GB1596498A - Resistance measuring circuits - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO RESISTANCE MEASURING CIRCUITS (7i) We, THE PLESSEY COMPANY LIMITED, Vicarage Lane, Ilford, Essex, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to resistance measurement apparatus and more particularly to apparatus for measuring the resistance of one path in a plurality of resistance paths such as may be found in a complex wiring system.

Resistance measuring circuits are normally based on Ohm's law directly, or on use of operational amplifier techniques.

Such circuits have been refined over many years, and are capable of yielding high precision.

When it is required to read, sequentially, a number of resistances connected to different test points, the measuring apparatus has to be connected to the unknown networks through cyclic switches; these may be implemented by means of rotary commutators, relays, reed relays, mechanical crosspoint switches or electronic switches.

Relays, which have the advantage that their contact resistance is low, have limited life when used in test systems required to scan continuously at high speed. Electronic switches have substantially unlimited life but, depending on their type, they may introduce finite voltage drop (possibly temperature dependent) or finite series resistance. Both of these effects introduce measuring errors. This invention provides apparatus for reducing these errors, with special reference to applications in automatic testing. According to the present invention resistance measurement apparatus for measuring the resistance of one of a plurality of resistance paths includes first switch means for applying a known current to the first end of a selected resistance path, second switch means for applying a known voltage potential or current drain to the other end of the selected resistance path, differential amplifier means, a first input of which is connected to the first switch means for detecting the voltage drop across the selected resistance path, third and fourth switch means having the same characteristics as the first and second switch means respectively and being connected to each other, the known current being applied to the third switch means and the known voltage potential or current drain being applied to the fourth switch means to provide a dummy load, the second input of the differential amplifier being connected to the third switch means for detecting the voltage drop across the dummy load, the third and fourth switch means substantially eliminating the effects of the first and second switch means whereby the output of the differential amplifier means is proportional to the resistance of the selected resistance path.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawings in which: Figure 1 shows a known basic sequential resistance measuring arrangement, Figure 2 shows a known more complex sequential resistance measuring arrangement, Figure 3 shows a sequential resistance measurement arrangement with switch error balancing, Figure 4 shows a known circuit for measuring randomly interconnected resistances using a further current injected through the meter, Figure 5 shows a circuit explaining the operation of the circuit of Figure 4, Figure 6 shows a resistance measurement apparatus according to the present invention Figure 7 shows a circuit explaining the effects of capacity on the circuit of Figure 6 and Figure 8 shows a third resistance measurement apparatus to overcome the effect of such capacitive loading.

Referring now to the drawings, Figure 1 shows a basic sequential resistance measuring arrangement which can be used when one end of each unknown resistor is commoned. It is clear that the resistance indicated by the meter M will include the resistance of the Switch S.

Figure 2 shows one method of avoiding this error by the use of two "ganged" switches in an arrangement analogous to known four-terminal connections. Here Si carries measuring current and the magnitude of r, is unimportant provided R ri. Switch S2 carries only the small amplifier input current and its resistance r2 is much smaller than the input resistance of amplifier A. Alternatively, the drop produced by iM across r2 is negligible compared with the drop across the unknown resistance Rl or R2 etc. This condition may be expressed as iM ilN. Thus the arrangement of Figure 2 eliminates series resistance effects and allows small values of unknown resistance to be rapidly measured.

Another arrangement for reducing the effect of switch resistance is shown in Figure 3. Here the voltage drop across r2 is balanced by the drop across r3 in an additional switch so that the difference amplifier A2 amplifies the drop across Rl etc. plus the voltage ilN (r2-r3) and it follows that the switch errors are small if (r2-r3) is small. It has been found that, in the case of modern field effect transistor circuits integrated as "transmission gates", the variation of r2 or r3 over a number of circuit packages, and over a range of temperatures rom 0 C to 600C can be of the order of 10 per cent of r2. Therefore, if Rl etc, is of the same order as r2, the measuring error will be only 10 per cent.

