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Testing of a DCS Converter

IP.com Disclosure Number: IPCOM000102828D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 1 page(s) / 44K

Publishing Venue

IBM

Related People

Cao, TM: AUTHOR [+4]

Abstract

Differential current switch (DCS) to single-ended converters are subject to stuck-near-threshhold faults which are difficult to detect with conventional test systems. This circuit employs a switchable biasing current source for the detection of faults unique to this circuit type.

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Testing of a DCS Converter

      Differential current switch (DCS) to single-ended converters
are subject to stuck-near-threshhold faults which are difficult to
detect with conventional test systems.  This circuit employs a
switchable biasing current source for the detection of faults unique
to this circuit type.

      The Figure is an example of a conventional differential to
single ended converter circuit 10 responsive to differential inputs
Il, I2, and I3, I4 for generating single-ended outputs 01, 02.  A
pair of voltage clamps 12, 14 are provided to limit the output
swings.  A test bias current source 16, 20 is connected in parallel
with the conventional current source 18, 19.  Transistor 16 is turned
on when input TO is 'HIGH' and is off when input TO is 'LOW' (about
equal to the VEE power supply).  When it is on, transistor 16 and
resistor 20 create a current source that may double the current in
the cascode current switch tree.

      Of particular interest are circuit faults that can cause the
current to split between the two collector nodes N1 and N2, causing
the output signals at nodes 01 and/or 02 to be at or near threshold.
By switching on transistor 16 so as to add a bias current during
testing, nodes N1 and N2 are pulled down such that nodes 01 and 02
are below threshold for the defective circuit, and a predictable
'stuck 0 error' is achieved.  A good circuit will still produce valid
logic levels.

      Disclosed anonymously.