Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Converging with plural inputs and single output
Reexamination Certificate
2001-04-12
2002-09-17
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Converging with plural inputs and single output
C327S059000, C327S078000, C327S590000
Reexamination Certificate
active
06452436
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to automatic test equipment (ATE) for electronics, and, more particularly, to electronic sources that automatically switch between voltage control and current control depending upon programming and load conditions.
BACKGROUND OF THE INVENTION
ATE systems commonly include a variety of electronic sources for setting bias conditions and testing DC characteristics of devices. A type of electronic source known as a “V/I” source combines both voltage-forcing and current-forcing modes in a single instrument. The two modes generally share a common control circuit and output stage, but employ different feedback paths. The different feedback paths can be engaged programmatically, for example by explicitly setting a force-voltage or force-current mode, or can be engaged automatically, as described below.
Automatically controlled V/I sources accept programmed values for voltage and current, and switch between voltage-controlled and current-controlled modes automatically, as required, to ensure that neither the programmed voltage nor the programmed current is exceeded. For example, an automatically controlled V/I source programmed for 5 Volts and 5 mA would operate in voltage-controlled mode (at 5 Volts) when connected to grounded loads greater than 1 K&OHgr;, but would automatically switch to current-controlled mode (at 5 mA) when driving grounded loads less than 1 K&OHgr;.
V/I sources often include greater than two control modes. One type of V/I source provides one current-controlled mode and two voltage-controlled modes. This type of source functions as a current source with positive and negative voltage clamps. Another type of V/I source provides one voltage-controlled mode and two current-controlled modes. This type of source functions as a voltage source with positive and negative current limits. Some V/I sources include four feedback modes—two current-controlled modes and two voltage-controlled modes. Only three modes are allowed to be active at a time. The source can be used as either a current source with two clamps or a voltage source with two current limits, depending upon how the source is programmed.
FIG. 1
is a simplified illustration of a conventional V/I source
100
. The source
100
is configured as a voltage source with two current limits. A digital-to-analog converter (DAC)
110
establishes a desired output voltage for the V/I source
100
. A summing circuit
116
subtracts a voltage feedback signal from an output voltage of the DAC
110
, to produce an error voltage, V
ERROR
. A crossover circuit
122
selects one feedback path for passage to a control circuit
124
. When the V/I source
100
is operating in voltage-controlled mode, the crossover circuit
122
passes the V
ERROR
to the control circuit
124
. The output of the control circuit
124
is fed to a gain stage
126
and to a shunt
128
before arriving at a device under test (DUT)
132
. When operating in voltage-controlled mode, the V/I source
100
forms a closed loop feedback system among the elements described above, and maintains an output voltage at the DUT
132
at the value prescribed by the DAC
110
.
The V/I source
100
also includes DACs
112
and
114
for establishing positive and negative current limits, respectively. A differential circuit
130
coupled to the shunt
128
produces a current feedback signal proportional to the voltage across the shunt. Summers
118
and
120
subtract the current feedback signal from the outputs of DACs
112
and
114
, to develop error signals I
PosError
and I
NegError
, respectively. When the V/I source
100
operates in positive current-controlled mode, the crossover circuit
122
passes I
PosError
to the control circuit
124
; when it operates in negative current-controlled mode, the crossover circuit
122
passes I
NegError
. The circuit elements combine to form feedback systems that maintain the output current of the V/I source
100
at the value prescribed by DAC
112
or DAC
114
, depending upon which of the two current modes is operative.
The control circuit
124
typically includes an integrating circuit for establishing dominant frequency characteristics of the V/I source. The gain stage
126
may provide voltage gain, current gain, or both. The shunt
128
generally includes an array of different resistors that can be individually selected to accommodate different current ranges.
FIG. 2
illustrates a conventional crossover circuit
122
commonly used with the V/I source
100
of FIG.
1
. The crossover circuit
122
includes operational amplifiers (op amps)
214
and
224
, buffers
212
and
222
, diodes
216
,
218
,
226
, and
228
, and resistors
210
,
220
, and
230
. The op amps
214
and
224
each have two distinct states of operation—an active state and an inactive state.
Taking the op amp
214
as an example, the op amp
214
assumes the active state whenever I
PosError
is less than V
ERROR
. Under these conditions, diode
216
becomes reverse-biased and diode
218
conducts in the forward direction. A feedback loop is formed consisting of op amp
214
, diode
218
, buffer
222
, and resistor
220
. The feedback loop tends to drive the input of the buffer
212
to a level equal to I
PosError
. The buffer
212
then provides I
PosError
to the input of the control circuit
124
.
Op amp
214
assumes the inactive state whenever I
PosError
is greater than V
ERROR
. Diode
218
becomes reverse-biased and diode
216
conducts. I
PosError
is thus cut off from the control circuit
124
, and feedback is closed locally around op amp
214
via diode
216
.
The negative polarity operates in an analogous manner. Op amp
224
assumes the active state whenever I
NegError
is greater than V
ERROR
. A feedback loop is formed consisting of op amp
224
, diode
228
, buffer
222
, and resistor
230
. The loop tends to drive the input of the buffer
212
to I
NegError
, and establishes the negative current-controlled mode. When I
NegError
is less than V
ERROR
, diode
228
becomes reverse-biased and diode
226
conducts, thus cutting off I
NegError
from the control circuit
124
and locally closing feedback around the op amp
224
.
The crossover circuit
122
thus engages a current-controlled mode when either of the op amps
214
and
224
operates in its active state. When both op amps operate in their inactive states, the crossover circuit
122
engages voltage-controlled mode. Voltage-controlled mode is thus engaged whenever V
ERROR
is greater than I
NegError
and less than I
PosError
. In voltage-controlled mode, the crossover circuit
122
passes V
ERROR
to the control circuit
124
via resistor
210
.
The crossover circuit
122
generally operates smoothly and accurately, making virtually seamless transitions between feedback modes. We have recognized, however, that the crossover circuit
122
can behave improperly during programming and output transients. For example, when programming a fast voltage step (via the DAC
110
), the signal V
ERROR
undergoes a voltage step from its steady-state value. The resulting step can momentarily cause V
ERROR
to cross I
PosError
or I
NegError
. These conditions cause the V/I source to inappropriately switch from voltage-controlled mode to one of its current-controlled modes. The mode change is “inappropriate” because it is not caused by excessive current flow; in fact, the output current may be zero. Rather, it is the natural consequence of applying a fast programming step.
FIG. 3
illustrates this condition.
FIG. 3
is a V/I plot of the V/I source
100
operating with the crossover circuit of FIG.
2
. The curve
300
represents the output of the V/I source during its three distinct feedback modes:
1. The upper, horizontal portion of the curve
300
represents the output of the V/I source when the positive current loop is engaged and programmed to a value I
ProgPos
;
2. The vertical portion represents the output when the voltage loop is engaged and programmed to a value V
Prog
; and
3. The lower, horizontal portion represents the output wh
Callahan Timothy P.
Englund Terry L.
Rubenstein Bruce D.
Teradyne, Inc.
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