Analog multi-channel probe system - Patent Review 5418470

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BACKGROUND OF THE INVENTION

The present invention relates to analog probe systems, and more particularly to an analog multi-channel probe system for embedment into a device under test (DUT) that provides for high speed analog and digital signals to be routed to points where they may be measured by conventional methods.

There exist today several integrated circuits that are controlled by a standard IEEE 1149.1 boundary scan interface. Current boundary scan based methods of testing integrated circuits work well where the integrated circuits to be tested are static and digital, but fall short when at-speed or analog testing is required. Optimum solutions to these problems would be to embed an "oscilloscope-on-a-chip" and a "VLSI-tester-on-a-chip" into the integrated circuit or system to be tested (DUT). Unfortunately such chips presently do not have the price/performance levels required to make them practical for most applications. Also many potential test points are externally inaccessible due to the spacing of leads on integrated circuits, the density of parts on a printed circuit board, or the burying of such points within a multi-layer multi-chip module or printed circuit board. Therefore testing with conventional instruments and automated test equipment (ATE) is possible only at interfaces where the circuit or board connections are accessible.

What is needed is an analog multi-channel probe system for providing data from inaccessible test points to points that are accessible for measurement by conventional test and measurement instruments.

SUMMARY OF THE INVENTION

Accordingly the present invention provides an analog multi-channel test probe system for embedment in a device to be tested. A plurality of input points are coupled via multiplexers input buffer amplifiers to respective analog multiplexers. The outputs from the analog multiplexers are input to output buffer amplifiers such that up to m of n input points may be connected to any of m output points. The m outputs are input to respective differential input/output amplifiers where they are compared to a reference signal and produce differential outputs. An analog multiplexer has as inputs desired reference voltage levels and an input from an uncommitted input buffer amplifier. One of these inputs to the analog multiplexer is selected as the reference signal. A programmable fifty ohm termination is provided for the differential outputs if desired.

The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a block diagram of an analog multi-channel probe system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the FIGURE, the illustrated analog multi-channel probe system has eight test inputs and four measurement outputs. Eight input signals INPUT 0-7 from test points of a device under test are input to respective input buffer amplifiers 12 via the eight inputs respectively of the probe system. Each input buffer amplifier 12 may be selectively enabled by an appropriate input enable signal. The input buffer amplifiers 12 provide high impedance and low capacitance to the input signals to allow minimum loading on the signal being measured, and convert the voltage at the input to a current at the output. The output from each input buffer amplifier 12 is input to respective analog multiplexers 14. The analog multiplexers 14 each have five outputs (one more than the number of measurement outputs of the probe system). The particular output line from any of the multiplexers 14 on which the signal at the input appears is determined by a select command for each multiplexer. Four of the outputs from the analog multiplexers 14 are input to respective programmable output buffer amplifiers 16. The extra output from each multiplexer 14 is for an uncommitted probe as a reference circuit. The multiplexers 14 steer the input currents to the desired outputs. Each output buffer amplifier 16 may be selectively enabled by an appropriate output enable signal, and converts the current at the input to a voltage at the output. In this manner between zero and four of the input signals INPUT 0-7 may be routed to between zero and four of the output buffer amplifiers 16, i.e., OUTPUT 0-3 as shown.

The remaining circuitry is for converting the single-ended output signal into a differential output signal. A reference input signal REF from the device under test is applied to an uncommitted input buffer amplifier 18 and an uncommitted output buffer amplifier 20 coupled in series. The reference input signal REF may be selected alternatively from among any of the input signals INPUT 0-7 by coupling the extra output from the multiplexers 14 to the uncommitted output amplifier 20. A reference source 22 provides many voltage references, such as ECL+, ECL-, TTL and GND. The voltage references and the reference input signal REF are applied as inputs to analog routing switches (or analog multiplexers) 24 the outputs of which are input to respective differential input/output amplifiers 26. A reference select signal is applied to the analog routing switches 24 to determine which reference levels are output. The output signals from the output buffer amplifiers 16 are applied as the other inputs to the differential input/output amplifiers 26. The multiple routing switches 24 may be replaced with a single routing switch having its output applied in parallel to all of the differential input/output amplifiers 26. In this way only a single reference level is selected which is applied to all of the differential input/output amplifiers 26. Differential output signals minimize the effects of local crosstalk and noise sources as they travel to a measurement point. The differential output signals from the differential input/output amplifiers 26 are input to respective selectable termination circuits 28, such as 50 ohm circuits, to provide appropriate connections for conventional test and measurement instruments at the measurement points. A termination enable signal determines whether the particular differential output signal is terminated or passed straight through to the differential output terminals. Thus the input signals may be referenced to the internally generated voltage levels or to the reference signal REF from the DUT.

A standard IEEE 1149.1 boundary scan interface 30, or similar program bus, is provided for programming the analog multi-channel probe system. A test access port (TAP) controller 32 provides appropriate signals from a test clock TCK and a test master signal TMS. Test input data TDI is loaded serially into a control register 34, an instruction register 36 and a bypass register 38. A test data output multiplexer 40 is coupled to have as inputs the outputs from the control register 34, the instruction register 36 and the bypass register 38 to provide test output data TDO back to the boundary scan interface. A decoder logic circuit 42 converts the contents of the control register 34 into respective enable/select signals for the input and output buffer amplifiers 12, 16, the routers and routing switches 14, 24, and the termination circuits 28.

Thus the present invention provides a programmable analog multi-channel probe system for embedment into a device under test, such as a printed circuit board, integrated circuit or multi-chip module, that couples any test point of the device under test to an external measurement point where conventional instrumentation may be used to measure the voltage(s) at the selected test point(s).

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