EP1132794A1 - Method to derive a temperature independent voltage reference and a circuit to derive such a voltage reference - Google Patents

Abstract

In a method for obtaining a temperature-independent voltage reference with a bandgap reference circuit using at least one bipolar transistor (4) and a voltage source (1), only one bipolar transistor (4) is connected in series with a resistor (R). Various voltages are optionally applied, and the voltages before (2) and after (3) the series resistance (R) are tapped and measured separately and fed to an analog-to-digital converter (ADC) (5). The amplification factor of the analog-digital converter (5) is calculated from the digitized measured values and used to correct the measured values. The circuit arrangement for obtaining such a temperature-independent voltage reference has a bipolar transistor (4) and a resistor (R) connected to the transistor (4) connected in series. An analog-to-digital converter (ADC) (5) is connected to taps (2, 3) on both sides of the resistor (R) via switches (S2, S3) in order to achieve digitized voltage measurement values. The digital signals of the ADC (5) are fed to a computer (7) for determining the gain factor, from which the corrected voltage signal can be read digitally. <IMAGE>

Description

The invention relates to a method for obtaining a temperature-independent voltage reference with a bandgap reference circuit using at least one bipolar transistor and a voltage source and on a circuit arrangement to obtain a temperature-independent voltage reference.

When using bipolar transistors and electronic components such as analog-digital converters, known temperature dependencies of the transistor parameters or the circuit must be taken into account if a temperature-independent voltage reference is to be created. In particular, the characteristics of a bipolar transistor are strongly temperature-dependent, the temperature-dependent relationship between the collector current I _c and the base emitter voltage U _{BE being} of particular importance. The dependence of U _BE on the temperature T results from the following equation: I. c = I S e q U BE kT

The reason for this temperature dependence of I _C is the temperature dependence of the reverse current I _S and the temperature voltage U _T = kT / q, taking into account the temperature dependence of the reverse current I. S = A e -QU G kT T x the following relationship I. c = A e -U G q kT + U BE q kT T x in which k is the Boltzmann constant (1.38 x 10 ^-23 VAs / K), q the elementary charge = 1.602 x 10 ^-19 As, U _G ≈ 1.12 V the gap voltage of silicon, T the temperature, x an empirical constant and A mean a proportionality factor. The temperature dependence of UG is usually neglected in the known circuit arrangements.

Most bipolar transistors result from these relationships with a temperature increase of 11 ° K an increase from Ic to double. In circuits to achieve Voltage references are already known, in principle the base emitter voltage of a bipolar transistor as a voltage reference to use. To compensate for the known high temperature dependence is in such known, analog circuits a voltage with a symmetrically equal positive Temperature coefficient added, this voltage with a second transistor is generated. The known bandgap reference circuits to achieve a voltage reference therefore usually two selected in their characteristics Transistors ahead, choosing with smaller tolerances must be done.

The invention now aims at a method of the beginning to create the type mentioned, in which only a bipolar transistor is used and therefore the selection of one the characteristic of this transistor tuned second transistor is unnecessary. The invention further aims to to further reduce the temperature dependence of the measured values and temperature compensation with much higher accuracy to achieve. To achieve this object, there is the invention Process essentially in that only a bipolar transistor is connected in series with a resistor that optionally different voltages are applied that the Voltages tapped before and after the series resistance and an analog-to-digital converter (ADC) and that the Gain factor of the analog-digital converter from the digitized Measured values are calculated and used to correct the measured values becomes. The fact that in the context of the inventive method an analog-digital converter is also used arrives and subsequently processes the signals in digital form the temperature dependence of such ADC circuits which has to be compensated. As part of the The method according to the invention is therefore made up of a plurality of Measured values of the amplification factor of the analog-digital converter for the prevailing temperature is determined and can be adjusted accordingly Form updated each time, so actually corrected measured values are available, which are characterized by a higher precision than that of analog circuits is possible.

According to a preferred implementation of the method according to the invention is done so that to correct the gain of the ADC a measured value for the base-emitter voltage of the Bipolar transistor and a measured value for the reverse current of the Bipolar transistor detected from the voltage drop across the resistor and that by using a calculation method, the temperature-dependent Parts of the two measured values eliminated and on more valid for the respective temperature prevailing at the time of measurement Gain factor is determined.

