Abstract:
A probe for connecting a device under test to a measurement device that corrects for dc errors and noise generated by the current flowing through the ground shield of a transmission line used by the probe. The probe identifies a voltage drop in the ground preferably using an additional line between the device under test and the measurement device. The signal provided to the measurement device is corrected based on the identified voltage drop.

Description:
This is a Continuation of application Ser. No. 10/117,653, filed on Apr. 5, 2002, now U.S. Pat. No. 6,806,697, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   A probe is a device that ideally non-invasively connects a measurement device to a device under test (also referred to as a “DUT”). Known measurement devices, that utilize probes, include: oscilloscopes; spectrum analyzers; network analyzers; logic analyzers; counters; and time interval meters. In such known measurement devices, minimally invasive probes are connected to an indeterminate source of a signal to be tested and a ground of a DUT. To function in a minimally invasive manner, a probe has high impedance relative to the source impedance such that the probe does not add a significant load to the circuit under test. This feature separates probes from standard interconnects, such as used by power supplies and the like. 
   Active probes are a class of probes that contain active circuits, typically designed to provide impedance buffering near the probe tip to drive a 50 ohm transmission line to the measurement equipment One example of such an active probe is the AGILENT 1158A which is a 4 GHz Active probe for the AGILENT INFINIIUM oscilloscope. Active probes generally comprise three parts: a probe pod with circuits powered by the measuring device; the probe tip for interfacing with the DUT; and a transmission line between the probe tip and pod typically comprising a coax line with one or more single line wires. 
   A reoccurring problem with probes are dc errors and noise generated by the current flowing through the ground shield of the transmission line used by the probe. Such problems, especially at DC or low frequencies (such as 60 Hz), can be significant. Most probes include instructions for manually checking for these effects by grounding the probe tip and ground tip to the DUT ground to give the user an understanding of the level of these problems. 
   Various techniques have been used in the past to minimize the ground shield noise. For example, it is known to connect the DUT to an AC supply as close as possible to the measurement equipment. However, this is not always practical and may not limit the current enough for cutting edge measurement equipment. Another solution has been to insert a choke (inductor) into the power line ground of either the DUT or the measurement equipment so as to limit the current flow into the ground. This solution has not proven satisfactory at low frequencies and is not effective at all for DC. One solution that may help for DC is the use of a heavy conductor for the transmission line, unfortunately this solution has not been successful for low frequency signals. One solution that actually works is to float either the DUT or the measurement equipment by cutting the power ground line. Unfortunately, such a solution is quite dangerous. 
   The present inventors have discovered an apparatus and method for automatically compensating for noise caused by currents flowing in the transmission line ground of a probe. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An understanding of the present invention can be gained from the following detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a simplified block diagram including a probe in accordance with a preferred embodiment of the present invention. 
       FIG. 2  is circuit diagram of a circuit including a probe in accordance with the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 1  is a simplified block diagram of a circuit  100  in accordance with the preferred embodiment of the present invention. It will be appreciated by those of ordinary skill in the relevant arts that the circuit  100 , as illustrated in  FIG. 1 , and the operation thereof as described hereinafter is intended to be generally representative such systems and that any particular system may differ significantly from that shown in  FIG. 1 , particularly in the details of construction and operation of such system. As such, the circuit  100  is to be regarded as illustrative and exemplary and not limiting as regards the invention described herein or the claims attached hereto. 
   The circuit  100  generally comprises a measuring device  102  connected to a DUT  104 . In accordance with the preferred embodiment of the present invention, the measuring device  102  may be an oscilloscope, such as an AGILENT INFINIIUM. The DUT  104  could comprise any number of devices, including PC mother boards, high speed digital circuits, cellular phone circuits, etc . . . . The measurement device  102  is generally connected to the DUT  104  via a signal wire  106  (comprising portions  106   a  and  106   b ) and a ground wire  108 . An additional wire  110  (comprising portions  110   a  and  110   b ), carrying very low current, is provided between the measurement device  102  and the DUT  104 . 
   A detection circuit  112  is interposed on the additional wire  110 . More specifically, a first portion  110   a  of the wire  110  connects the ground of the DUT  104  to a detection circuit  112 , while a second portion  110   b  of the wire  110  connects the detection circuit  112  to the ground of the measurement device  102 . The detection circuit  112  detects a voltage difference between the ground of the Measurement device  102  and the ground of the DUT  104  and outputs a signal proportional to the detected difference. 
   A summing circuit  114  is similarly interposed on the signal wire  106 . More specifically, a first portion  106   a  of the signal wire  106  connects the probe tip to an input of the summing circuit  114  while a second portion  106   b  of the signal wire  106  connects the measurement device  102  to an output of the summing circuit  114 . A second input of the summing circuit  114  is connected to the output of the detection circuit  112 . The summing circuit  114  outputs a signal proportional to the sum of the voltages on the first and second inputs thereof, e.g. the output of the detection circuit  112  and the signal measured on the DUT  104  via the signal wire  106   a.    
   In operation, the detection circuit  112  senses the voltage drop due to the current flowing on the ground shield of the ground wire  108  which the sum circuit  114 , in effect, subtracts from the signal voltage. In this manner the EMF generated on the shield of the ground wire  108  does not interfere with the operation of the measurement device  102 . 
     FIG. 2  is circuit diagram of probe in accordance with the preferred embodiment of the present invention. It will be appreciated by those of ordinary skill in the relevant arts that the circuit  200 , as illustrated in  FIG. 2 , and the operation thereof as described hereinafter is intended to be generally representative such systems and that any particular system may differ significantly from that shown in  FIG. 2 , particularly in the details of construction and operation of such system. As such, the circuit  200  is to be regarded as illustrative and exemplary and not limiting as regards the invention described herein or the claims attached hereto. 
   The circuit  200  generally comprises a measuring device  202  connected to a DUT  204  by a probe  206 . In accordance with the preferred embodiment of the present invention, the measuring device  102  may be an oscilloscope, such as an AGILENT INFINIIUM. The DUT  204  could comprise any number of devices, including PC mother boards, high speed digital circuits, cellular phone circuits, etc . . . . The probe  206  comprises a probe tip  208 ; a cable  210 ; and an electronics pod  212 . The measurement device  202  is connected to the DUT  204  via a signal wire  214  and a ground wire  216  in the probe  206 . An additional wire  218 , carrying very low current, is provided between the measurement device  202  and the DUT  204 . Those of ordinary skill in the art will recognize that only the components necessary for an understanding of the present invention are represented in  FIG. 2 . For example, other signal processing electronics are present in the pod and the tip. 
   In the example shown in  FIG. 2  taking the voltage signal of the DUT  204  to be 1v and the voltage on the ground wire  216  to be 0.1v, the voltage of the signal measured exiting the DUT  204  would be 1.1v with out the corrective action of the present invention. With R 2  and R 5  set at 90 k ohm and R 3  and R 4  set at 10 k ohm the signal entering high frequency buffer amp  220  would be at 0.11v. The voltage between the amp  220  and the amp  224  (with a gain set to 1), e.g. the correction voltage, would be 0.01v. The voltage seen on the measurement device  202 , across R 1 , would be 0.1v (as opposed to 0.11v which it would otherwise measure). 
   Although a few embodiments of the present invention has been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.