GB2124785A - Circuit module for impedance measuring circuit - Google Patents

Abstract

A circuit module can be used as source or sink for connection to source or sense nodes respectively of a network under test for impedance measurement. The module is a high-gain amplifier 10 with a feed back impedance R1 to the inverting input and a switch S1 which is operated to connect the non-inverting input either to the source voltage level or to earth so that the inverting input can be connected to a source node or a sense node respectively. <IMAGE>

Description

SPECIFICATION Circuit module for impedance measuring circuit The present invention relates to impedance measuring circuits and in particular to a circuit module for use is such measuring circuits.

There is a known type of impedance measuring circuit in which the impedance of a selected electrical circuit element connected in a network of elements forming a circuit under test can be measured by connecting a low impedance current source at a first predetermined voltage to a source node of the circuit under test on one side of the selected element, a low impedance current sink at a second predetermined voltage to a sense node of the circuit under test on the other side of the selected element and a further low impedance current sink at said second predetermined voltage to any or each further node of the circuit under test which node has an independent current path to said sense node, and then measuring the current flowing into the low impedance current sink connected to the sense node.With such an arrangement it can be seen that the current flowing into the current sink connected to the sense node is equal to the current flowing between the source and sense nodes through the selected circuit element as a result of the difference between the first and second predetermined voltages. Thus, the impedance of the selected element can readily be determined by measuring this sensed current. The or each further low impedance current sink ensures that the respective further node is held at the same potential as the sense node.

According to the present invention, a circuit module for use in an impedance measuring circuit of the above described type comprises a high gain differential amplifier having a predetermined feedback impedance arranged to interconnect its output and inverting input, and a switch means for selectively connecting the non-inverting input to a potential representing said first predetermined voltage or to a potential representing said second predetermined voltage, whereby, when said inverting input is connected to a node of a circuit under test, the module can provide said current source or said current sink on selection by said switch means.It can be seen that the high gain amplifier operates in the usual way to produce a voltage on its output providing a current through the feedback impedance which brings the voltage on the inverting input of the amplifier to be closely similar to that on the non-inverting input as selected by the switch means. Accordingly, when the switch means is selected to apply a potential representing said first predetermined voltage to the non-inverting input, the amplifier operates to maintain the inverting input at a corresponding potential so that on connection of the inverting input to a node of a circuit under test, the module operates as a current source at said first predetermined voltage.On the other hand, when the switch means is selected to apply a potential representing said second predetermined voltage to the noninverting input, the amplifier operates to maintain the inverting input at this second predetermined voltage. In a typical example, the second predetermined voltage is earth so that the module then operates as a current sink and the node connected to the inverting input is the sense node.

Assuming the current absorbed from the sense node by the module when operating as a current sink is equal to the current flowing through the selected circuit element of which the impedance is to be measured, it can be seen that the high-gain amplifier operates so that the current through its feedback impedance is also equal to the current in the selected element of the circuit under test.

Thus, the voltage on the output of the amplifier is inversely representative of the impedance of the selected circuit element in the circuit under test.

Conveniently, the module includes an output switch for selectively connecting the output of the amplifier to, or disconnecting the output from, an output terminal of the module.

In one arrangement, the switch means comprises a resistance connecting the non-inverting input of the amplifier to said potential representing said second predetermined voltage and a source switch selectively connecting said non-inverting input to or disconnecting it from said potential representing said first predetermined voltage. Then when the switch is closed, the non-inverting input is held at the potential representing said first predetermined voltage, whereas when the switch is open the non-inverting input is at the potential representing said second predetermined voltage, typically earth potential.

The modules can also be used to eliminate the parasitic currents flowing to or from the sense node from adjacent nodes of the circuit under test. A respective module is connected to each adjacent node with the non-inverting input of the amplifier of each module connected to the potential representing said second predetermined voltage. In this way the adjacent nodes are maintained at the same voltage as the sense node to eliminate the parasitic currents.

It will be appreciated that, in certain arrangements, the impedance of the electrical lead interconnecting the node of the circuit under test to the inverting input of the amplifier of the module may become significant so that there is a potential drop across the lead.

This is undesirable since it is important that the voltages at the various nodes of the circuit under test are closely controlled in accordance with the predetermined voltages applied to the modules. Accordingly, a module may have a first electrical lead connected to said inverting input of the amplifier and a second electrical lead connected in series with the feedback impedance to the output of the amplifier, the two leads being for connection of said inverting input and said feedback impedance respectively to a node of a circuit under test.

With this arrangement, the currents flowing through the feedback impedance of the module are carried by said second electrical lead so that there is insignificant current flowing in said first lead so that the node of the circuit under test is at the same potential as the inverting input of the amplifier.

