JP2000031823A - A/d converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-accuracy digital output against the fluctuation of a reference voltage without providing any complicated and expensive high- stability reference voltage generating circuit. SOLUTION: This device is provided with a reference voltage generating circuit 7 having a Zener diode 13, thermistor 11 having dependency on power supply voltage and temperature, resistors R1-R5 having power supply voltage dependency and Schottky barrier diode(SBD) 12 of a diode having temperature dependency. A CPU 6 performs A/D conversion respectively to a voltage Vt of the thermistor 11 and a voltage Vd from the voltage dividing circuit of resistors R2 and R3 with a reference voltage Vref' fluctuated by an A/D converter circuit 1, compares the values of respective digital signals and finds a resistance value Rt of the thermistor 11 having no power supply voltage dependency. A temperature T at that time is found from these temperature characteristics, and a voltage Vf provided from the SBD 12 is found. The reference voltage Vref' fluctuated by the voltage Vf and A/D converted digital signal converted by the reference voltage Vref' with the fluctuated voltage are estimated and A/D conversion is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D (Analog / Digital) converter for converting an analog input into a digital output, and more particularly to an A / D converter for preventing a decrease in digital output accuracy due to a reference voltage fluctuation. About the vessel.

[0002]

2. Description of the Related Art Conventionally, various types of A / D converters have been proposed. Basically, a ratio between an analog input and a reference voltage is obtained, and the value of this ratio is output as a digital value. The method is adopted. For example, in an A / D converter that outputs a 10-bit digital value, the analog input Vi
Is divided by the reference voltage Vref, and the quotient is 2 ¹⁰ = 1024
, A digital output can be obtained.
Therefore, in this type of A / D converter, the reference voltage Vre
When f is fluctuated, there is a problem that the value of the digital output with respect to the same analog input fluctuates.

[0003]

In order to solve such a problem, the reference voltage Vref is conventionally kept stable. One of the methods is to provide a high-stability reference voltage generating circuit. Although this type of high-stability reference voltage generation circuit has been proposed, an element that does not cause characteristic fluctuation due to temperature fluctuation or a circuit configuration for correcting this is used, so the circuit configuration is complicated and expensive. There is a problem of becoming something. Even when such a high-stability reference voltage generating circuit is used, if the reference voltage actually fluctuates, it cannot be dealt with, and the value of the digital signal output as a result fluctuates. , High-precision A / D
It is difficult to perform the conversion.

An object of the present invention is to provide an A / D converter capable of obtaining a digital output with high accuracy with respect to a change in reference voltage without providing a highly stable reference voltage generation circuit.

[0005]

An A / D converter according to the present invention includes an A / D (analog / digital) conversion circuit for comparing an analog signal with a reference voltage and converting the analog signal into a digital signal, a constant voltage element, and a power supply. A first element having voltage dependency and temperature dependency, a second element having power supply voltage dependency but no temperature dependency
A third element having little temperature dependency while having little power supply voltage dependency, and the reference voltage, power supply voltage and temperature-dependent voltage from the constant voltage element and the first to third elements, respectively. A reference voltage generating circuit for obtaining a power supply voltage dependent voltage and a temperature dependent voltage, and selecting the analog signal, the temperature dependent voltage or the temperature independent voltage to select the A / D
A selecting means for inputting to the conversion circuit; and a control means for selectively operating the selecting means and monitoring a digital signal output from the A / D conversion circuit to correct the reference voltage. In particular, the control unit sequentially selects the power supply voltage and the temperature-dependent voltage, the power supply voltage-dependent voltage, and the temperature-dependent voltage obtained from the first to third elements by the selection unit, and performs the A / D conversion. Input to a circuit, infer a fluctuated reference voltage of the reference voltage based on a value of a digital signal for each of the selected voltages obtained by the A / D conversion circuit, and estimate the fluctuated reference voltage. The value of a digital signal that is A / D converted by the A / D conversion circuit is corrected based on

