ACTIVE PROBE FOR MEASURING VOLTAGE AT HIGH IMPEDANCES

Abstract

The present invention relates to an active probe (1) for measuring a voltage across a high value impedance, said probe (1) comprising: an impedance matching circuit (10) configured, on the one hand, to have an input impedance at least 100 times higher than the impedance to be measured and, on the other hand, to deliver a low impedance signal at the output; a probe tip (6) configured to take a measurement and connected to said impedance matching circuit (10); a ground reference (G) connected to said impedance matching circuit (10); said impedance matching circuit (10) comprising, connected in cascade, at least one input circuit (14) and a non-inverting amplifier circuit (16) adapted to high impedance values.

Claims

1. Active probe (1) for measuring a voltage across a high-value impedance, said probe (1) comprising: an impedance matching circuit (10) configured, on the one hand, to have an input impedance at least 100 times higher than the impedance to be measured and, on the other hand, to deliver a low impedance signal at the output; a probe tip (6) configured to take a measurement and connected to said impedance matching circuit (10); a ground reference (G) connected to said impedance matching circuit (10); said impedance matching circuit (10) comprising, in a cascade, at least one input circuit (14) and a non-inverting amplifier circuit (16) adapted to high impedance values.

2. Probe (1) according to the preceding claim, characterised in that the low impedance output of said impedance matching circuit (10) has an impedance that is equal to or less than 50 ohms.

3. Probe (1) according to the preceding claim, characterised in that the non-inverting amplifier circuit (16) comprises an operational amplifier (AO1), as well as a resistor (R5) arranged at the inverting input of said operational amplifier (AO1).

4. Probe (1) according to the preceding claim, characterised in that the amplifier circuit (16) comprises a guard ring (20) surrounding the inverting and non-inverting inputs of the operational amplifier (AO1) and at least partially the fifth resistor (R5).

5. Probe (1) according to any one of the preceding claims, characterised in that the impedance matching circuit (10) comprises at least one component (C2, C3, R4) and/or a filter circuit (18).

6. Probe (1) according to any one of the preceding claims, characterised in that said probe (1) is configured to measure a direct voltage and/or a voltage having a frequency of less than 60 Hz.

7. Probe (1) according to any one of the preceding claims, characterised in that it is configured to measure an impedance higher than 10 gigaohms (G), preferably between 10 and 50 gigaohms (G).

8. Probe (1) according to any one of the preceding claims, characterised in that the impedance matching circuit (10) has a transfer function H(p) including three poles (.sub.1-3) and a zero (.sub.z).

9. Probe (1) according to the preceding claim, characterised in that the input circuit (14) has an input impedance higher than 50 gigaohms (G).

10. Probe (1) according to any one of the preceding claims, characterised in that the input circuit (14) comprises a divider bridge (R1 and R2).

11. Probe (1) according to any one of the preceding claims, characterised in that said input circuit (14) comprises at least one resistor (R1) including at least one guard ring (100) fixed to the body of said resistor (R1).

12. Probe (1) according to the preceding claim, characterised in that the guard ring (100) of said at least one resistor (R1) comprises a stainless steel ring (102) fixed to the middle of the body of the resistor (R1) and a metal braid (104) connecting the said ring (102) to a ground (G).

Description

[0041] The invention will be better understood and other aims, details, features and advantages thereof will become clearer in the following description of a particular embodiment of the invention, given solely by way of illustration and with no limitations, with reference to the accompanying drawings in which:

[0042] FIG. 1, referred to as [FIG. 1], is a highly schematic representation of a measurement probe according to the invention;

[0043] FIG. 2, referred to as [FIG. 2], is a schematic representation of the impedance matching circuit of the probe of FIG. 1;

[0044] FIG. 3, referred to as [FIG. 3], is a schematic and enlarged representation of a resistor in the circuit of FIG. 2, and

[0045] FIG. 4, referred to as [FIG. 4], is an equivalent electrical schematic representation of the resistor of FIG. 3.

[0046] FIG. 1 is a highly schematic representation of an active probe 1 for measuring a voltage across a high impedance.

[0047] It should be noted that high impedances means impedances that are higher than 10 gigaohms (G), and preferably impedances between 10 and 50 gigaohms (G).

