JPS61114173A - Method and device for measuring impedance value - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The impedance value measuring device of the invention can be used even when the device under test is powered down, i.e. when the measuring device provides whatever power is required for the measurement. It is desirable to have the ability to make measurements of

However, the devices to be tested exhibit a wide range of voltage requirements and M limits. For example, if a l)n junction is included in the circuit under test, any applied voltage must be low enough to prevent damage to the pn junction or unwanted turn-on. Because of the variable value of the required excitation voltage, and because of the desire to avoid switching between quadruple ranges if possible, it is advantageous to scale the impedance ratioally, ie by the standard impedance rather than by the applied voltage. be.

Although devices have been available for making impedance ratio measurements, these devices typically required fairly complex and expensive circuitry to provide digital output readings. Conventional bridge circuits or electronic analogues can thus measure impedance in seconds or standard impedance.

Computer circuits are also available that measure the voltage across an unknown impedance and a standard impedance, and then calculate the ratio between the two. This strategy has a speed subdivided by the speed and complexity of the combination.

The desired elimination of switching between different measuring ranges has also hitherto required some mechanism to compress the measuring scale. Logarithmic amplifiers have been used in the past, but unfortunately these amplifiers tend to be unstable, inaccurate, expensive, and occupy considerable printed circuit board real estate.

SUMMARY OF THE INVENTION In accordance with the present invention in particular embodiments, a predetermined voltage source, adjusted according to the requirements of the device under test, is coupled to a series combination of an unknown impedance and a standard impedance.

The same input voltage is also adjustably attenuated according to the digital value derived from the continuous approximation.

The attenuated voltage is compared to a voltage drop that is correlated to a standard impedance, and as a result of the comparison, a digital value is finally determined to provide a measurement of the unknown impedance in proportion to the standard impedance.

aTlllk: A digital value of continuous approximation is provided by a register, where binary digits continuously determine the degree of attenuation of a predetermined voltage during comparison with the voltage across the unknown impedance. For example, 2
The binary digits are generated progressively starting with the most significant digit, each providing an attenuation of the upper voltage inversely proportional to the binary digit value. Successive comparisons are made and binary numbers are accumulated to represent the value of the unknown impedance ratio in terms of standard impedance. This method is very fast and requires a number of clock cycles equal only to the number of output digits (resolution) provided by the successive approximation register.

This technique is economical in terms of component parts and relatively uncomplicated compared to prior art devices. Compression of the desired scale is also achieved with considerable accuracy, avoiding the use of logarithmic amplifiers and the like.

It is therefore an object of the invention to provide an improved method and apparatus for determining the value of impedance.

Another object of this invention is to provide an improved method and apparatus for measuring unknown impedances and quickly providing output readings as digital values.

Yet another object of the invention is to provide an improved impedance measurement method and apparatus that is substantially independent of the test voltage applied to the unknown impedance.

Yet another object of the invention is to provide an improved method and apparatus for accurate impedance measurement that is uncomplicated and inexpensive to implement.

Yet another object of the invention is to provide an improved impedance measurement method and apparatus that avoids range switching and provides scale compression while maintaining accuracy.

The subject matter of this invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, the mechanism and method of operation, together with further advantages and objects, may best be understood by reference to the following description taken in conjunction with the accompanying drawings. Similar reference characters here represent similar elements.

"1 Rainbow 1" Referring to the drawings, and in particular to FIG. 1, which illustrates the invention in block diagram form, an excitation voltage Vr is applied from an amplifier 12 to a node 10 of the circuit, and this excitation voltage is connected between node 10 and ground. This same excitation voltage is applied to a series circuit comprising an unknown impedance with the value Zx and a standard with the value 7r placed between the known impedance 16. This same excitation voltage is multiplied by the
It is applied as a reference input to a /A converter or programmable attenuator 18, where the value Vr is extracted to provide a value to lead 20 in accordance with digital attenuation input 21. In particular, the multiplier 0/Δ includes an R/2R circuit, described more fully below, which receives a plurality of binary attenuation decision inputs provided by a continuous approximation register 24. Register 24, also described more fully below, consists of two consecutive
The base digits are provided in a predetermined order, e.g., descending order, and each digit undergoes a proportional degree conversion within the D/A converter.