In the present invention, for example, the reference voltage generating circuit includes a Zener diode as the constant voltage element, a thermistor as the first element, and a second element.
And a diode as the third element, wherein the reference voltage is generated as a breakdown voltage of the Zener element, and the power supply voltage and the temperature-dependent voltage depend on the thermistor and the resistance. As the divided voltage, the power supply voltage dependent voltage is generated as a divided voltage by a plurality of resistors, and the temperature dependent voltage is generated as a forward voltage of the diode. Then, the control means converts the voltage obtained from the thermistor into an A / D using the fluctuation reference voltage.
The value of the converted digital signal is compared with the value of the digital signal obtained by A / D-converting the voltage obtained from the voltage dividing circuit of the resistor with the variation reference voltage to obtain the resistance value of the thermistor independent of the power supply voltage. The temperature at that time is obtained from the resistance value of the thermistor and the temperature characteristic of the thermistor, the voltage obtained from the diode is obtained based on the obtained temperature, and the voltage obtained from the diode and the voltage The fluctuation reference voltage is estimated with the digital signal A / D-converted with the fluctuation reference voltage, and the A / D conversion in the A / D conversion circuit is corrected based on the estimated fluctuation reference voltage, thereby obtaining a highly accurate digital signal. Is output.

[0007]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an A / D converter according to the present invention. Here, the A / D conversion circuit 1 is configured to output an input analog signal as a 10-bit digital signal. That is, a dividing circuit 2 for dividing the analog input voltage Vin input to the analog input port Pa by a reference voltage Vref generated by a reference voltage generating circuit described later and input to the reference voltage input port Pr, and the dividing circuit 2 Is multiplied by a ¹⁰ -bit binary number, that is, 2 ¹⁰ = 1024, to obtain a digital value V
and a multiplication circuit 3 that outputs out. That is,
The division circuit 2 and the multiplication circuit 3 perform the operation of the equation (1) to output a 10-bit digital signal from the 10-bit output data bus 4 connected to the digital output port Po. Dout = K · (Vin / Vref) × 1024 (1) Here, K is a coefficient, and usually K = 1.

An analog multiplexer 5 is connected to an analog input port Pa of the A / D conversion circuit 1. A plurality of analog signals input to the analog multiplexer 5 are selected to select the analog signal. One analog input port Pa is configured to be input.
Further, a CPU 6 is connected to the output data bus 4 of the A / D conversion circuit 1 and receives a digital signal output to the output data bus 4, and performs a required operation based on the data value of the digital signal. And the switching of the analog multiplexer 5 is controlled to select an analog signal input to the analog input port Pa of the A / D conversion circuit 1. For example, when the A / D converter of the present invention is used in a camera, a printer, or the like, the CPU 6 controls the camera or the printer.
It is possible to use PU. The CPU 6
Is configured to be able to control the division circuit 2 of the A / D conversion circuit 1 and to set the coefficient K of the equation (1) by calculation. The coefficient K can be switched to a slightly different value of a plurality of coefficients K by a switching circuit built in the A / D conversion circuit 1, for example, a switching circuit built in the division circuit 2.

The reference voltage port P of the A / D conversion circuit 1
A reference voltage generation circuit 7 for generating a reference voltage Vref input to r includes a thermistor 11, a Schottky barrier diode (hereinafter, SBD) 12, a Zener diode (hereinafter, ZD) 13, and resistors R1 to R5. ing. The thermistor 11 is a first element, the resistors R2 and R3 are second elements, and the SBD 12 is a third element.
, The ZD 13 is configured as a constant voltage element. The reference voltage Vref is generated as a voltage obtained by dividing the power supply voltage VDD by the resistor R5 and ZD13. Another element is the power supply voltage VDD.
And a circuit for measuring the fluctuation of the reference voltage Vref caused by the fluctuation of the reference voltage Vref caused by the fluctuation of the characteristic, the fluctuation of the characteristic of the ZD and the fluctuation of the temperature. That is, the power supply voltage VDD is divided by the resistor R1 and the thermistor 11, and the voltage Vt whose value changes depending on the fluctuation of the power supply voltage VDD and the temperature.
Generate The power supply voltage VDD is determined by the resistors R2 and R3.
To generate a voltage Vd whose value changes only depending on the fluctuation of the power supply voltage VDD. Further, a resistor R4 and SB
D12 generates a voltage obtained by dividing the power supply reduced voltage VDD, that is, a forward voltage Vf of the SBD that depends only on temperature fluctuation. These voltages Vt, Vd, and Vf are input to the analog multiplexer 5 respectively. Thus, the analog multiplexer 5 selects the voltages Vt, Vd, and Vf by the CPU 6 together with the analog signal to be A / D-converted, and the A / D conversion circuit 1
Is input to the analog input port Pa, and the A / D conversion circuit 1 can perform A / D conversion.

