Circuit for measuring in-circuit resistance and current

A circuit for measuring alternating current (a.c.) and direct current (d.c.) in a conductor without breaking the conductor is provided. A modulated current source is coupled in parallel with a segment of the conductor to inject a test current. A synchronous demodulator is also coupled in parallel across the segment to separate the test voltage drop induced by the test current from the voltage drop due to the current in the conductor. The test voltage drop and the voltage drop are measured by a voltmeter and the resistance of the segment and the current flowing through the segment can then be calculated.

BACKGROUND OF THE INVENTION 
This invention relates generally to circuits for measuring electrical 
current flow and in particular to a method and apparatus for measuring 
electrical current flow in a conductor of an operating circuit. 
Measuring electrical current flow through a conductor may be done in a 
number of different ways. The traditional invasive approach is to break 
the conductor and insert a current measuring device such as an ammeter in 
series with the conductor. However, in many cases, breaking the conductor, 
which may be in the form of a copper wire, printed circuit (p.c.) board 
trace, or integrated circuit lead, is difficult and inconvenient, and may 
also cause damage to the conductor, particularly in the case of p.c. board 
traces. 
Non-invasive approaches have been developed that allow for making 
in-circuit current measurements without breaking the conductor. Current 
clamps and magnetic pickup probes are used to detect the magnetic field 
surrounding the wire in order to determine the current passing through the 
wire. Such techniques allow the current to be determined without touching 
the conductor. However, current clamps and magnetic pickup probes 
generally suffer from the effects of interference from adjacent magnetic 
fields and require a substantial amount of physical clearance around the 
conductor, making their use in testing current in p.c. board traces 
limited. 
Another non-invasive approach is the use of a pair of two-wire probes 
("Kelvin clips") which make electrical contact with the conductor at two 
separate points along its length, commonly called a four-wire measurement, 
which eliminates the effect of resistance in the probes. Each two-wire 
probe has a source contact and a sense contact. The voltage drop between 
the two points is measured between the sense contacts. The measured 
voltage is nulled to zero using a balancing current injected through the 
source contacts. The balancing current, which is adjusted to equal the 
current through the conductor, may now readily be measured with an 
external ammeter without breaking the conductor. Current balancing 
techniques, however, provide only limited measurement accuracy and range. 
At the low end of the current range, the limitation is the ability of the 
instrument to sense the relatively small voltage drops in the millivolt 
range that are developed across a short section of conductor. At the high 
end of the current range, the limitation is ability of the measurement 
instrument to generate the balancing current. 
Another non-invasive technique for measuring current is described in U.S. 
Pat. No. 5,386,188, "In-Circuit Current Measurement", issued Jan. 31, 
1995, to Minneman et al., and assigned to Keithley Instruments, Inc. 
Minneman et al. teach injecting first and second currents through an 
element of the conductor using the four-wire measurement technique and 
measuring the first and second voltage drops across the element. The 
values of the first and second currents and the first and second voltage 
drops are then used to calculate the current through the conductor. 
However, the circuit provides only a limited ability to handle a.c. 
currents and noise present in the conductor current. 
Therefore, it would be desirable to provide a circuit for non-invasively 
measuring current in a conductor with improved measurement range and 
without regard to the a.c. or d.c. content of the current. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a circuit for measuring 
alternating current (a.c.) and direct current (d.c.) in a conductor 
without breaking the conductor is provided. The circuit is coupled to 
several points on the conductor, usually in close physical proximity to 
define a segment of the conductor, via a pair of two-wire probes, each 
two-wire probe having a sense contact and a source contact that are both 
coupled to the conductor. 
The circuit is first configured to measure resistance by coupling a 
modulated current source in parallel with the segment through the source 
contacts to inject a test current. At the same time, a synchronous 
demodulator is also coupled in parallel across the segment to separate the 
test voltage drop across the segment induced by the test current from the 
voltage drop due to the conductor current. The modulated current source 
and the synchronous demodulator are both coupled to a reference waveform 
generator to receive a modulation waveform to achieve synchronous 
detection of the test voltage drop developed by the test current of the 
modulated current source. Synchronous demodulation allows for the 
separation of low level signals from a relatively high amplitude 
interference signals. The frequency of the modulation waveform may be 
selected so that noise and interference due to the conductor current does 
not substantially interfere with or degrade the measurement of the test 
voltage drop signal and is below a predetermined noise floor level. 
The test voltage drop separated by the synchronous detector is measured by 
a voltmeter configured to measure d.c. voltage. Because the test current 
I.sub.t is known and the voltage drop with the current source on, 
V.sub.on, and the voltage drop with the current source off, V.sub.off, can 
be measured, the resistance of the segment can thus be calculated 
according to the following equation. 