The lead 200 reduced output from the D/A converter 18 is connected to an unknown impedance 14 within a comparator 22.
and a known impedance 16 and compared with the voltage at the - node 25. Continuous approximation register 2
4 provides the output digits in a predetermined order, eg, starting with the most significant bit, and comparator 22 indicates whether the output voltage on lead 20 is greater than the voltage on lead 25. If the voltage on lead 20 is higher, the first (top) pit in the output of register 24 is cleared. On the contrary, the lead is 25 and 1! If ff is higher (if
It is held as an input of 8. In either case, register 24 then outputs the next most significant bit to D/Δ converter 1.
8 and the sequence is repeated. Digital output 24 from continuous approximation register 24 ultimately selects attenuation within D/ATI inverter 1B, causing a substantial correspondence of the attenuated output 20 with the voltage at tap 26.

The digital output of resistor 24 is an accurate representation of the fractional excitation voltage present at tap 26 and thus an accurate measurement of the unknown impedance.

Assuming the impedance is resistance, the conversion result N
is expressed as follows; Vx /Vr = N/2' [1] Also, t;L
N=Vx 2' /Vr where n is equal to the resolution of the D/A converter 18 expressed as an integer. The conversion result Ni is an integer value falling within the following range: 0<=N<2°. Furthermore, due to the nature of the circuit network, Vx<Vr.

As seen from Figure 1: Vx - Vr Rr / (Rx + Rr)
By flowing [21 equation [2] into equation 11], Rx = Rr ((2°/N>-1) r3]
It will be noted that the immediately preceding equation for the value of Rx is independent of the **ri pressure Vr. The only consideration regarding the reference voltage is that it must be large enough to reduce noise, bias, and other non-ideal real-world effects. At the same time, Vr must be small enough to avoid unwanted conduction through pn junctions or the like connected to the resistor under test.

In a detailed embodiment, Vr is provided by amplifier 12 and can vary from 40 mV to 10 V without affecting system accuracy. The unknown value IJ' in equation [3] is the integer output of continuous approximation register 24, denoted N, and I! Quasi 11 [Anti-Rr value.

The result 1.1 is obtained very quickly, with the continuous approximation register 2/l cycling through a number of clock cycles corresponding to its resolution, which in this example is 12. The final resistance value (determined in proportions, i.e. by the standard resistance rather than the voltage value, is extremely accurate. Furthermore, the circuit is linear and uncomplicated. Furthermore, the effect of the scale contraction This is achieved by the equation [
3], an asymptotic relationship with two boundary values is noted. The first boundary value of N is 2'=1, and the 21st boundary value is O. In the boundary region, there is a range of component values that shores up the old numerical value of the narrowing range. By differentiating equation [3], we can provide the expression for the scale as a function of the reference component N: (d /d N) (Rx) = (d /d N
) (Rr((2°/N1-1) -Rr(2”)(N-2).

The self-rate error as a function of the measured component and using a 12-bit implementation is shown graphically in Figure 3, where Rx is the unknown resistance, Rr is the standard resistance, and the II of Rx Differences are graphically represented logarithmically. The curve provides the one bit error as a function of the resistance at which it occurs.

It will be observed that the error is less than 1% over a wide range of unknown resistances.

Referring still to FIG. 2, a circuit according to the invention is illustrated in more detail, particularly with respect to continuous approximation register 24 and I)/A converter 18. A particular example of a continuous approximation register is manufactured by Motorola and is shown in the drawing
It includes a pair of registers such as an MC14559B type register shown at 2 and an MC14549B type register shown at 34. The output of comparator 22 is connected to the data input of each register, while the conversion end output number of register 32 is connected to the conversion start input of register 34. The end of conversion output of register 34 (not shown) is J:
<Used in an understood manner to indicate that the continuous approximation is complete. The two registers are used together to
A 12-bit output is provided for continuous operation of switch 36 of Δ converter 18.