In the reference voltage generating circuit 7, the reference voltage V
Since ref is set to the breakdown voltage of ZD13, a constant reference voltage Vref can be output even when the power supply voltage VDD fluctuates, and the A / D conversion of the formula (1) in the A / D conversion circuit 1 is performed. Run stably. However, since the breakdown voltage fluctuates due to variations in the characteristics of the individual ZDs due to manufacturing factors of the ZD 13 and fluctuations in the characteristics of the ZDs due to temperature changes, the reference voltage Vref also fluctuates to the reference voltage Vref ′. Vref '
As a result, A / D conversion in the A / D conversion circuit 1 is performed. For this reason, the accurate A / D conversion circuit 1
/ D conversion becomes impossible. Therefore, this fluctuation reference voltage V
By estimating ref 'and correcting the A / D conversion performed in the A / D conversion circuit 1 based on the fluctuation reference voltage Vref', an accurate digital output can be obtained.

In order to estimate the fluctuation reference voltage Vref ′, the thermistor 1 in the reference voltage generation circuit 7
1, the voltage V obtained from the SBD 12 and the resistors R1 to R4
t, Vd, and Vf are used. Hereinafter, a method of estimating the fluctuation reference voltage Vref ′ and an operation of A / D conversion using the estimated fluctuation reference voltage will be described. First, in the A / D conversion in the A / D conversion circuit 1, it goes without saying that the equation (1A) is obtained by replacing the reference voltage Vref in the equation (1) with the variable reference voltage Vref '. Dout = (Vin / Vref ′) × 1024 (1A) where Vin is an input analog signal (voltage), D
out is an A / D converted digital signal.

As a procedure for estimating the variation reference voltage Vref ', first, as shown in the flowchart of FIG. 2, the voltage Vt of the thermistor 11 and the voltage Vd of the resistors R2 and R3 which depend on the variation of the power supply voltage VDD are calculated. Power supply voltage V
The resistance value Rt of the thermistor 11 that cancels out the DD fluctuation element and depends only on the temperature fluctuation is obtained, and the temperature at that time is estimated from the resistance value Rt (step S10). Next, using the estimated temperature, the voltage Vf of the SBD 12 that depends only on the temperature fluctuation is obtained (step S11). Then, from the value of the digital signal obtained by A / D conversion of the voltage Vf by the A / D conversion circuit 1, it is estimated by calculating a fluctuation reference voltage Vref 'which is a reference of the A / D conversion. (Step S12).

Next, the above procedure will be described with reference to the flowchart of FIG. First, each of the voltages Vt and Vd obtained by the reference voltage generation circuit 7 is selected by the analog multiplexer 5 and input to the analog input port Pa of the A / D conversion circuit 1. The fluctuation reference voltage Vre
The values of the digital signal when the A / D conversion is actually performed by f ′ are Dt and Dd, respectively. These Vt, Vd
Dt = (Vt / Vref ′) × 1024 (2) Dd = (Vd / Vref ′) × 1024 (3) is obtained when A / D-converted Dt and Dd are obtained (step S101). ). These (2),
By dividing equation (3) and calculating (2) / (3), Dt / Dd = Vt / Vd (4) is obtained (step S102).