EQU R.sub.seg =V.sub.on -V.sub.off !/I.sub.t 
V.sub.on and V.sub.off are both measured at the output of the synchronous 
detector by the voltmeter as a d.c. voltage and is the test voltage drop 
V.sub.on -V.sub.off !. 
The circuit can be further configured to provide in-circuit current 
measurements. The voltmeter is coupled directly across the segment to 
measure voltage drop with the current source turned off. Because the 
conductor current may contain a.c. content, the voltmeter may be 
configured for a.c. volts rms (root mean squared) to obtain the correct 
voltage drop V.sub.0. With R.sub.seg and V.sub.0 now known, the current I 
flowing through the segment may be calculated according to the following 
equation. 
EQU I=V.sub.seg /R.sub.seg 
In this way, the current flowing through a conductor may be obtained 
regardless of the a.c. content of the current. 
One object of the present invention is to provide an apparatus for 
measuring in-circuit a.c. and d.c. current in a conductor. 
Another object of the present invention is to provide an apparatus for 
measuring resistance in a conductor in which a.c. current is present. 
An additional object of the present invention is to provide an apparatus 
for measuring in-circuit a.c. and d.c. current using synchronous 
demodulation of the test current. 
A further object of the present invention is to provide a method of 
synchronously modulating and demodulating the test current in a conductor 
to reject the a.c. components of the conductor current.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a block diagram of the circuit for measuring current and 
resistance according to the present invention. A conductor 10 may comprise 
a wire, a printed circuit board trace, an integrated circuit lead, or any 
other type of metal capable of conducting electrical current. A two-wire 
probe 12 is placed at point 14 along the length of the conductor, with 
adjacent source and sense leads making electrical contact with the 
conductor 10. A two-wire probe 16 is placed at point 18 with adjacent 
source and sense leads making electrical contact with the conductor 10. 
The length between the points 14 and 18 of the conductor comprises a 
segment which has a certain amount of resistance modeled by the resistor 
11. The resistor 11 is typically not a discrete component but rather a 
model of the trace resistance which varies according to the length of the 
segment of the conductor 10. A voltage drop V.sub.0 appears across the 
resistor 11 as a result of a current I flowing through the conductor 10. 
A modulated current source 22 is coupled to the source lead of the 2-wire 
probe 12 via a switch 20a. The switch 20a, along with switches 20b and 20c 
comprise a ganged switch with upper and lower positions corresponding to a 
resistance measuring mode and a current measuring mode respectively. The 
switches 20a-c are shown in the upper position for resistance measuring 
mode. The modulated current source 22 produces a test current I.sub.t that 
has modulation components responsive to a modulation signal produced by a 
reference waveform generator 24. 
The reference waveform generator 24 produces the modulation signal at a 
desired frequency in order to minimize the noise floor received from the 
conductor current. A set of modulation frequencies may be maintained and 
selected as needed to obtain a noise floor below a predetermined level, as 
explained in more detail in FIG. 2. In the preferred embodiment, the 
modulation signal is in the form of a square wave with a frequency of 
approximately 61 hertz. 
A synchronous demodulator 26 is coupled across the conductor 10 at the 
points 14 and 18 via the sense leads of the two-wire probes 12 and 16 to 
receive a voltage drop signal at a pair of input terminals. The 
synchronous demodulator 26 also receives the modulation signal from the 
reference waveform generator 24 in order to synchronously demodulate the 
voltage drop signal, which may contain both the test voltage drop signal 
and voltage drop from the current I. 
A voltmeter 28 is coupled to an output of the synchronous demodulator 26 to 
receive the test signal as V.sub.on and V.sub.off via the switches 20b and 
20c in resistance measurement mode. V.sub.on is measured with the 
modulated current source 22 turned on and V.sub.off is measured with the 
modulated current source 22 turned off. The values of V.sub.on and 
V.sub.off are converted to digital measurement values in the voltmeter 28 
and coupled to a microprocessor 30 which stores V.sub.on and V.sub.off. 
The voltmeter 28 may comprise a self-contained digital voltmeter or 
alternatively may comprise an analog to digital converter with a sample 
clock and appropriate filtering circuitry to produce the digital 
measurement values. 
The microprocessor 30 provides signals capable of controlling the mode of 
the circuit for the resistance measuring mode and the current measuring 
mode. The microprocessor 30 provides a MODE signal to the switches 20a-c 
which sets each of the switches 20a-c up for resistance measuring mode and 
down for current measuring mode. In resistance measuring mode, the 
modulated current source 22 and synchronous demodulator 26 are coupled in 
parallel across the segment. While in the resistance measuring mode, the 
modulated current source 22 is turned off to measure V.sub.off and on to 
measure V.sub.on according to an OFF/ON signal provided by the 
microprocessor 30. The microprocessor 30 may also be used to select the 
appropriate frequency of the test signal in response to a FREQUENCY signal 
provided to the reference waveform generator 24 while comparing the signal 
level of V.sub.off against a predetermined maximum noise floor level. 
