: http: / / ednjapn.com / edn / articles / 1611 / 08 / news012.html) [Non-patent document 3] [Retrieved June 10, 2021], Internet (https: / / service.macnica.co.jp / library / 107577) Summary of the Invention [Problem to be solved by the invention]

[0005] The R-2R ladder circuit can configure a high-resolution, high-precision variable signal source with a relatively small number of resistor elements. However, to improve the output precision for the setting code, a high-precision resistor is required on the MSB side.

[0006] Resistor string circuits have low power consumption and high monotonically increasing characteristics, but the linearity of the output relative to the set code depends on the uniformity and layout of the resistor elements, so trial and error in layout design and manufacturing is required.

[0007] PWM circuits have the advantage of stable performance because they do not require a resistor element string like an R-2R ladder circuit or resistor string circuit and can be manufactured using only digital circuits, but they require the design and manufacture of a high-order low-pass filter with high frequency accuracy to remove ripple noise that appears in the output.

[0008] For R-2R ladder circuits and resistor string circuits, it is possible to improve linearity and accuracy by adjusting the resistor elements in the final manufacturing stage and correcting the relationship between the setting code and output. However, in this case, adjustments and corrections are made while checking the output of the R-2R ladder circuit and resistor string circuit, so a reference measuring device outside the circuit is required.

[0009] Furthermore, the offset voltage of the comparator, the unit voltage of the DA converter, and its linearity change over time. Therefore, to maintain conversion accuracy over a long period of time, periodic calibration is essential (Non-Patent Document 2).

[0010] However, adjusting the offset voltage of the comparator and the unit voltage of the DA converter requires a reference measuring instrument external to the AD converter. For example, to calibrate an AD converter located in a remote location, the measuring instrument must be carried with the user. This poses the challenge of making it difficult to calibrate multiple AD converters in remote locations.

[0011] The present invention has been made in view of this problem, and has as its object to provide an AD converter with a self-calibration function that does not require a measuring device for calibration. [Means for solving the problem]

[0012] An AD converter with self-calibration function according to one aspect of the present invention comprises: a first reference voltage unit that generates a temperature-compensated first reference voltage; a second reference voltage unit that generates a second reference voltage that is calibrated by the first reference voltage; an integrator that, during calibration, generates an integrated voltage by integrating a unit voltage using one of the first reference voltage, the second reference voltage, and a ground voltage as an initial value; a comparator that compares the integrated voltage with a threshold voltage and outputs a judgment signal; a calibration control unit that, during calibration, measures the integrated time from the initial value until the integrated voltage exceeds the threshold voltage and calibrates the unit voltage and an offset voltage of the comparator; and a conversion control unit that, during conversion, converts the input voltage into a digital value using a conversion integrated time, which is the integrated time when an input voltage is used as an initial value, and the second reference voltage. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide an AD converter with a self-calibration function that does not require an external measuring device for calibration. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a functional block diagram showing a configuration example of an AD converter with a self-calibration function according to an embodiment of the present invention; [Figure 2] 2 is a diagram illustrating the operation of an integrating unit shown in FIG. 1. FIG. [Figure 3] FIG. 10 is a diagram illustrating switching of unit voltages. [Figure 4] 2 is a diagram showing the relationship between the integrated voltage and current values ​​and the number of integrations in the integrating section shown in FIG. 1. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same reference numerals are used to denote the same parts, and the description thereof will not be repeated.

[0016] Fig. 1 is a functional block diagram showing an example of the configuration of an AD converter with a self-calibration function according to an embodiment of the present invention. In the AD converter 100 shown in Fig. 1, an integrator / converter integrates unit voltages using an input voltage as an initial value to generate an integrated voltage, and a comparator compares the integrated voltage with a threshold voltage to convert it into a digital value.

[0017] The AD converter 100 includes a first reference voltage unit 10, a second reference voltage unit 11, an integrating and converting unit 30, and a control unit 20. The integrating and converting unit 30 includes a switching unit 31, an integrating unit 32, a threshold voltage unit 33, and a comparator .

[0018] The control unit 20 is made up of a calibration control unit 21 and a conversion control unit 22. The calibration control unit 21 includes an offset measurement unit 210, a correlation measurement unit 211, a unit voltage measurement unit 212, and a reference voltage correction unit 213.

[0019] The first reference voltage unit 10 generates a temperature compensated first reference voltage, and the second reference voltage unit 11 generates a second reference voltage that is calibrated with the first reference voltage.

[0020] During calibration, the integrating section 32 generates an integrated voltage by integrating unit voltages using one of the first reference voltage, the second reference voltage, and the ground voltage as an initial value.

