Amplifiers – With periodic switching input-output
Reexamination Certificate
1999-08-02
2001-06-05
Mottola, Steven J. (Department: 2817)
Amplifiers
With periodic switching input-output
C327S554000
Reexamination Certificate
active
06242974
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to solid state devices and methods for providing accurate voltage reference sources. More particularly, the present invention relates to devices, circuits and methods for providing bandgap voltage reference sources. Still more particularly, the present invention relates for devices, circuits and methods for providing simple low-cost bandgap voltage reference sources for use in applications such as analog-to-digital (ADC) digital-to-analog (DAC) and temperature measuring circuits and systems.
BACKGROUND OF THE INVENTION
Solid-state temperature sensors are widely used to provide reliable temperature measurement for many applications. In fact, silicon sensors often provide superior performance at a much lower cost than resistance-temperature-detectors, thermocouples and thermistors. Silicon-based temperature measurement typically involves an integrated circuit (hereinafter “IC”) and a sensor diode as shown in FIG.
1
. The IC
100
in
FIG. 1
applies current to diode
150
using on-chip current sources
110
and
115
of standard design and corresponding switches S
1
and S
2
(
120
and
125
, respectively). Clock and switch control circuit
140
provides alternating clock control signals to alternately close switches S
1
and S
2
and apply the respective currents to diode
150
. Measurement circuit
130
then measures the voltages, V
BE
, appearing across the diode when the currents are applied, which voltage is proportional to temperature. Specifically, switch control circuit
140
is shown generating the alternating sampling signals, &PHgr;
1
and &PHgr;
2
. During clock phase
1
, &PHgr;
1
, switch S
1
is closed and the diode is biased by I
1
to produce a voltage, V
BE1
. During clock phase
2
, &PHgr;
2
. switch S
2
is closed and the diode is biased by I
2
to produce voltage, V
BE2
. The measurement circuit samples and stores the diode voltage during each clock phase.
The measurements are based on a diode's voltage-current relationship, which is governed by the equation
V
BE
=V
T
*ln(I
D
/I
s
) (1)
where, I
D
is the forward diode current, I
S
is the diode reverse-saturation current, V
BE
is the forward diode voltage, and V
T
is the diode's thermal voltage given by
V
T
=K*T/q (2)
where, K=Boltzmann's Constant=1.38066*10
−23
J/° K
T=Temperature in degrees Kelvin, ° K, and
q=Electron Charge=1.602*10
−19
Coulombs.
It can be shown that the change in voltage measured across a diode, &Dgr;V
BE
, when the diode is excited with two different currents, I
1
and I
2
, is
&Dgr;V
BE
=V
T
*ln(I
1
/I
2
) (3)
Equation (3) is based on the assumption that the diode is operating in its linear, 60 mV/decade (Gummel-Poon) region. If the currents, I
1
and I
2
, are precisely matched, then &Dgr;V
BE
can be used to provide a very stable, well-defined thermometer signal by substituting Equation (2) in Equation (3) and solving Equation (3) for T.
FIG. 1
illustrates a “single-wire” diode temperature measurement system in which only one conducting path connects a single pin of IC
100
and diode
150
. The structure of
FIG. 1
can, of course, be expanded for use with a plurality N of diodes, each connected to a respective pin on an IC such as
100
in
FIG. 1
, and each monitoring temperature at a respective off-chip location. A standard N:1 multiplexer is then controlled by the clock and control circuit
140
to connect the measurement circuitry
130
to each of the N diodes in turn.
By way of contrast,
FIG. 2
illustrates the use of a transistor
250
to provide a more accurate “two-wire” differential measurement. The IC
200
in
FIG. 2
contains an excitation circuit comprising current sources I
1
(
215
), I
2
(
210
), and respective switches S
1
(
220
), and S
2
(
225
), operating under the control of clock and switch control circuit
240
, as in FIG.
1
. Again, the voltages V
BE1
and V
BE2
are sampled and stored. The two-wire connection of transistor
250
(which may be a PNP or NPN transistor) to IC
200
includes the ground sense path
270
connecting the base of transistor
250
to ground pin at IC
200
. Because of the transistor connection, the current flowing in the ground sense wire is divided by the &bgr; of transistor
250
. Also, in the circuit of
FIG. 2
current flows from the excitation circuit output through pin
245
of the IC
200
and through a wire connection
260
having resistance R
WIRE
to the emitter of transistor
250
. If the resistance of the wire
260
causes an excessive voltage drop, then it may be desirable to provide a second conducting path
265
connecting the emitter to the IC measuring pin. Multiplexing techniques can be applied to the circuitry of
FIG. 2
in the same manner as for the diode monitor arrangement of FIG.
1
. Either PNP or NPN sensor transistors can be used in such applications.
If the currents I
1
and I
2
in the circuits of
FIGS. 1 and 2
are precisely in the proportion I
1
=M*I
2
, and the measurement circuit is designed to subtract V
BE2
from V
BE1
, then the residual signal, &Dgr;V
BE
=V
T
*ln(M), provides an accurate measure of temperature. A key to achieving accurate temperature measurement using this technique therefore is the matching of the current sources, I
1
and I
2
. It is generally not possible to exactly match two independent current sources on a chip with any high degree of reproducibility.
Ratio matching in the range of 0.1% to 1.0% can be achieved through the use of very precise design and manufacturing techniques, but many applications require better accuracy. One-time factory calibration is often used to reduce the current source mismatch error, but this approach relies on the use of expensive manufacturing techniques including thin-film laser trimming, so-called zener-zapping, or fuse blowing. These solutions require additional active silicon for implementation, which increases the size and cost of the chip. Furthermore, the extra trim circuitry and testing results in a reduction of product yield and increase in manufacturing cost.
Central to many prior art temperature measurement systems of the type described above is a stable, reliable, robust voltage reference. Classic bandgap voltage references are described, e.g., in, Horowitz, P and W. Hill, The Art of Electronics, 2
nd
Ed., Cambridge Univ. Press, 1989, especially pp. 335-339. Input offset voltages, V
OSI
, present at the input of op-amps used in standard bandgap voltage references introduce errors that can prove troublesome for many applications. As with other operational amplifiers (op-amps) and other analog circuit designs, means are often provided to trim or otherwise adjust imbalances and variations encountered in particular integrated circuit (IC) implementations of bandgap references. The design and manufacturing complexities of introducing such trimming and adjusting proves to be time consuming, wasteful of IC device area and generally more costly.
SUMMARY OF THE INVENTION
The limitations of the prior art are overcome and a technical advance is made in accordance with the present invention as described in illustrative embodiments herein.
In accordance with an illustrative embodiment of the present invention, a new auto-zero circuit arrangement is provided intermediate, an input network—such as a standard reference diode network—and an op-amp exhibiting an input voltage offset, V
osi
. This combination provides a voltage reference solution for sampled data systems that avoid prior art limitations. In response to a periodic “auto-zero pulse,” the new auto-zero circuit arrangement nulls the op amp's V
OSI
while functioning in an “auto-zero” mode. After the auto-zero circuit returns to its normal mode, the overall combination is then in condition to function without the troublesome V
OSI
effect. As the potential for V
OSI
problems again develops, another auto-zero pulse causes the circuit arrangement to switch again to the auto-zero mode. Th
Micrel,Inc
Mottola Steven J.
Ryan William
LandOfFree
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