Thermal measuring and testing – Temperature measurement – By electrical or magnetic heat sensor
Reexamination Certificate
2000-07-24
2001-12-25
Cunningham, Terry D. (Department: 2816)
Thermal measuring and testing
Temperature measurement
By electrical or magnetic heat sensor
C327S512000
Reexamination Certificate
active
06332710
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature sensor circuit, and in particular, to a multi-channel temperature sensor having a single negative input terminal configured to receive input from multiple remote temperature-sensing diodes.
2. Description of the Related Art
When operated at a constant current, the voltage (V
for
) of a forward-biased P/N diode exhibits a negative temperature coefficient of about −2 mV/° C. This property can be utilized to detect temperature. Unfortunately, the absolute value of V
for
varies according to diode composition and hence the process conditions under which the diode was fabricated. One approach to overcome such process-based variation in V
for
is to calibrate the current supplied across the P/N junction to match the variation in V
for
exhibited by a particular diode. However, such a calibration of individual current supplies is impractical for mass produced devices. Another approach is to detect a change in forward-biased diode voltage (&Dgr;V
for
) for two different applied currents,
1
X and NX, where NX is an integer multiple of
1
X. Specifically:
T
=
q
Δ
V
for
η
K
ln
(
N
)
,
where
T
=
absolute
temperature
in
°
Kelvin
)
;
q
=
the
charge
on
the
carrier
(
electron
charge
)
;
Δ
V
for
=
change
in
forward
-
biased
voltage
;
K
=
Boltzmann
'
s
constant
;
N
=
ratio
of
the
two
applied
currents
;
and
η
=
ideality
factor
of
diode
.
(
I
)
The premise of this approach is the principle that any uncertainty in diode behavior introduced by process variation is eliminated (i.e., cancelled out) by detecting a voltage change for two different currents flowing across the same diode.
Accordingly,
FIG. 1
shows a schematic diagram of a conventional temperature sensor circuit utilizing this principle to detect ambient temperature. Temperature sensor circuit
100
includes remote diode
102
positioned in remote device
104
and connected with temperature sensor
106
through output line
108
at positive data pin (DxP) and through input line
110
at negative data pin (DxN). While
FIG. 1
depicts remote diode
102
as a simple diode remote diode
102
can also take the form of the forward-biased emitter-base P/N junction of a PNP or NPN bipolar transistor.
Temperature sensor
106
also includes variable current supply
112
configured to communicate a current to positive data pin DxP. The output from variable current supply
112
is varied between a base current (
1
X) and an integer multiple (NX) of the base current, as controlled by logic block
116
.
Current output from the positive data pin DxP is communicated through output line
108
to remote diode
102
. Current flows across remote diode
102
, and is returned back through input line
110
to temperature sensor
106
at the negative data pin DxN.
Temperature sensor
106
includes analog-to-digital (ND) converter
114
having first input terminal
114
a
, second input terminal
114
b
, and output terminal
114
c
. A first current is flowed into remote diode
102
, and first input terminal
114
a
experiences a first voltage corresponding to the flow of this first current into remote diode
102
.
A/D converter
114
receives, at second input terminal
114
b
, a second voltage corresponding to the current flowed across remote diode
102
. This second voltage experienced at the second input terminal
114
b
is a function of (i.e., depends on) the potential drop across remote diode
102
. A/D converter
114
samples the difference between the first and second voltages, converts this difference into a digital signal, and communicates this digital signal to logic block
116
.
Precise operation of the conventional temperature sensor circuit of
FIG. 1
is explained in conjunction with
FIGS. 1A-1B
.
FIG. 1A
is a detailed view of the circuitry at the DxP pin and DxN pin of temperature sensor
106
.
FIG. 1B
is a detailed view of the circuitry of A/D converter
114
of temperature sensor
106
.
First constant current source
118
provides a base current
1
X along first force line
120
to the DxP pin. First switch
122
selectively connects second constant current source
124
to first force line
120
. Second constant current source
124
provides a supplemental current (N−1)X constituting an integer multiple of the base current
1
X output by first constant current source
118
. The currents output by the first and second constant current sources
118
and
124
will vary somewhat with temperature, but the ratio of these currents will retain the integer relationship described herein.
First switch
122
is controlled by logic block
116
. Initially, first switch
122
is deactivated, and first constant current source
118
alone communicates base current
1
X to the DxP pin. The base current
1
X flows out of the DxP pin, through output line
108
, and across remote diode
102
. The resulting voltage on the DxP pin is communicated along first sense line
126
to first input terminal
114
a
of A/D converter
114
.
Current flowing across remote diode
102
is conveyed through input line
110
back to temperature sensor
106
at the DxN pin. This current, then, flows through second force line
128
, exhibiting a parasitic resistance represented by resistor
130
in series with diode
132
, into ground. Third current supply
170
is also in electrical communication with second force line
128
. The resulting voltage on the DxN pin is communicated to second input terminal
114
b
of AID converter
114
along second sense line
134
.
The voltage difference between the DxP and DxN pins represents the voltage drop (V
for1
) across forward-biased remote diode
102
at the base current
1
X. This voltage difference is sampled by A/D converter
114
, as shown in FIG.
1
B.
A/D converter
114
includes voltage reference
148
in electrical communication with non-inverting input node
150
a
of operational amplifier
150
, and also in electrical communication with first plate
152
a
of sampling capacitor
152
through second switch
154
. First input terminal
114
a
of A/D converter
114
is in electrical communication with first plate
152
a
of sampling capacitor
152
through third switch
156
. One skilled in the art will recognize that A/D converter
114
is representative of a variety of analog-to-digital converters suitable for use in temperature sensor circuits.
Second input terminal
114
b
of A/D converter
114
is in electrical communication with second plate
152
b
of sampling capacitor
152
through fourth switch
158
. Second plate
152
b
of sampling capacitor
152
is in electrical communication with inverting input node
150
b
of operational amplifier
150
through fifth switch
160
.
First plate
162
a
of feedback capacitor
162
is in electrical communication with inverting input node
150
b
of operational amplifier
150
. Second plate
162
b
of feedback capacitor
162
is in electrical communication with output node
150
c
of operational amplifier
150
.
The A/D converter
114
depicted in
FIG. 1B
also includes a parasitic capacitor
164
in electrical communication with second plate
152
b
of sampling capacitor
152
. Parasitic capacitor
164
represents the parasitic capacitance arising due to existence of the fourth and fifth switches
158
and
160
. In reality, these switches are MOS transistors that experience some form of parasitic capacitance. The charge retained as a result of this parasitic capacitance must be considered during operation of A/D converter
114
, and is thus represented as parasit
Aslan Mehmet
Henderson Richard
Cunningham Terry D.
National Semiconductor Corporation
Pillsbury & Winthrop LLP
Tra Quan
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