DE3124231A1 - Arrangement for representing a variable electrical precision resistor - Google Patents

Description

May 4, 1981 122/13

Company

M.K. Juchheim GmbH & Co.

Moltkestrasse 13 - 31

6400 Fulda

"Arrangement for the representation of a variable electrical precision resistor"

The invention relates to an arrangement for representing a variable precision electrical resistor using at least one fixed reference resistor.

Precision resistors are often used in technology, for example in the calibration of measuring and control devices. The implementation of physical quantities such as temperature, pressure, Humidity, distance or angle in a directly measurable electrical

May 4, 1981 122/13

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Size. Examples of this are temperature measurement using sensors with temperature-dependent resistances and the Implementation of pressure, humidity, path to the angular position a potentiometer.

In order to be able to use a measuring and control circuit, both the transducer and the data acquisition system must can be adjusted precisely and independently of one another due to the necessity of replacing the assemblies. the The calibration instruments used must be at least one size class more precise than the accuracy of the one used System.

In the event that the measured value transmitter system is a variable resistor, its calibration is still an option today complex process that can only be carried out with precision resistors. There is a special oath for each transmitter curves in the form of tables. In order to calibrate a device, the physical quantity must be found in the associated table, the associated resistance value can be read off and set on a precision resistor. This measure is time consuming and easily leads to transmission errors.

Precision potentiometers, for example, are known as precision resistors. In these devices, the terminal resistance is from the one set on the potentiometer shaft Angle of rotation dependent. The angle adjustment can here by means of an electronic control circuit.

However, the disadvantages of such a circuit are as follows: When the potentiometer is wound, the resistance changes leaps and bounds from turn to turn. The resulting

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Grid dimension is relatively large. This disadvantage does not exist with potentiometers with layered resistors, however, in the case of such resistors, the temperature coefficient of the resistance material does not meet the requirements Requirements. In general, potentiometers cannot be manufactured with sufficient linearity. As a result, lets the electromechanical angle adjustment does not work with achieve sufficient precision.

So-called precision decades are also known. These consist of a large number of possibly different ones Individual resistors connected to one another by switching elements in a wide variety of combinations can be, whereby a different total resistance can be achieved. For making a Precision decade, extremely high demands are placed on the selector switches or changeover relays. Also will requires a large number of precision resistors, which must also be precisely adjusted individually. The contacts the changeover switch or relay are usually susceptible to corrosion, which results in a change in resistance. One electronic control of the resistance value would be through an electromechanical actuator is possible, but is very expensive.

The invention is therefore based on the object of producing a precision resistor without a large number individually adjusted resistors and without a corresponding number of switching elements and yet extremely fine

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the preselection or setting of a precise resistance value in stages permitted. It should have a high level of reproducibility and Stability of the resistance value can be given as well as the possibility of the resistance through a computer or precisely set the information generated by Mikroprozes9Dr to be able to.

The object is achieved in the arrangement described at the outset according to the invention in that a with a tap connected to a first terminal to a reference voltage source applied reference resistance via a reference voltage line an input of a controllable Digital-to-analog converter is switched on that the output of the Digita! analog converter to an output amplifier for converting the output current into a proportional one Voltage is applied and that the output of the output amplifier together with a tap on the reference voltage line containing a resistor, a summation amplifier is switched on, its output with a second Terminal is connected in such a way that the total resistance between the first and the second terminal of the arrangement depending on the setting on the Digita! analog converter Binary number a multiple proportional to the binary number of the reference resistance.

The invention is based on the principle, preferably by means of a single fixed precision resistor, the reference resistor is to be designated, and a circuit that can be controlled with different binary numbers Digital-to-analog converter, at the output of this switching

May 4, 1981 122/13

circuit to form a total resistance, which is a multiple of the reference resistance according to the given binary number represents, whereby the "multiple" is to be understood as meaning both whole-number multiples and decimal fractions are.

Such a precision resistor is proportionate to form simple components available as catalog items, as detailed in the detailed description will be. An essential prerequisite for this is the presence of a reference resistor, which is largely the Ideal case should correspond to Ohm's law. Such However, resistors can be produced. As will be demonstrated by means of a numerical example, can be found on this way an adjustable or programmed resistor in the case of a 15-bit Digita! -Analog converter Set in a resistance range of 0.00 ohms to 655.35 ohms with a pitch of 0.01 ohms leaves. With such precise and extremely finely adjustable resistance values, calibrations and balancing processes can be carried out perform extremely reliably of all kinds.