[0048] Said active probe 1 thus comprises a casing 3, a man-machine interface 5, integrated into the casing 3 for example, a probe tip 6 configured to take a voltage measurement, as well as a ground reference 7 connected to the probe tip 6 and intended to be connected to a reference ground G.

[0049] Said probe 1 comprises advantageously a battery (not shown) for powering various circuits and components of said probe 1, thus enabling it to be autonomous and not have to be connected to the mains when in use.

[0050] It should be noted that the term man-machine interface refers to all of the elements that enable the user to interact with the probe according to the invention, more particularly to control the probe according to the invention and exchange information with it.

[0051] The probe tip 6 ends in an electrically conductive part intended to be applied to the point of a circuit having a high impedance for which the voltage is to be measured, while reference 7 is for example in the form of an alligator clip.

[0052] It should be noted that the term active probe refers to the fact that the probe comprises one or more active components configured to process the measurement signal, which are located as close as possible to the probe tip 6, in particular in order to be as insensitive as possible to various disturbances and thus reduce measurement errors.

[0053] As illustrated in FIG. 2, said probe 1 thus comprises an impedance matching circuit 10 and a voltmeter 12. Said impedance circuit 10 has an input 10a corresponding to the tip 6 of the active probe 1, while the ground G of said circuit 10 is the ground to which reference 7 of said probe 1 is connected.

[0054] Said impedance matching circuit 10 comprises at least one input circuit 14, to which the input 10a is connected, and a non-inverting amplifier circuit 16 adapted to high impedance values, the output 10b of which is connected to the voltmeter 12 or to an oscilloscope input. The input circuit 14 and the amplifier circuit 16 are connected in a cascade.

[0055] The circuit 10 advantageously has an input impedance and an output impedance with a ratio greater than or equal to 1000, and preferably greater than or equal to 5000. The input impedance of said circuit 10 is therefore very high compared with the output impedance of the circuit 10.

[0056] Said at least one input circuit 14 comprises more particularly a circuit of the resistive voltage divider bridge type (or divider bridge). Said divider bridge thus comprises at least two resistors R1 and R2 (suitably arranged), also referred to respectively as a first resistor R1 and second resistor R2.

[0057] It should also be noted that the input circuit 14 has an input impedance higher than 50 gigaohms (G).

[0058] The second resistor R2 is a foot resistor, one end of which is connected to ground G, while the first resistor R1 is connected, on the one hand, to the input 10a, and on the other hand, to the second resistor R2. One of the ends of the resistors R1 and R2 being connected to the output 10c of the divider bridge (the output 10c of the divider bridge also corresponding to the input of the circuit 16).

[0059] Said input circuit 14 also comprises a capacitor C1 (also referred to as the first capacitor), preferably variable or adjustable, the value of the capacitance of the capacitor C1 being able to vary over a given range of intervals. The capacitor C1 advantageously has a capacitance value of between 1.5 and 55 picofarads.

[0060] Said capacitor C1 is also arranged in parallel to the resistor R1 of the divider bridge. Said capacitor C1 is therefore connected, on the one hand, to the input 10a, and, on the other hand, to the output of the divider bridge 10c.

[0061] It should be noted that the impedance matching circuit 10 may also comprise a capacitor C2, also referred to as a second capacitor, arranged in parallel to the second resistor R2 of the divider bridge.

[0062] This second capacitor C2 makes it possible in particular to filter the harmonics and/or reduce the noise present at the input 10a of the probe 1, said capacitor C2 therefore acts as a filtering component.

[0063] Said amplifier circuit 16, thus comprises an operational amplifier AO1, at least three resistors R3, R4 and R5, as well as a capacitor C3. These elements are also referred to respectively as a third resistor R3, fourth resistor R4, fifth resistor R5 and third capacitor C3.

[0064] The output 10c of the input circuit 14 is connected to one of the inputs of the amplifier circuit 16, and more particularly to the non-inverting input of the operational amplifier AO1.

[0065] The third resistor R3 is connected to the ground G at one of its ends, while the other end is connected to the inverting input of the operational amplifier AO1. The third resistor R3 can also be described as a foot resistor and contributes to the gain provided by the circuit 16 with the fourth resistor R4.