A more complex problem is that of measuring a number of randomly interconnected resistance which do not have a common terminal. This problem has been partially solved by the circuit shown in Figure 4, where the switches S5 and S6 operate asynchronously with respect to each other and measuring current is injected through the scanner switch S6.

Figure 5 shows a section of Figure 4 where one resistance (between modes 1 and 5) only. is measured at the instance when switch S, is set to mode 1 and Switch S6 is set to mode 5. It is clear that the apparent resistance measured will be Ras+rs+r6 This restricts the resistance of the lowest practical value of Rl, (or other unknown resistance) which can be measured. It has been found in practice that switch resistance such as that of CMOS transmission gate type CD4067, for example, can be 200 ohms and a 10 per cent variation of this resistance causes the lowest value of unknown resistance measured to be about 20 ohms. In many cases, however, it is required to measure lower resistances. When the electronic switches employed are bipolar rather than MOS, the resistances r, and r6 are replaced mainly by offset voltages which we have found to be of the order of 100 to 400 millivolts.

The arrangement of Figure 6 shows a means of overcoming the errors of the circuit of Figure 5. Four switches S10, S12, SA and SB are used, S12 applying a known constant current Im to the first end of the resistance path being measured and S10 applying a known voltage potential or current drain to the other end of the resistance path, in this case earth. The switch S12 is connected to a first input of a differential amplifier A20. Each of the switches S10 and S12 comprises a plurality of isolating switches operative to subdivide a single resistance path.

The switches SA and SB are static switches of the same type as the switches S12 and S10 respectively and are directly connected to each other. The same current Im is applied to the switch SA and the switch SB is connected to earth. The switch SA is connected to the second input of the differential amplifier A20. This circuit acts as a dummy load for the differential amplifier A20 so that the amplifier output is proportional to the resistance of the resistance path being measured only, and is independent of the resistances of the switches.

The known constant current Im can cover a range of the order of 100 microamperes to 10 mil1iamperes.

This results in a voltage at the output of amplifier A20 which is given by 10 ImRX, where Rx is the resistance of the resistance path being measured. When Im is set to a constant 1 milliampere, the amplifier output voltage is equal to 10RX Rx or 1000 100 volts, which is directly proportional to the value of Rx and sensibly independent of the internal resistance of the electronic switches S,0 to Sl2. Every one ohm change in R, produces an output of 10 millivolts which can easily be read on a digital voltmeter. When the measuring current I is increased to 10 milliamperes, a change of 0.1 ohms is detected as a 10 millivolt output.

Referring now to the "off" characteristics of MOS switches, it is well known that their "off" resistance is sensibly infinite but that their "off" capacitance is finite and can be of order of 5 picofarads per switch position. This capacitance affects the maximum speed of multi-point scanning systems. If, for example 1600 measuring points are to be examined as shown in Figure 7, the constant current of 1 picofarads and a time interval of the order of 50 microseconds is required to produce a change of 5 volts at the input of the amplifier summing resistance. To increase the maximum speed of a multi-point scanning system, the switches can be sectionalised as shown in Figure 8 where each group of 160 measuring points can be charged up in 5 microseconds. The outputs of each section are fed through respective transistor amplifier circuits TR1, TR2...TR16 to the positive input of the differential amplifier and the output of the switches SA, SB is fed through a transistor TRO to the negative input of the differential amplifier.

While the arrangements and methods herein before described have been related mainly to resistance and inter-connection networks, it should be noted that the applications are more general and relate, for example, to those cases where a number of transducers is interconnected. An example of such a transducer array is represented by networks of resistance thermometers and another example is represented by networks of thermistors and yet another example is represented by networks of strain gauges.