The arrangement can be constructed in such a way that it does not have any mechanical components, for example no switching contacts contains, so that no aging due to the effects of corrosion is possible. If the arrangement is for example for switching over the measuring range contains mechanical components, it can be designed so that the contact resistance of the changeover switch is only effective to a small extent.

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Depending on the design of the arrangement, the inaccuracy for special applications to 10 ppm of the maximum setting range be restricted. For general use cases However, 30 ppm can be achieved without any special effort if the properties are commercially available today Components are taken into account. In extreme cases, the grid can be wet or the precision can be spread to 1: 250,000, for example by using an 18-bit D / A converter. The subject of the invention can also be used to achieve negative resistance values produce.

The subject of the invention can also be used as a preliminary stage for use a potentiometer output with little additional effort. The Angular position of the potentiometer wiper controlled. Of the The output of the grinder is extremely low-resistance.

The resistance or the angular position of the potentiometer is controlled by digital information. The information is in the form of a binary number that is immediate determines the resistance or the angular position. in particular advantage. Wei se can do the control of the Digita! -Analog converter by a microprocessor which is connected upstream of the digital-to-analog converter is. In the microprocessor, for example, various Linearization tables are stored so that a Calibration can take place directly in physical quantities. Such a device can be used as an interface module for any computer system.

Further advantageous configurations of the subject matter of the invention result from the other subclaims.

May 4, 1981 122/13

An embodiment of the subject invention is explained in more detail below with reference to FIGS. 1 to 5.

Show it:

Figure 1 is a basic circuit diagram of the subject of the invention,

Figure 2 shows a breakdown of the circuit diagram

Figure 1 in more details,

Figure 3 is a block diagram of a complete calibration source, in which the precision resistor according to Figures 1 and 2 is integrated and the additionally contains current and voltage outputs, which are also precise to multiples of Reference values are adjustable,

Figure 4
15th

the current output circuit of Figure 3 in larger resolution and

FIG. 5 shows the voltage output circuit from FIG. 3,

also in higher resolution.

In Figure 1, terminals 1 and 2 are shown, which are used to tap the resistor R set. A Reference voltage source connected, which supplies an external reference voltage U. Terminal 1 is connected to a reference resistor 3 connected, which is the real core of the invention, and from which one by means of the remaining details of the circuit

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May 4, 1981 122/13

Multiple is formed. The current I (x) flowing through the resistor can be measured electronically. If the current is known I (x) and the specified resistance R, the voltage U (x) applied to terminals 1 and 2 can be calculated. The Andes Voltage applied to terminals is generated by a controllable voltage source.

The well-known relationship of Ohm's law applies: U (x) = I (x) · R (1).

The reference resistor consists of the material zeranine and has a sufficiently good temperature coefficient of 0.4 ppm / K.

The current flowing through the system flows completely through the reference resistor 3 Voltage is applied to an amplifier 4 with a transmission ratio of 1: 1. It follows as a result

^U 1 ^{= U} 3 ^{= R} ref ^{I (x) (2)}

The output voltage of the amplifier is applied as reference voltage Uf via a connection point 5 and a reference voltage line 6 to an input 7 of a digital-to-analog converter 8 (DAC). The DAC is a 16-bit converter _5, ie the maximum number that can be set is 2 -1 = 65,535. Such a converter is available on the market, for example, under the type designation DAC 9331-16 from Hybrid Systems.

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The DAC is connected to 9 cuo üinsrzah ?. via a 16-core control line. fed. The output 10 of the DAC is connected to an output amplifier 11. With regard to the voltage present at its output, which is proportional to the binary number entered, the following applies:

U ₄ = -U _ref - _ ± _ = "U ₃ Z (3). ^{4 reT} · Zmax. ^J Zmax.

Here Z is the set binary number. The output amplifier 11 is via an intermediate resistor 13 and a Line 14 connected to a summation amplifier 15.

From the connection point 5, a tap 16 leads via a resistor 17 to the line 14 and thus to the same input of the summation amplifier 15. The intermediate resistor 13 has, for example, 30.0000 kilo ohms, while the resistor 17 has, for example, 5.0000 kilo ohms. Further resistors 18, 19 and 20 - in the order they are listed - are laid out as follows: 6,000; 10.0000 and 10.0000 kilohms.