[0066] The fifth resistor R5 is arranged at the inverting input of the operational amplifier AO1. One end of the resistor R5 is therefore connected to the inverting input, while the other end is connected to a node 10d, the node 10d to which the third resistor R3 is also connected (as well as to the fourth resistor R4).

[0067] The fourth resistor R4 and the third capacitor C3, arranged in parallel with one another, are connected, on the one hand, to the node 10d, and, on the other hand, to the output of the operational amplifier AO1 (the output of the amplifier AO1 corresponding to the output 10b of the impedance matching circuit 10).

[0068] It should be noted that the association of the fourth resistor R4 and third capacitor C3 in parallel to the said fourth resistor R4 (and its position in the impedance matching circuit 10) is a filter circuit making it possible in particular to filter the noise associated with the amplification of the (measured) signal.

[0069] Advantageously, the operational amplifier AO1 is an operational amplifier of the CMOS (Complementary Metal Oxide Semiconductor) type, this type of amplifier having very low input currents, of in the order of about ten femtoamperes.

[0070] The amplifier circuit 16 also comprises a guard ring 20 surrounding the inverting and non-inverting inputs of the operational amplifier AO1 and (at least) partially the fifth resistor R5. Said guard ring 20 prevents in particular the migration of leakage currents to other components of the circuit 10 where there is a potential difference. The guard ring 20 is also connected to the ground G of the impedance matching circuit 10.

[0071] The guard ring 20 is for example a metal strip, such as copper, placed on the printed circuit board (PCB) on which the electronic components are arranged. This guard ring 20 makes it possible in particular to protect the elements that it surrounds from parasitic leakage currents.

[0072] In addition, the impedance matching circuit 10 comprises advantageously a surge voltage protection circuit 18.

[0073] Said protection circuit 18 comprises two diode-connected transistors T1 and T2, for example JFET type transistors. It is particularly advantageous to use transistors of this type, as they have very low leakage currents which will not interfere with the measurements.

[0074] The impedance matching circuit has a transfer function H(p) including three poles and a zero, more particularly the transfer function has the following form:

[00001]$H\left(p\right)=H_0\left(\frac{1+\frac{p}{{}_1}}{1+\frac{p}{{}_2}}\right)\left(\frac{1+\frac{p}{{}_Z}}{1+\frac{p}{{}_3}}\right)$

[0075] with

[00002]$H_0=\left(\frac{R_2}{R_1+R_2}\right)\left(1+\frac{R_3}{R_4}\right);$${}_1=\frac{1}{R_1{C}_1};$${}_2=\frac{1}{\left(R_1//R_2\right)\left(C_2+C_p\right)};$${}_3=\frac{1}{R_4{C}_3};$${}_z=\frac{1}{\left(R_3//R_4\right)C_3};$

[0076] where C.sub.p is the input capacitor of the operational amplifier AO1; .sub.1, .sub.2, .sub.3 and .sub.z are the pulsations (generally referred to as cut-off pulsations) relative to the poles and zero of the transfer function H(p) of the circuit 10.

[0077] Advantageously, the pulsations .sub.1 and .sub.2 are equal, pulsation .sub.3 is largely greater than .sub.1 and .sub.2 respectively, while the pulsation .sub.z is greater than .sub.3 (advantageously at least a ratio of 10, and preferably at least a ratio of 50).

It should be noted that largely greater, means at least a ratio of 100 between the pulsation .sub.3 and the pulsations .sub.1 and .sub.2.

[0078] Said probe 1 is thus configured to measure a direct voltage and/or a voltage having a frequency of less than 60 Hz.

[0079] As illustrated more particularly in FIGS. 2 and 3, the first resistor R1 comprises advantageously a guard ring 100 fixed to the body of said resistor R1.

[0080] Thus, the guard ring of said at least one resistor R1 comprises a stainless steel ring 102 fixed to the middle of the body of the resistor R1 and a metal braid 104 connecting said ring to a ground G.

[0081] As shown in the equivalent electrical diagram in [FIG. 4], the guard ring makes it possible to break down the leakage resistance R.sub.f of the resistor R1 into two leakage resistances R.sub.f1 and R.sub.f2 arranged parallel to one another, thereby greatly reducing the leakage resistance associated with the resistor R1.