Within the scope of the method according to the invention, the procedure for determining the amplification factor is such that the amplification factor with S = - 1 - lnI x + x + ln A + xln q dlnI x k + ln R - 1 + dlnI x U G + x is calculated, in which lnI _{x is} the natural logarithm of the measured value for the collector current, x and A constant, R the resistance value and U _G the bandgap voltage (for Si ≈ 1.12 V). Since the amplification factor is always recalculated from a plurality of measured values by the algorithm which will be explained in more detail below, it is possible within the scope of the method according to the invention, as is in accordance with a preferred development, that the value for S is updated continuously or at regular time intervals and is used to calculate the actual reference voltage and, if necessary, for the exact determination of test voltages.

The circuit arrangement according to the invention for obtaining a temperature-independent voltage reference can be particularly simple be trained and comes with a small number of components out. The circuit arrangement is essentially as a result characterized in that they have a bipolar transistor and a has the resistor R connected in series, that on taps on both sides of the resistor R over Switch to achieve an analog-to-digital converter (ADC) digitized voltage measurements is connected and that the digital signals from the ADC to a computer to determine the gain factor are supplied, from which the corrected Voltage signal can be read out digitally.

The switch can be used as a multiplexer component in a particularly simple manner be formed, the inputs of a control signal of the computer are switched and have connections which voltages to measure by actuating the assigned Switch. The multiplexer thus transmits each after setting the switch, the analog signals to the analog input of the ADC. In principle, the circuit arrangement can be found under Can be built using PNP or NPN transistors. in the The case of PNP transistors is the emitter with the resistor connected, and the collector connected to the base to ground placed with the adjustable voltage source on the other Connection of the resistor is connected.

A preferred use of the circuit arrangement according to the invention represents use in a digital voltmeter, the basic procedure as well as the circuit arrangement in no way on such digital voltmeters is limited.

The invention is based on the calculation of the Gain factor selected algorithm and based a circuit example for a digital voltmeter closer explained.

Starting from the basic relationship, which shows the dependence of U _BE on the temperature T in a bipolar transistor, I. c = I S e q U BE kT it is now further taken into account that not only the collector current but also the reverse current I _{S is} temperature-dependent. The temperature dependence of the reverse current follows the relationship I. S = A e -QU G kT T x the meanings already mentioned apply in these relationships.

By inserting the meaning I _S according to equation (2) into equation (1), the relationship can be I. c = A e -U G q kT + U BE q kT T x receive.

If an analog-digital converter is used, the analog measured values in the ADC experience a temperature-dependent gain S, which would result in corresponding errors if temperature compensation were not carried out. For the computational elimination of such errors, U _BE is first replaced by U _x , which gives the relationship U _BE = U _x / S and U _{x means} the measured voltage, which has to be corrected by using the correct amplification factor. Likewise, I _C can be replaced by the actual measured value I _x , which is measured as a voltage drop across resistor R and must have the same gain factor S. The relationship results from appropriate substitution I. c = e lnI x RS subsequently following the relationship lnIx = In [A e -U G q kT + U x q kT RST x ] the natural logarithm of this current measurement is expressed. In this respect, the function of I and U _{x x} thus is represented graphically, wherein plotted on the y-axis lnI _x and the x-axis U _x. A straight line is obtained with the slope dlnI _x , which intersects the y-axis at the corresponding value dlnI _x at U _x = 0. The slope of this straight line is thus dlnI x = q kST By solving this relationship according to T one obtains T = q dlnI x k p At the point U _x = 0 can now by inserting in lnI x = ln [A e - U G q kT + U x q kT RST x ] the relationship lnI x = ln [A e - U G q kT RST x ] be derived. Appropriate transformations of this equation lead to the relationship lnI x = - U G q kT + ln A + ln R + ln S + xln T and further on lnI x = -dlnI x U G S + ln A + ln R + xln q dlnI x k p + ln p and finally lnI x = -dlnI x U G S + ln A + xln q dlnI x k + ln R + ln S From this relationship it can now be clearly seen that the absolute temperature T no longer appears for the determination of the true value of the amplification factor S, since this relationship only contains universal constants U _G , q, k, and the known values and temperature-independent expressions x, A and contains the only slightly temperature-dependent value R. If the temperature dependence of R is also to be taken into account, this can be done, for example, by modifying the value x accordingly.