[0021] The comparator 34 compares the integrated voltage with a threshold voltage and outputs a determination signal.

[0022] During calibration, the calibration control unit 21 measures the integrated time from the initial value until the integrated voltage exceeds the threshold voltage, and calibrates the unit voltage and the offset voltage of the comparator 34.

[0023] During conversion, the conversion control unit 22 converts the input voltage into a digital value using a conversion integrated time, which is an integrated time when the input voltage is set to an initial value, and a second reference voltage.

[0024] The operation of each functional component of the AD converter 100 will now be described in detail.

[0025] (Integration section) 2 is a diagram showing a circuit model of the integrating unit 32. The integrating unit 32 includes a current source 320, SW1, and a capacitance C0.

[0026] FIG. 2 shows a1 and d1 of SW1, which are necessary for explaining the integration operation of integrating unit voltages, and omits other terminals.

[0027] When a charge equivalent to Vo,0 is stored in capacitance C0, the voltage Vo,1 of capacitance C0 can be expressed by the following equation after one integration operation in which a1 and d1 of SW1 are connected for Δt seconds and then a1 and d1 are disconnected.

[0028]

number

[0029] In this case, (I0 / C0)Δt is defined as the unit voltage VG (VG=(I0 / C0)Δt). The voltage Vo,k across the capacitance C0 when the above integration operation is repeated k times can be expressed by the following equation.

[0030]

number

[0031] When the initial value of the voltage of the capacitance C0 is Vo,0 and the integration operation is performed k2 times with the coarse adjustment unit voltage VG2 of the large unit voltage VG and k1 times with the fine adjustment unit voltage VG1 of the small unit voltage VG, the voltage Vo of the capacitance C0 can be expressed by the following equation.

[0032]

number

[0033] The unit voltage VG and the offset voltage of the comparator 34 need to be calibrated. This calibration is performed by controlling the integration / conversion unit 30 with a signal from the calibration control unit 21 to measure the number of integrations.

[0034] The calibration control unit 21 includes an offset measurement unit 210, a correlation measurement unit 211, a unit voltage measurement unit 212, and a reference voltage correction unit 213. The offset measurement unit 210 performs offset measurement processing.

[0035] The offset measurement processing measures the number of integrations until the voltage Vo of the capacitor C0 reaches the threshold voltage Vth with the second reference voltage Vrefs as the initial value. The offset voltage Vofc of the comparator 34 satisfies Vth + Vofc > Vo before the start of integration, but Vth + Vofc < Vo when the integration operation is repeated.

[0036] Since the determination signal, which is the output of the comparator 34, changes between the cases of Vth + Vofc > Vo and Vth + Vofc < Vo, the control unit 20 measures the number of integrations until the determination signal changes after the initial value is set and the integration operation is repeated.

[0037] In the offset measurement processing, after setting the initial value of the second reference voltage Vrefs, if the number of integrations with the coarse adjustment unit voltage VG2 and the fine adjustment unit voltage VG1 repeated until the determination signal changes are ko2 and ko1 respectively, the following equation holds.

[0038]

Equation

[0039] The switching between the coarse adjustment unit voltage VG2 and the fine adjustment unit voltage VG1 can be performed by the following method.

[0040] FIG. 3 shows a circuit for explaining the switching operation between VG2 and VG1. Let the current values of the current sources 320 for VG2 and VG1 be I2 and I1 respectively. The switching of the current value is performed with a signal MAGP from the control unit 20.

[0041] 4A and 4B are diagrams showing the relationship between the number of integrations and the integrated voltage and current values ​​in the integration unit 32. Fig. 4A shows the relationship between the number of integrations and the integrated voltage, and Fig. 4B shows the relationship between the number of integrations and the current value.

[0042] When integrating with VG2, a1 and d1 of SW1 are connected and a current I2 from current source 320 flows not only to capacitance C0 but also to resistor section 321. If the resistance of resistor section 321 is Rg, then at the moment when current I2 is flowing, a voltage of RG·I2 is superimposed on the integrated voltage.

[0043] When a1 and c1 of SW1 are connected and the current I2 stops flowing, the voltage generated in resistor unit 321 becomes zero. In integrator 32, when the voltage generated in resistor unit 321 is superimposed on the integrated voltage when the current generating VG2 flows from current source 320, exceeds the sum of threshold voltage Vth and the offset voltage of comparator 34, integrator 32 switches the current of current source 320 to the current generating VG1.

[0044] The correlation measurement unit 211 performs a correlation measurement process in which, after setting the reference voltage to an initial value, it accumulates the coarse adjustment unit voltages for the number of times obtained by subtracting 1 from the first coarse adjustment number, and then accumulates the fine adjustment unit voltages until the accumulated voltage exceeds the threshold value Vth.