If the power supply voltage VDD is also fluctuated and the unknown power supply voltage is assumed to be VDD ', Vt = VDD' × (Rt / (Rt + R1)) (5) Vd = VDD '× (R2 / (R2 + R3)) (6) Substituting Vt and Vd of (5) and (6) into (4) and canceling VDD ′, Dt / Dd = [Rt / (Rt + R1)] / [R2 / (R2 + R3)] (7) Obtained (Step S103). Then, when the equation (7) is solved for Rt, Rt = (Dt · R3 · R1) / [(R2 + R3) · Dd−Dt · R3] (8) (step S104).

On the other hand, the thermistor 11 has a resistance value Rt
Has a temperature dependency, and as its characteristics, Rt = R ₂₅ × exp [B / (T + 273) −1 / (25 + 273)] (9) is obtained. Here, R ₂₅ is Rt at 25 ° C.,
B is a constant and T is temperature. By substituting equation (8) into equation (9) and solving for T, the temperature T is obtained (step S).
105). Note that the operation formula of T becomes complicated, and thus is omitted here.

In the SBD 12 configured as a silicon epitaxial planar type, Vf is proportional to T if If is constant. For example, If = 3 mA
And 0.4 V when T = 25 ° C. and 0. 0 when T = 40 ° C.
38V. From this, Vf = −T / 750 + 13/30 [V] (10) is obtained (step S106). Actually, if Vf changes, If changes, but the error due to it changes can be ignored.

Therefore, Vf can be obtained by substituting T obtained from the above equations (8) and (9) into the equation (10).
This Vf is selected by the analog multiplexer 5 and A / D
The signal is input to the conversion circuit 1 and is obtained by A / D conversion.
By substituting f into the expression (1A), Df = (Vf / Vref ′) × 1024 (1B), from which Vref ′ = (Vf / Df) × 1024 (1C) is obtained, and Vref ′ is obtained. (Step S10
7).

Therefore, in this A / D converter, A / D
When performing D conversion, the CPU 6 selects an analog signal to be subjected to A / D conversion in the analog multiplexer 5, inputs the analog signal to the analog input port Pa of the A / D conversion circuit 1, and also obtains the obtained fluctuation reference voltage Vre.
The coefficient K is calculated and corrected based on f ′, and this is calculated as A /
The signal is input to the division circuit 2 of the D conversion circuit 1. That is, K = Vref ′ / Vref (11) is corrected. As a result, the A / D conversion circuit 1 uses the variation reference voltage Vref 'to convert the analog signal into an analog signal.
Even when the D conversion is performed, by multiplying the quotient obtained from the division circuit 2 by the coefficient K corrected as described above,
The obtained value is the same value as the quotient obtained by division using the original reference voltage Vref. This gives Z
Even when the reference voltage fluctuates due to manufacturing variations of D or variations in characteristics due to temperature variations, accurate A / D conversion is realized.

As described above, according to the present invention, the thermistor, S
An A / D converter capable of obtaining a high-accuracy digital output with respect to fluctuations in the reference voltage can be configured using only inexpensive circuit components such as a BD, a ZD, and a resistor. This eliminates the need for a high-stability reference voltage generation circuit, a high-precision temperature measurement circuit, etc.
This makes it possible to simply and inexpensively configure the / D converter. FIG. 4 shows an A / D converter according to the present invention.
It is a figure which shows the simulation of A / D conversion in different temperature, and the measured value, It can be confirmed from the result that the A / D converter of the present invention functions effectively.

Note that each voltage Vr,
Vd and Vf are not limited to the above-described circuit configuration, but can be appropriately changed. Further, instead of the thermistor and the SBD, another active element or a passive element having a temperature dependency can be used.