D/A converter 18 is suitably an AD7541 type device manufactured by Analog Devices. This includes a conventional R/2R network as shown, where an R-value serial connection 38 is joined between node 10 and ground via a final 2R leg 40, while a 2R resistor legs 42 are each connected by a respective switch 36 to an input of an output amplifier 44 of the device. Switches 36 actually include CMO8 switching circuits contained within the converter, each of which is electrically closed by a binary digital output from continuous approximation register 24. In the inactive state, switch 36 is suitably connected to ground.

Reviewing the operation of the circuit, registers 32 and 34 are:
It operates to provide a continuous output on lead 46 starting with the R high order bit output. Such an output is connected to resistor 4
2 to the input of amplifier 44, closing switch 50;
If comparator 22 supplies a data signal indicating that the voltage at tap 26 is higher than the voltage provided by amplifier 44, the output bit on lead 46 is held and the top switch 50 of switch 36 remains closed. . On the other hand, if the converse is true, the output on lead 46 will drop and switch 50 will be opened. The next most significant bit is then asserted on lead 48, closing switch 52, thereby causing resistor 54
(which provides substantially half the current that would have been provided by resistor 42) also provides an input to amplifier 44. Again, if the voltage at tap 26 is higher than the output of amplifier 44, then
Output 48 remains on and switch 52 remains closed. Otherwise, the output on lead 48 drops and switch 52 is opened. This process proceeds in a binary series through the 12 output bits provided by the successive approximation register 24, such that the final digital output at 21 is from which the value of the unknown resistor 14 can be easily derived. ”-represents the stated value N.

Capacitor measurements can also be made using H, I-H techniques for highly accurate and rapid results. In this case, the measurement), the unknown volume 1C× and the reference volume 1itCr
Depends on the charge distribution across the circuit network.

Once both capacitors are discharged, both are charged until Vr reaches a predetermined value and the charging process is stopped. Since both are charged with the same current and for the same time,
The electric current ffv x at the tap 26 in FIG.
] by substitution into equation [11], Cx −Cr N/
(2' - N) [5] Again, the voltage or current voltage by which the net work is charged has no effect on the capacitance measurement. In addition, this technique eliminates parasitic effects of line and switch impedances since no current flows during the measurement process.

Inductor measurements are also accomplished in a ratio fashion. A known value of the reference component 1 r is drawn at position 7 r within the series of networks. By applying a voltage Vr to the series network and following a period of stabilization, the ratio of the two components ]-x and lr can be derived.

V=l (d+/dt) Teartame [61Vr(Lx
+1-r) (di/dt) [7] or VX -l
-r (di/dt) [8] [7]
And by substituting [8] into [1], N= (Lr) (di/dt) (2”)
(1/ (1-r×L x ) (di/dt) ) = (1-r >(2'> (1/ (1-r
+l−X ) ) or Lx −Lr ((2°/
N)-1) [91 The excitation voltage Vr is assumed to have no current limit for applying equation [9].

That is, as long as the drive circuit withstands sufficient voltage/dt, equation [9] can be applied. However, in practice, the III limit of the voltage drive current should be accounted for. Consider the following example for the excitation voltage r-0,25 at a current limit of 15 mA.
Let it be v. I--10mh (synthetic inductor lx→-
1-r). Substituting these values into equation [6] yields (q; di/dt - 25 al per second
lllll.

Considering the above current limit: 25A/5ea-1
There are 5IllA10.6IllS'c. Koreha,? J
f Application of excitation voltage to the network 0. '6n+
It means something to be done within s.