[0045] In the correlation measurement process, after setting the initial value Vrefs, VG2 is integrated ko2-1 times, and then VG1 is integrated until the judgment signal changes. At this time, if the integration at VG1 until the judgment signal changes is kp1, the following equation is established.

[0046]

number

[0047] The unit voltage measurement unit 212 performs a unit voltage measurement process to measure a second coarse adjustment accumulation count, which is the number of times the accumulated voltage obtained by accumulating the coarse adjustment unit voltage with the ground voltage as the initial value exceeds the threshold voltage Vth, and a second fine adjustment accumulation count, which is the number of times the accumulated voltage obtained by accumulating the fine adjustment unit voltage exceeds the threshold voltage Vth.

[0048] The unit voltage measurement process repeats the integration operation with the ground voltage as the initial value until the judgment signal changes. If the number of integrations for the coarse adjustment unit voltage VG2 and the fine adjustment unit voltage VG1 are ki2 and ki1, respectively, the following equation is established.

[0049]

number

[0050] During conversion, the input voltage Vi is used as the initial value and the integration operation is repeated until the judgment signal changes. If the number of integrations for the coarse adjustment unit voltage VG2 and the fine adjustment unit voltage VG1 are kv2 and kv1, respectively, the following equation is established:

[0051]

number

[0052] The input voltage can be expressed by the following equation using ko2, ko1, kp1, ki2, and ki1 measured during calibration and kv2 and kv1 measured during conversion.

[0053]

number

[0054] The right side of equation (8) represents the converted voltage.

[0055] The offset voltage Vofc of the comparator 34 and the current I0 and capacitance C0 that calibrate the unit voltage fluctuate with temperature and time, so if Vofc, I0, and C0 are stored in a memory or the like and converted using equation (7), there is a possibility that the conversion error will be large.

[0056] According to the AD converter 100 of this embodiment, the conversion error can be improved by calibrating Vofc and the unit voltage during calibration.

[0057] (Fluctuation of reference voltage) It is known that the reference voltage output from a low-power reference voltage source configured with an integrated circuit, such as a bandgap reference, fluctuates with temperature and time. In the conversion operation of this AD converter 100, the reference voltage Vrefm is stored in advance, so if the actual standard reference voltage Vrefs fluctuates from the stored reference voltage Vrefm, an error occurs between the actual input voltage and the converted voltage. If the conversion value for the actual standard reference voltage Vrefs is Vi,s and the conversion value for the stored reference voltage Vrefm is Vi,m, the conversion error is expressed by the following equation:

[0058]

number

[0059] (Calibration using a high-precision reference voltage source) High-precision reference voltage sources that are temperature-compensated and use Zener diodes, etc., are known to have small variations in reference voltage with temperature fluctuations and little drift. However, because they use Zener diodes, they have a large voltage and require a heater for temperature compensation, so they consume a lot of power. For this reason, they are not suitable for use as a reference voltage source for constant calibration, as this would shorten the continuous use time of battery-powered devices.

[0060] By using it infrequently to calibrate the standard reference voltage Vrefs (second reference voltage), the continuous use time and accuracy of the device can be ensured.

[0061] In this AD converter 100, the high-precision reference voltage Vrefo (first reference voltage) is set equal to the stored reference voltage Vref. When calibrating using a high-precision reference voltage source, a reference voltage calibration process is performed in addition to the offset measurement process, correlation measurement process, and unit voltage measurement process. In the reference voltage calibration process, the high-precision reference voltage Vrefo is set as the initial value, and the accumulation operation is repeated until the judgment signal changes. If the number of accumulations for the coarse adjustment unit voltage VG2 and the fine adjustment unit voltage VG1 are kr2 and kr1, respectively, the following equation holds:

[0062]

number

[0063] |Vrefo - Vrefs| < When it is VG2, kr2 = ko2. In this case, by taking the difference between equations (4) and (10), the relational expression between Vrefo and Vrefs can be obtained.

[0064]

Number

[0065] VG1 can be derived as follows from equations (4) to (6).

[0066]

Number

[0067] Vrefs can be expressed by the following equation from equations (11) and (12).

[0068]

Number

[0069] When equation (13) is substituted into equation (8), the converted voltage can be obtained by the following equation.

[0070]

Number

[0071] Since the stored reference voltage Vrem and Vrefo are equal, the conversion value Vi,m at the stored reference voltage Vrem is equal to equation (14), so the error from the actual conversion value Vi,s becomes zero.

[0072] Thus, the AD converter 100 according to this embodiment has high long - term stability of conversion accuracy and low power consumption.