Based on these design data, the positive input results of the summation amplifier 15 has a voltage Ur as follows:

U ₅ = U ₃ + U ₄ = U ₃ - U ₃ -Z = U ₃ f \ _ Z \ (4)

Based on the known relationships, the following now arises:

BATH ORIGINAL

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May 4, 1981 122/13

J ₂ = U ₃ Λ __Ζ __ ^ With U ₃ = R _ref - Kx) ν max

Kx) Z ₁ .

⁶ * ^Z max; = U (x) = R _ref Kx) - R _ref Kx)

U (x) = R _ref Kx)

U (x) = R _ref ■ Kx)

With U ₁ = R _f Kx)

6 Z

Max

TOT ^{= R} x ^{= R} ref -

-Λ

^Z max

It results from the fact that the reference _{resistor 3 R f,} due to the DAC used with 16 bits, has to be designed to be exactly 3,932.1 ohms in order to achieve the desired pitch of 0.01 ohms. It follows that the circuit according to the figure represents a programmable resistor which can _{be set exactly from O 3} OO ohms to 655.35 ohms with a pitch of 0.01 ohms. By changing the design, other resistance ranges or other grid dimensions can of course be achieved.

As shown, the DAC must have an externally accessible reference voltage input. The reference voltage must be freely selectable in a large area, for example between - 12 volts. Within a tolerance of - 1.2 volts, the accuracy of the output voltage U »may be below the Minimum requirements decrease. However, the output voltage IL must

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May 4, 1981 122/13

the function (3) suffice. Z is the maximum one

ma χ

adjustable number, i.e. in the present case 65,535.

In order to achieve maximum accuracy, the arrangement must of course be balanced. Since measures to match the However, they are familiar to a person skilled in the art, no further details are presented.

In Figure 2, the same parts as in Figure 1 are provided with the same reference numerals; however, the following is additionally To see: With the summation amplifier 15, each input is amplified by fan 1. The amplifier 4 provides one Represents the voltage follower, the at the reference resistor 3 applied voltage leads to the summation amplifier with low resistance. The same voltage also serves as a reference voltage Uf for the DAC. By a switch 21, a

Voltage source 22 to the voltage follower or amplifier be placed, for example, a fixed voltage of Puts out 10,000 volts. This fixed reference voltage is used for the current and voltage output circuits described below needed.

Between the connection point 5 and the reference voltage line 6, a further changeover switch 23 is provided, with which the positive or negative reference _{voltage U f} can optionally be applied to the input 7 of the DAC via an intermediate element 24 to generate positive or negative output currents or voltages and positive ones or to generate negative output resistances.

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The DAC generates from the reference voltage and the set voltage Binary number an output current in the subsequent output amplifier 11 is converted into a voltage. The gain can be switched by a switch 25. Is the If switch 25 is closed, the output voltage at 10 volts reference voltage and Z "= 65,535 is 1.3557 volts.

ITIaX

If the switch 25 is open, the gain is increased by a factor of 10.

Figure 2 is out of itself by the usual circuit symbols understandable; it is therefore based on figuring each Components omitted.

In FIG. 3, the same elements as previously are provided with the same reference numerals. In addition, the following applies recognize:

The output 12 of the output amplifier 11 is connected via a line 26 in parallel to a current output circuit and a voltage output circuit 28 connected to the corresponding clamps 27a, 27b or 28a, 28b. Further details are explained in more detail with reference to FIGS. 4 and 5.

As soon as the changeover switch 21 is moved from the position shown in the other position in which it contacts the contact 22a a fixed reference voltage is applied to the DAC. As a result, at the terminals 27a / 27b or 28a / 28b tap currents or voltages that are proportional to the binary number given to the DAC.

The arrangement shown defines a calibration source that is suitable for current, voltage and resistance. The arrival

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Order supplies voltages in the range between 65 mV to 10 V. The resolution in the 65 mV range is 1 pV. the Currents are in the range from 65 μΑ to 20 mA. The resolution is 1nA for the 65 nA range. The resistance is in the 2 ranges between 650 ohms and 6.5 kilo ohms adjustable. In the 650 ohm range, the resolution is 10 milliohms.