While preferred embodiments of the invention have been shown and described,
It will be apparent to those skilled in the art that many changes and modifications may be made in its boundary aspects without departing from the invention. It is therefore intended that the appended claims cover all such changes and modifications as fall within the true spirit and scope of the invention.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an impedance measuring device according to the present invention. FIG. 2 is a more detailed block and schematic diagram of the invention. FIG. 3 is a plot of the error of measurements according to this method and apparatus as a function of the measured impedance. In the figure, 12 is an amplifier, 14 is an unknown impedance,
16 is a known impedance, 18 is a D/A converter,
21 is an attenuation input, 22 is a controller, 24 is a continuous approximation register, and 26 is a tap.

Claims (12)

[Claims] (1) A method of measuring an impedance value, comprising: applying a first voltage; applying the product of the first voltage and a selected number; and varying the value of the number through a series of steps; compare the voltage corresponding to the drop with said product, and indicate when the last mentioned voltage and said product are in a predetermined relationship, and when said predetermined relationship is achieved, determine the impedance; The method in which said number is used to indicate. (2) applying said first voltage to said impedance in series with another standard impedance for carrying a common current, said number being the first mentioned impedance and said other standard impedance; 2. A method according to claim 1, wherein the ratio between (3) A method of measuring an impedance value, the method comprising: providing a first voltage; coupling the first voltage to the impedance; providing a product of the first voltage and a selected number; generating portions of said selected number by successive steps proceeding in a value-by-value manner from one value to a last value; and including said number of portions as an indication of said impedance in response to a predetermined comparison. 4. The method of claim 3, wherein the number portions include individual digits. (5) A method for determining a value of a first impedance proportional to a value of a standard impedance, the method comprising: applying a first voltage of a circuit to the first impedance and the standard impedance in series; variably attenuating the voltage in accordance with the digital input value, generating the digital input value by successive approximation providing digits of the digital input value in a predetermined order, each digit representing a successive degree of attenuation; comparing a variably attenuated first voltage to a voltage drop across said first impedance and responsively causing the generation of a digital input value to output a sequence of digits to said attenuated first voltage; the digital value representing the first impedance proportional to the standard impedance; (6) A method for measuring an impedance value, comprising: converting a digital output of a continuous approximation register into an analog value; comparing the analog value with a voltage developed in relation to the impedance; and determining that the analog value is related to the impedance. and using the digital output as a measure of the impedance. 7. The method of claim 6, wherein the same voltage source is applied to provide a reference input to the continuous approximation register and to provide a current through the impedance. (8) coupling a standard impedance in series with a first mentioned impedance for receiving said current, said voltage developed in relation to said first mentioned impedance being drawn at an interconnection between said impedances; 8. The method of claim 7, wherein the digital output represents the first mentioned impedance value proportional to the standard impedance value. (9) An apparatus for measuring the value of a first impedance, comprising: means for applying a first voltage; means for coupling the first voltage of a circuit with the first impedance; and the first voltage. means for producing the selected value by successive digital steps; and comparing the first voltage at each step with the product of the selected value. means for determining whether a predetermined comparison has been achieved in response to a voltage across the impedance of the first impedance, thereby including the digits of said step in a final digital value indicative of said first impedance; Equipment including. (10) An impedance measuring device for determining a first impedance value proportional to a standard impedance value, comprising: means for providing a voltage reference; and coupling the first impedance and the standard impedance in series to the voltage reference. means; and programmable attenuation means receiving said voltage reference as an attenuation input, said programmable attenuation means having a plurality of attenuation determining inputs arranged in a predetermined digital order, and having a continuous value. a continuous approximation register adapted to provide a succession of digital outputs, said digital output coupled to provide an attenuation determining input for said programmable attenuation means; means for providing a conversion decision input to said continuous approximation register to provide said voltage reference attenuated to a fixed relationship with the voltage at said tap, , wherein the final digital output of the continuous approximation register represents a value of the first impedance that is proportional to a value of the standard impedance. 11. The apparatus of claim 1, wherein the programmable attenuation means comprises an R/2R network. (12) The value Zx of the first impedance is expressed by the formula Zx
2. The device of claim 1, wherein the final digital output N of the continuous approximation register and the value Zr of the standard impedance are related by =Zr((2^n/N)-1).