In order to solve this equation, a Taylor expansion of first order for ln S around 1.0 can be carried out, which results in lnI x = -1 + S + dlnI x U G S - (-1 + S) x + ln A + xln q dlnI x k + ln R results. Solving this equation yields S = -1-lnI x + x + ln A + xln q dlnI x k + lnR -1 + dlnI x U G + x

In total, x, A and R can be used individually for each circuit arrangement can be calibrated using particularly suitable values a simulation can be calculated in advance.

In a continuously self-calibrating system the value for the gain factor S is continuous in each case or updated at regular intervals so that increasingly accurate values are obtained iteratively. Because of a such an iteration process is also straightforward allowed, only a first order Taylor expansion in the above Use calculation.

Such calculations can be carried out without special calibration an accuracy of about 1% can be achieved. If the values can be optimized accordingly for x, A and R, the Accuracy to less than 0.1% over a working temperature range of about 100 ° K.

The invention is described below with reference to a drawing illustrated embodiment of a digital voltmeter explained in more detail.

In the drawing, 1 denotes a variable voltage source with which different voltages can be generated. The voltage is applied to terminal 2 of a resistor R, a PNP transistor being used in the circuit arrangement shown, the emitter E of which is connected to the tap 3 of the resistor. The base and collector of the bipolar transistor 4 are in turn connected to ground or zero potential, the voltage values which can be tapped at 2 and _{3 being} alternatively fed to the analog-digital converter as analog signals via switches S ₂ and S ₃ . The signal digitized in the ADC 5 is sent via the signal line 6 to a computer 7, in which the corresponding corrections are made in accordance with the computing algorithm mentioned above. A further switch S _{1 is} provided for use as a digital voltmeter, via which a test voltage can be applied to the ADC 5 via the connection 8 and measured .

The switches S ₁ , S ₂ and S ₃ are now each closed alternatively, it being possible for these switches S ₁ , S ₂ and S ₃ to be contained in a multiplexer and the switch positions themselves to be controlled by the computer 7. In principle, the voltages at taps 2 and 3 must be determined for the measured value V _x = I _x · R and subtracted from one another, the measured variable V _{x being able to} be determined via switch S3 with switches S1 and S2 open. Since the voltage source 1 can be set to different voltages, different measuring points can be formed for the above-mentioned evaluation, from which the current value for S can be calculated.

Overall, digital reference voltage technology applied, which is a continuous recalibration of the ADC allowed, by appropriate frequency of this calibration not only temperature effects but also others from which Operating time-dependent effects are largely compensated for can.

Claims (5)

Method for obtaining a temperature-independent voltage reference with a bandgap reference circuit using at least one bipolar transistor and a voltage source, characterized in that only one bipolar transistor is connected in series with a resistor, that optionally different voltages are applied, that the voltages before and after the series resistor tapped and fed to an analog-to-digital converter (ADC) and that the gain factor of the analog-to-digital converter is calculated from the digitized measured values and used to correct the measured values. Method according to Claim 1, characterized in that, to correct the amplification factor of the ADC, a measured value for the base emitter voltage of the bipolar transistor and a measured value for the reverse current of the bipolar transistor are detected from the voltage drop across the resistor and that the temperature-dependent components of the two measured values are eliminated by using a computing method and a gain factor valid for the respective temperature prevailing at the time of measurement is determined. A method according to claim 1 or 2, characterized in that the gain factor with S = -1-lnI x + x + ln A + xln q dlnI x k + lnR -1 + dlnI x U G + x is calculated in which lnI _{x is} the natural logarithm of the measured value for the collector current, x and A constant, R is the resistance value and UBG is the band gap voltage (for Si ≈ 1.12 V). Method according to Claim 3, characterized in that the value for S is updated continuously or at regular time intervals and is used to calculate the actual reference voltage and, if appropriate, test voltages. Circuit arrangement for obtaining a temperature-independent voltage reference, characterized in that it has a bipolar transistor (4) and a resistor R connected to the transistor (4) in series, that on taps (2, 3) on both sides of the resistor R via switches ( S ₂ , S ₃ ) an analog-digital converter (ADC) (5) is connected to achieve digitized voltage measurements and that the digital signals of the ADC (5) are fed to a computer (7) for determining the gain factor, from which the corrected voltage signal can be read digitally.

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