[0073] As described above, the AD converter 100 includes a first reference voltage unit 10 that generates a temperature-compensated first reference voltage, a second reference voltage unit 11 that generates a second reference voltage calibrated by the reference voltage, an integrator 32 that generates an integrated voltage by integrating a unit voltage using one of the first reference voltage, the second reference voltage, and the ground voltage as an initial value during calibration, a comparator 34 that compares the integrated voltage with a threshold voltage and outputs a determination signal, a calibration control unit 21 that measures the integrated time from the initial value until the integrated voltage exceeds the threshold voltage during calibration and calibrates the unit voltage and the offset voltage of the comparator 34 during conversion, and a conversion control unit 22 that converts the input voltage into a digital value using the second reference voltage and the conversion integrated time, which is the integrated time when the input voltage is set to the initial value. This makes it possible to provide an AD converter with a self-calibration function that does not require an external measuring device for calibration.

[0074] The calibration control unit 21 also includes an offset measurement unit 210 that measures a first number of integrations until an integrated voltage obtained by integrating the coarse adjustment unit voltage with the second reference voltage as an initial value exceeds the threshold voltage, and a second number of integrations until an integrated voltage obtained by integrating the fine adjustment unit voltage exceeds the threshold voltage, and a correlation measurement unit 211 that measures a third number of integrations until an integrated voltage obtained by integrating the fine adjustment unit voltage exceeds the threshold voltage after integrating the coarse adjustment unit voltage a number of times obtained by subtracting 1 from the first number of integrations after setting the second reference voltage as an initial value. a unit voltage measurement unit 212 that measures a fourth number of integrations until the integrated voltage obtained by integrating the coarse adjustment unit voltage with the ground voltage as an initial value exceeds the threshold voltage, and a fifth number of integrations until the integrated voltage obtained by integrating the fine adjustment unit voltage exceeds the threshold voltage, and a reference voltage correction unit 213 that measures a sixth number of integrations until the integrated voltage obtained by integrating the coarse adjustment unit voltage with the second reference voltage as an initial value exceeds the threshold voltage, and a seventh number of integrations until the integrated voltage obtained by integrating the fine adjustment unit voltage exceeds the threshold voltage.

[0075] According to the embodiment described above, even if the offset of the comparator 34 or the capacitance constituting the integration unit fluctuates over time, it is possible to calibrate the offset of the comparator 34 and the integration unit 32, which is the analog value output unit of the DA converter, and it is possible to provide an AD converter with high long-term stability. [Explanation of symbols]

[0076] 10: First reference voltage section 11: Second reference voltage section 20: Control unit 21: Calibration control section 22: Conversion control section 30: Accumulation and conversion section 31: Switching section 32: Integration section 33: Threshold voltage section 34: Comparator 100: AD converter with self-calibration function 210: Offset measurement unit 211: Correlation measurement unit 212: Unit voltage measurement unit 213: Reference voltage correction unit 320: Current source

Claims

[Claim 1] a first reference voltage unit that generates a temperature compensated first reference voltage; a second reference voltage unit that generates a second reference voltage; an integrating unit that generates an integrated voltage by integrating a coarse adjustment unit voltage and a fine adjustment unit voltage using any one of the first reference voltage, the second reference voltage, a ground voltage, and an input voltage as an initial value; a comparator that compares the integrated voltage with a threshold voltage and outputs a determination signal; an offset measurement unit that measures a first integration count ko2 of the coarse adjustment unit voltage and a second integration count ko1 of the fine adjustment unit voltage, which are integrated using the second reference voltage as an initial value until the determination signal changes, in an offset measurement process; a correlation measurement unit that, in the correlation measurement process, integrates the coarse adjustment unit voltage by a number of times obtained by subtracting 1 from the first integration number ko2 using the second reference voltage as an initial value, and then measures a third integration number kp1 by integrating the fine adjustment unit voltage until the determination signal changes; a unit voltage measurement unit that measures a fourth integration count ki2 of the coarse adjustment unit voltage and a fifth integration count ki1 of the fine adjustment unit voltage, which are integrated using the ground voltage as an initial value until the determination signal changes, in a unit voltage measurement process; a conversion control unit that, during conversion, measures a sixth number of integrations kv2 of the coarse adjustment unit voltage and a seventh number of integrations kv1 of the fine adjustment unit voltage, which are integrated using the input voltage as an initial value until the determination signal changes; During conversion, the conversion control unit converts the input voltage into a digital value using the first integration number ko2, the second integration number ko1, the third integration number kp1, the fourth integration number ki2, the fifth integration number ki1, the sixth integration number kv2, the seventh integration number kv1, and the second reference voltage. AD converter.

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