[0021]

As described above, according to the present invention, the value of a digital signal obtained by A / D-converting a voltage obtained from a power supply voltage such as a thermistor and a temperature-dependent element with a fluctuated reference voltage, A voltage obtained from an element having a power supply voltage dependency is compared with a digital signal obtained by A / D conversion with a changed reference voltage, and a resistance value of a thermistor having no dependency on a power supply voltage change is determined. The temperature at that time is obtained from the resistance value and the temperature characteristics of the thermistor.Furthermore, an element having no temperature dependency, such as a diode, and having a temperature dependency, and a voltage obtained from the obtained temperature are obtained. Since the fluctuated reference voltage is estimated with a digital signal obtained by A / D-converting the voltage with the fluctuating reference voltage, the A / D conversion circuit in the A / D conversion circuit based on the estimated fluctuating reference voltage. Strange It is possible to output a highly accurate digital signal by correcting. Therefore, there is no need to provide a complicated circuit configuration using elements that are highly stable with respect to temperature, such as a high-stability reference voltage generating circuit. An A / D converter capable of performing the above can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an A / D converter according to an embodiment of the present invention.

FIG. 2 is a flowchart showing basic steps of an operation of estimating a fluctuation reference voltage in the A / D converter of FIG.

FIG. 3 is a flowchart illustrating a process of estimating a reference voltage based on the basic process of FIG. 2 using a mathematical expression.

FIG. 4 is a diagram showing simulation and measurement results of the A / D converter of the present invention.

[Explanation of symbols]

 Reference Signs List 1 A / D conversion circuit 2 Divider circuit 3 Multiplier circuit 5 Analog multiplexer 6 CPU 7 Reference voltage generation circuit 11 Thermistor 12 Schottky barrier diode (SBD) 13 Zener diode (ZD) R1 to R5 Resistance VDD Power supply voltage Vref Reference voltage Vref 'fluctuation Reference voltage

Claims (4)

[Claims] An A / D (analog / digital) conversion circuit for converting an analog signal to a reference voltage and converting it into a digital signal; a constant voltage element; a first element having power supply voltage dependency and temperature dependency; A second element having power supply voltage dependency and no temperature dependency; and a third element having almost no power supply voltage dependency and temperature dependency, wherein the constant voltage element and the first to third elements are provided. A reference voltage generating circuit for obtaining the reference voltage, the power supply voltage and the temperature-dependent voltage, the power supply voltage-dependent voltage, and the temperature-independent voltage from each element, and the analog signal, the power supply voltage and the temperature-dependent voltage, and the power supply voltage-dependent voltage Or a selection means for selecting a temperature-dependent voltage and inputting the selected voltage to the A / D conversion circuit, and selectively operating the selection means and monitoring a digital signal output from the A / D conversion circuit. An A / D converter comprising: a control unit for correcting the reference voltage. 2. The power supply voltage and temperature dependent voltage obtained from the first to third elements and the power supply voltage dependent voltage and temperature dependent voltage are selected in order to select the A /
Input to a D conversion circuit, the A / D conversion circuit estimates a fluctuated reference voltage of the reference voltage based on the value of the digital signal for each of the selected voltages, and The A / D converter according to claim 1, wherein the value of the digital signal subjected to the A / D conversion is corrected based on the value. 3. The reference voltage generating circuit includes a Zener diode as the constant voltage element, a thermistor as the first element, a voltage dividing resistor as the second element, and a third element. With a diode as
The reference voltage is generated as a breakdown voltage of the Zener element, the power supply voltage and the temperature-dependent voltage are divided voltages by the thermistor and a resistor, and the power supply voltage-dependent voltage is a divided voltage by a plurality of resistors. The A / D converter according to claim 2, wherein the dependent voltage is generated as a forward voltage of the diode. 4. The control means according to claim 1, wherein a value of a digital signal obtained by A / D-converting a voltage obtained from said thermistor with said variable reference voltage and a voltage obtained from said voltage dividing circuit of said resistor are converted into A by said variable reference voltage. The value of the thermistor, which does not depend on the power supply voltage, is obtained by comparing the value of the / D converted digital signal, and the temperature at that time is obtained from the resistance value of the thermistor and the temperature characteristic of the thermistor. The voltage obtained from the diode is obtained based on the voltage, and the variable reference voltage is estimated from a voltage obtained from the diode and a digital signal obtained by A / D converting the voltage with the variable reference voltage. A / D converter.