WHAT WE CLAIM IS: 1. Resistance measurement apparatus for measuring the resistance of one of a plurality of resistance paths including first switch means for applying a known current to the first end of a selected resistance path, second switch means for applying a known voltage potential or current drain to the other end of the selected resistance path, differential amplifier means, a first input of which is connected to the first switch means for detecting the voltage drop across the selected resistance path, third and fourth switch means having the same characteristics as the first and second switch means respectively and being connected to each other, the known current being applied to the third switch means and the known voltage potential or current drain being applied to the fourth switch means to provide a dummy load, the second input of the differential amplifier means being connected to the third switch means for detecting the voltage drop across the dummy load, the third and fourth switch means substantially eliminating the effects of the first and second switch means whereby the output of the differential amplifier means is proportional to the resistance of the selected resistance path.

2. Resistance measuring apparatus as claimed in claim 1 in which the known current is a constant current.

3. Resistance measuring apparatus as claimed in claim 1 or claim 2 in which the known voltage potential or current drain is earth.

4. Resistance measurement apparatus as claimed in any preceding claim in which each of the first and second switch means comprises a plurality of isolating switches operative to subdivide a plurality of switching paths.

5. Resistance measurement apparatus as claimed in claim 4 in which only one of the isolating switches of each of the first and second switch means is used at a time to subdivide one of the plurality of switching paths.

6. Resistance measurement apparatus as claimed in any preceding claim in which the first and second, third and fourth switch means are static switches.

7. Resistance measurement apparatus substantially as hereinbefore described with reference to Figures 6, 7 and 8 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. known that their "off" resistance is sensibly infinite but that their "off" capacitance is finite and can be of order of 5 picofarads per switch position. This capacitance affects the maximum speed of multi-point scanning systems. If, for example 1600 measuring points are to be examined as shown in Figure 7, the constant current of 1 picofarads and a time interval of the order of 50 microseconds is required to produce a change of 5 volts at the input of the amplifier summing resistance. To increase the maximum speed of a multi-point scanning system, the switches can be sectionalised as shown in Figure 8 where each group of 160 measuring points can be charged up in 5 microseconds. The outputs of each section are fed through respective transistor amplifier circuits TR1, TR2...TR16 to the positive input of the differential amplifier and the output of the switches SA, SB is fed through a transistor TRO to the negative input of the differential amplifier. While the arrangements and methods herein before described have been related mainly to resistance and inter-connection networks, it should be noted that the applications are more general and relate, for example, to those cases where a number of transducers is interconnected. An example of such a transducer array is represented by networks of resistance thermometers and another example is represented by networks of thermistors and yet another example is represented by networks of strain gauges. WHAT WE CLAIM IS:

1. Resistance measurement apparatus for measuring the resistance of one of a plurality of resistance paths including first switch means for applying a known current to the first end of a selected resistance path, second switch means for applying a known voltage potential or current drain to the other end of the selected resistance path, differential amplifier means, a first input of which is connected to the first switch means for detecting the voltage drop across the selected resistance path, third and fourth switch means having the same characteristics as the first and second switch means respectively and being connected to each other, the known current being applied to the third switch means and the known voltage potential or current drain being applied to the fourth switch means to provide a dummy load, the second input of the differential amplifier means being connected to the third switch means for detecting the voltage drop across the dummy load, the third and fourth switch means substantially eliminating the effects of the first and second switch means whereby the output of the differential amplifier means is proportional to the resistance of the selected resistance path.

2. Resistance measuring apparatus as claimed in claim 1 in which the known current is a constant current.

3. Resistance measuring apparatus as claimed in claim 1 or claim 2 in which the known voltage potential or current drain is earth.

4. Resistance measurement apparatus as claimed in any preceding claim in which each of the first and second switch means comprises a plurality of isolating switches operative to subdivide a plurality of switching paths.

5. Resistance measurement apparatus as claimed in claim 4 in which only one of the isolating switches of each of the first and second switch means is used at a time to subdivide one of the plurality of switching paths.

6. Resistance measurement apparatus as claimed in any preceding claim in which the first and second, third and fourth switch means are static switches.

7. Resistance measurement apparatus substantially as hereinbefore described with reference to Figures 6, 7 and 8 of the accompanying drawings.