Since the value of the test current I.sub.t generated by the modulated 
current source 22 is known and the values of V.sub.on and V.sub.off have 
been stored, the microprocessor 30 may then calculate the value of the 
resistor 11 for the segment between points 14 and 18 as shown according 
the following formula. 
EQU R.sub.seg =V.sub.on -V.sub.off !/I.sub.t 
The segment resistance R.sub.seg, may be assumed to be constant over a 
limited range of frequencies, typically less than 1 MegaHertz. The segment 
resistance R.sub.seg may then be sent to a display 32 for visual display. 
The microprocessor 30 may then configure the circuit for current 
measurement mode by setting the switches 20a-c to the lower position. Now, 
the voltmeter 28 is connected across the segment to directly measure Vo. 
The modulated current source 22 and synchronous 26 are disconnected from 
the conductor 10. Because the current I may contain a.c. components, the 
voltmeter can be configured to measure a.c. volts in order to accurately 
measure the voltage drop V.sub.0. across the segment. Because the voltage 
drop V.sub.0, as well as V.sub.on and V.sub.off, is a low level voltage 
signal, special techniques for measuring low level voltage, such as 
shielding and guarding techniques known in the art, may need to be 
employed. 
Once the value for V.sub.0 has been determined and provided by the 
voltmeter 28 to the microprocessor 30, the value of the current I may then 
be calculated according to the following equation. 
EQU I=V.sub.seg /R.sub.seg 
The current I may then be sent to the display 32 for visual display. 
FIG. 2 is a graph of amplitude versus frequency illustrating the process of 
selecting an appropriate test frequency according to the present 
invention. The trace 50 labeled CURRENT NOISE shows the amplitude versus 
frequency content of the voltage drop Vo which contains a substantial 
amount of high frequency energy. Any of a variety of distributions of 
amplitude versus frequency may be possible, thus making the selection of 
multiple test frequencies desirable in order to obtain a resistance 
measurement at the desired accuracy level. A set of test frequencies 52, 
labeled F1, F2, and F3, may be selected in order to obtain a noise floor 
below the predetermined maximum level labeled NOISE FLOOR. The actual 
noise floor for a test frequency is determined by the amplitude of the 
trace 50 at that test frequency. In this case, only the frequency F3 
provides an acceptable noise floor level below the predetermined maximum 
level NOISE FLOOR and would be employed to make the measurement. 
FIG. 3 is a schematic diagram of the modulated current source 22 according 
to the preferred embodiment of the present invention. Current sources 50 
and 52 are both coupled to contacts of the switch 54 which alternatively 
couples the test current I.sub.t from each of the current sources 50 and 
52 to the segment of the conductor 10 responsive to the modulation signal 
received from the reference waveform generator 24. In this way, the test 
current I.sub.t contains a.c. content at the desired test frequency. The 
modulation signal is preferably a square wave to ensure accurate actuation 
of the switch 54. 
FIG. 4 is a schematic diagram of the synchronous detector according to the 
preferred embodiment of the present invention. Switches 60a-d are coupled 
across the segment of the conductor 10 and each of the switches 60a-d 
actuate in a manner synchronized to the modulation signal from the 
reference waveform generator 24. A capacitor 62 operates as a sample and 
hold circuit to store the voltage value received from across the segment. 
The switches 60a-d collectively form a commutator switch in order to 
couple the capacitor 62 across the segment in opposing polarities in a 
manner synchronized to the modulated current source 22 shown in FIG. 3 so 
that a test voltage drop is developed across the capacitor 62. The voltage 
drop due to the current I, which may contain d.c. and a.c. components 
which are unsynchronized and out of band signals, is thus separated from 
the test voltage drop due to the test current I.sub.t. 
It will be obvious to those having ordinary skill in the art that many 
changes may be made in the details of the above described preferred 
embodiments of the invention without departing from the spirit of the 
invention in its broader aspects. For example, the modulation signal may 
take a variety of waveshapes such as sawtooth and sine waves. The test 
frequency need not be constant and the modulation waveform may consist of 
pseudo-random intervals or employ agile frequency hopping in the manner of 
spread spectrum communications technology in order to more effectively 
reject the current noise from the current I using synchronous demodulation 
and obtain a resistance measurement of the desired accuracy. The choice of 
modulation signal characteristics may be arrived at through a reasonable 
amount of experimentation. Therefore, the scope of the present invention 
should be determined by the following claims.