Figure 3 can also be seen that the DAC 8 a Microprocessor 29 and this in turn an operating part is upstream. Furthermore, the j-microprocessor is an analog-digital -Converter 31 connected to the input amplifier 32 a sensor 33 for the terminal temperature is connected upstream. With these additional devices the following can be achieved:

The set values are implemented in the microprocessor 29 Values of the operating part in binary coded numbers. It can the system can be set to the variable to be measured (current, voltage, resistance) or to the type of sensor concerned. Furthermore, the measuring ranges can be switched over by control lines 34 which correspond to those described above Lead switches (range switches). This is done by means of corresponding signals from the operating section 30th

Correction factors can be stored in the microprocessor, whereby a delinearization can take place, for example. By the sensor 33, the terminal temperature may be included so that a related 0 ⁰ C thermoelectric voltage can be generated. Furthermore, the microprocessor 29 causes the

BATH ORIGINAL

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May 4, 1981 122/13

would send the desired numerical value to the DAC 8.

The current output circuit according to FIG. 4 is is a current source consisting of an input amplifier 35 (chopper amplifier), an operational amplifier 36, which supplies a sufficient voltage level, and from an unspecified short-circuit-proof Transistor output stage. A negative feedback runs over all three stages, so that the overall circuit is an operational amplifier with the following data:

Offset voltage approx. 1 pV, offset drift approx. 10 nV / K, output voltage > 10 V, output current> 20 mA.

The trimmers are used for offset adjustment and adjustment the exact reinforcement. The indicated area switches were implemented by relays. The circuit is dimensioned such that it operates at an input voltage of 1.5567 volts supplies a current of 65.535 μΑ, 655.35 μΑ or 6.5535 mA as required.

The voltage output circuit of Figure 5 provides a The voltage source is analogous to FIG. 4 with an input amplifier 37 and an operational amplifier 38 is equipped. Here, too, the area switches were relays realized. The circuit is dimensioned in such a way that it has an input voltage of 1.5657 volts depending on the range Provides output voltages of 65.535 mV, 655.35 mV and 6.5535 V.

The statement also applies to FIGS. 4 and 5 that because of the representation of unambiguous symbols on a further detail

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May 4th 1981

Spelling can be dispensed with. In general, the following applies, that the measuring currents or measuring voltages are direct currents or DC voltages must be. It can be act positive or negative tensions. Noise is allowed. The measuring currents may - just like the measuring voltages - certain Do not exceed limit values. In order to achieve the maximum possible accuracy, you also need to be in a particular Area lie. For example, the ratio should I- / I. do not exceed approx. 10/1. An out-wed a χ min

The arrangement was based on the selected design data suitable for resistance currents from 0.3 to 3 mA.

Claims (1)

May 4, 198 '122/13 PATENT CLAIMS: Arrangement for representing a variable electrical precision resistor using at least one fixed reference resistor, characterized in that a tap (3a), connected to a first terminal (1), of a reference resistor (3) applied to a reference voltage source (IL) via a reference voltage line (6) an input (7) of a controllable digital-to-analog converter (8) is connected so that the output (10) of the digital-to-analog converter (8) is an output amplifier (11) for converting the output current into a proportional voltage (LL ) is activated and that. ^; the output (12) of the output amplifier (11) together with a tap (16) containing a resistor (17) on the reference voltage line (6) is connected to a simulation amplifier (15) whose output (15a) is connected to a second terminal (2 ) is connected in such a way that the total resistance of the arrangement between the first (1) and the second terminal (2) is a multiple of the reference resistance ( 3) forms. i
2. / Arrangement according to claim 1, characterized in that ■ an intermediate resistor (13) is arranged between the output amplifier (11) of the digital-to-analog / converter (8) and the summation amplifier (15), the resistance of which is a multiple of that of the resistor (1 7) located in the tap (16) / the reference voltage line (6) f is. May 4th 1981
122/13 3. Arrangement according to claim 1, characterized in that the digital-to-analog converter (8) is a 16-bit converter
is. 4. Arrangement according to claim 1, characterized in that the output (12) of the output amplifier (11) of the digital-to-analog converter (8) connected to a voltage output circuit (28) for generating a variable output voltage is that of the Digita! -Analog converter (8) given binary number is proportional. 5. Arrangement according to claim 1, characterized in that the output (12) of the output amplifier (11) of the
Digita! -Analog converter (8) is connected to a current output circuit (27) for generating a variable current which is proportional to the binary number given to the Digita! -Analog converter (8). Arrangement according to Claim 1, characterized in that the digital-to-analog converter (8) is used for control
Microprocessor (29) is connected upstream.