Conveniently, the module has a feedback switch for selectively shorting out at least a major part of said feedback impedance. This switch is useful when the module is connected to adjacent nodes of a circuit under test for eliminating parasitic impedance, i.e.

used to apply a guard potential to a node of a circuit under test. Closing the switch enhances the guard capability of the module.

The present invention also envisages an impedance measuring circuit including at least three circuit modules of the kind described above, all having common sources of said potentials representing said first and second predetermined voltages. Where the modules have output switches as described above, the output terminais of the modules of the measuring circuit are preferably connected together at a common output signal terminal.

Then, the inverting inputs of the amplifiers of the three modules can be connected to respective nodes of a circuit under test and the modes of the modules are set by the various switching means and switches so that one module provides the source potential to a selected source node, a second module connected to a selected sense node operates in the sense mode and the third and any further modules each operate in the guard mode. In the guard mode, the non-inverting input of the amplifier of the module is connected to the potential representing said second predetermined voltage, but the output switch of the module is open circuit.

It can be seen then that only the voltage on the output of the amplifier of the module in the sense mode is connected via its output switch to the output signal terminal so as to give an indication of the impedance value of the selected circuit element between the source and sense nodes of the circuit under test.

Examples of the present invention will now be described in more detail with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a circuit module embodying the present invention; Figure 2 is a block schematic diagram of an impedance measuring circuit incorporating three circuit modules and Figure 3 is a circuit diagram of a modified form of module.

Referring to Fig. 1, the module comprises a high-grain differential amplifier 10 having a feedback resistance R1 connected between its output 11 and its inverting input 1 2. The high-gain amplifier 10 may be a standard operational amplifier having a gain sufficiently high that the input voltage across its inputs for maximum output voltage swing is negligible.

The non-inverting input 1 3 of the amplifier 10 is connected via an impedance R2 to earth. The non-inverting input 1 3 is also connected via a switch S1 to a potential Vjn. The output 11 of the amplifier is connected via a second switch S2 to an output terminal 1 4 of the module. Switch S1 is controlled to open and close by signals on a control line 15, and switch S2 is controlled by signals on a line 1 6.

It can be seen that when switch S1 is open, the non-inverting input 1 3 of the amplifer 10 is at earth potential so that the amplifer can operate to maintain the inverting input 1 2 also substantially at earth potential. In this way, when the inverting input 1 2 of the amplifier is connected by a lead 1 8 to a node 1 7 of a circuit under test, the module operates to maintain the node 1 7 substantially at earth potential and serves as a low impedance current sink connected to the node 1 7.

On the other hand, when the switch S1 is closed, the non-inverting input 1 3 of the amplifier 10 is held at V, and the amplifier 10 operates to maintain the inverting input 1 2 also at Viz Then the module operates to keep the note 1 7 at Vjn and provides a low impedance current source at the voltage V,.

When the switch S1 is closed, switch S2 is normally open so that any voltage on the output 11 of the amplifier is not connected to the output terminal 14. However, when switch S1 is open and the module is operating in the sense mode switch S2 can be closed so that the output terminal 14 is at the output voltage of the amplifier. It can be seen that in the sense mode, the amplifier 10 operarates so that the output voltage of the amplifier produces a current in the feedback impedance R1 equal to the current flowing into the node 1 7 so that the node 1 7 is held at earth potential. Provided all the current flowing into the node 1 7 is flowing through the element of the circuit under test of which the impedance is to be measured, then the voltage Vout on the output terminal 14 is inversely proportional to the impedance of the selected circuit element.

In Fig. 1 a switch S3 is illustrated connected in parallel across the feedback impedance R 1. The switch S3 is opened or closed by control signals on a control line 1 9. The switch S3 may be closed when the module is used in the guard mode so as to reduce the effective impedance of the current sink provided by the module to ensure effective guarding action. The switch S3 may also be closed when the module is used in the source mode.

Fig. 2 shows how an impedance measuring circuit incorporating three modules of the form shown in Fig. 1 may be connected simultaneously to different nodes of a circuit under test indicated schematically by the elements in the dotted line box 20. The three modules are identified in Fig. 2 by the letters A, B and C. The inverting input of the amplifier of each of the modules A, B and C is connected to a respective node, 21, 22 and 23 of the network of elements forming the circuit under test 20. The potential Vi, is applied from a common source 24 to the switches S1 of each of the modules A, B and C. Further, the output terminals 1 4 of each of the modules A, B and C are connected together at a common output signal terminal 25.The control signals to the switches S1, S2 and S3 are applied to each of modules A, B and C along respective control lines 26, 27 and 28 for each module.

It can be seen therefore that by applying suitable control signals on the lines 26, 27 and 28 of the three modules A, B and C, the modules can be operated in the source, sense or guard modes as desired without altering connections between the measuring circuit as a whole and the circuit under test 20. Thus, in the example illustrated, if it is desired to determine the impedance of the element R of the circuit under test 20, module A may be put in the source mode by opening switch S2 and closing switches S1 and S3 so that the node 21 is held at the voltage Vjn. Module B is then put in the sense mode with switches S1 and S3 open and switch 52 closed. Node 22 is then held at earth potential.Module C may be put in the guard mode with switches S1 and S2 open and switch S3 closed. Then node 23 is also held at earth potential to prevent any parasitic current flowing to the node 22. It can be seen then that a voltage drop of Vi is applied across the circuit eement R of the circuit under test 20 and the current flowing to the module B is representative of the value of the impedance of the element R. The output voltage of the amplifier 10 of the module B is supplied by the switch S2 to the common signal terminal 25 and is representative of the impedance of the element R.

If the impedance of one of the other elements of the circuit under test was required, it is a simple matter to alter the programming of the modules A, B and C so that the signal on the terminal 25 is representative instead of the impedance of this other element.

Referring to Fig. 3, this illustrates a modified form of the module of Fig. 1. Instead of connecting the feedback impedance R1 directly to the inverting input 1 2 of the amplifier 10, separate electrical leads 30 and 31 are provided for connection of the module to the respective node of the circuit under test.

Lead 30 connects the inverting input 1 2 to the node 1 7 whilst lead 31 is in series with the feedback impedance R1. The remaining elements of the module are as described with reference to Fig. 1. With the Fig. 3 arrangement, the current flowing to or from the node 1 7 to the module flows along the lead 31 through the feedback impedance R1. There is insignificant current flowing along the lead 30 connected to the inverting in put 1 2 of the amplifier 10. As a result, the amplifier 10 operates to maintain the node 1 7 closely at the selected potential, earth or Vjn as required.

By this arrangement, proper operation of the impedance measuring circuit is ensured irrespective of any significant impedance in the connecting leads to the circuit under test. A high value resistance 32 may be connected between the leads 30 and 31 to stabilise and prevent saturation of amplifier 10 when the leads 30 and 31 are not connected together at a node 17.

Claims (10)

1. A circuit module for use in an impedance measuring circuit of the type in which the impedance of a selected electrical circuit element connected in a network of elements forming a circuit under test can be measured by connecting a low impedance current source at a first predetermined voltage to a source node of the circuit under test on one side of the selected element, a low impedance current sink at a second predetermined voltage to a sense node of the circuit under test on the other side of the selected element and a further low impedance current sink at said second predetermined voltage to any or each further node of the circuit under test which node has an independent current path to said sense node, and then measuring the current flowing into the low impedance current sink connected to the sense node; the circuit module comprising a highgain differential amplifier having a predetermined feedback impedance arranged to interconnect its output and inverting input, and switch means for selectively connecting the non-inverting input to a potential representing said first predetermined voltage or to a potential representing said second predetermined voltage whereby, when said inverting input is connected to a node of a circuit under test, the module can provide said current source or said current sink on selection by said switch means.

2. A module as claimed in Claim 1 and including an output switch for selectively connecting the output of the amplifier to, or disconnecting the output from, an output ter minal of the module.

3. A module as claimed in Claim 1 or Claim 2 wherein the switch means comprises a resistance connecting the non-inverting input of the amplifier to said potential representing said second predetermined voltage and a source switch selectively connecting said noninverting input to, or disconnecting it from, said potential representing said first predetermined voltage.

4. A module as claimed in any preceding Claim and having a first electrical lead connected to said inverting input of the amplifier and a second electrical lead connected in series with a feedback impedance to the output of the amplifier, the two leads being for connection of said inverting input and said feedback impedance respectively to a node of a circuit under test.

5. A module as claimed in any prededing Claim and including a feedback switch for selectively shorting out at least a major part of said feedback impedance.

6. An impedance measuring circuit including at least two circuit modules as claimed in any preceding Claim all having common sources of said potentials representing said first and second predetermined voltages.

7. A measuring circuit as claimed in Claim 6 including three or more said circuit modules.

8. A measuring circuit as claimed in Claim 6 or Claim 7 as dependent directly or indirectly on Claim 2, wherein the output terminals of the modules are connected together at a common output signal terminal.

9. A circuit module substantially as hereinbefore described with reference to and as illustrated in Figs. 1 or 2 of the accompanying drawings.

10. An impedance measuring circuit substantially as hereinbefore described with reference to and as illustrated in Fig. 3 of the accompanying drawings.