Circuit for detecting an impedance change at a circuit input

Details Circuit for detecting an impedance change at a circuit input Circuit for detecting an impedance change at a circuit input

1998-09-21

2002-08-06

Le, N. (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

C324S649000, C324S691000, C365S189011

Reexamination Certificate

active

06429665

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to electrical circuitry and, in particular, to an electrical circuit for detecting an impedance at a circuit input.
2. Description of the Related Art
Currently, complementary metal-oxide semiconductor (CMOS) technology is nearly universally used to fabricate memories and other electronic components that require high switching speed and low power consumption. As integrated circuit fabrication techniques have continued to improve, it has been possible to produce integrated circuits having ever smaller feature sizes and increasing performance. However, recently there has been a growing realization that CMOS technology has fundamental scaling limitations and that diminishing performance enhancements are gained by standard geometric scaling. Accordingly, there is currently great interest in operating CMOS systems at sub-ambient temperatures to extract greater performance from current design points.
When operating a CMOS system at sub-ambient temperature, practical system reliability concerns dictate that no moisture be permitted to condense on the surfaces of system boards and low temperature cooling assemblies. Thus, it is imperative to maintain such surfaces at a temperature greater than the dew point of water vapor. In conventional systems intended for operation at sub-ambient temperatures, optical hygrometers such as that depicted in
FIG. 1
are utilized for dew point detection.
As shown in
FIG. 1
, a conventional optical hygrometer
10
includes two optical channels, a control channel
12
and a detection channel
14
, that each contain a light emitting diode (LED)
16
and a photodetector (PD)
18
. In control channel
12
, the amount of light emitted by LED
16
that is received by photodetector
18
is controlled by an optical balance
22
, which determines how far opaque barrier
20
extends into control channel
12
. Light emitted by LED
16
of detection channel
14
, on the other hand, is received by photodetector
18
after being reflected by a mirror
24
located on a cooled surface for which temperature control is desired. An output of each of photodetectors
18
is connected to an input of differential amplifier
26
, which generates an amplified output that forms an input to a defrost control circuit.
In operation, optical hygrometer
10
is first calibrated through the adjustment of optical balance
22
when mirror
24
is free from condensation such that the output of differential amplifier
26
is essentially zero. Then, as the surface on which mirror
24
is cooled below the dew point, moisture will begin to condense on mirror
24
, scattering the light emitted by LED
16
in detection channel
14
and causing a differential voltage to develop between the inputs of differential amplifier
26
. Thus, when condensation reaches a level determined by the setting of optical balance
22
, differential amplifier
26
signals the defrost control signal that condensation has formed on the cooled surface and that remedial action, such as heating the cooled surface, is required.
While conventional optical hygrometers such as that illustrated in
FIG. 1
are effective in providing an imprecise indication of the condensation of moisture on a cooled surface, such conventional optical hygrometers are subject to a number of drawbacks. First, conventional optical hygrometers tend to be expensive and bulky. It is therefore inconvenient and costly to utilize a large number of optical hygrometers in a practical computer system. In addition, conventional optical hygrometers cannot be used to detect moisture condensation over small surface areas. Second, conventional optical hygrometers do not generate digital output signals. Optical hygrometers therefore cannot easily be integrated into or interconnected with conventional digital circuitry, for example, to permit the gathering of statistical data pertaining to moisture condensation. Third, conventional optical hygrometers have poor sensitivity to the formation of condensation. As a result, damage can result to system components if the optical balance is calibrated incorrectly.
As should thus be apparent, it would be useful and desirable to provide an improved hygrometer for detecting moisture condensation in a computer system operating at sub-ambient temperature.
SUMMARY OF THE INVENTION
In accordance with the present invention, an impedance detection circuit is provided that includes a circuit input having a first contact and a second contact, a reference voltage rail coupled to the first contact, and a memory cell having a data node coupled to the second contact and an output. When the memory cell is read, the logic state of the output provides an indication of an impedance coupling the first and second contacts at the circuit input. The impedance detection circuit can be utilized to sense resistive and capacitive inputs and has any number of applications, including use as a digital hygrometer and as a fingerprint sensor.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.


REFERENCES:
patent: 4761769 (1988-08-01), Saito
patent: 4879681 (1989-11-01), Miwa et al.
patent: 5835420 (1998-11-01), Lee et al.
Carver Mead, “Analog VLSI and Neural Systems”, Addison-Wesley Publishing, Part III, Dynamic Functions, pp. 194-197, 1989.
Dan Strassberg, “Biometrics: You Are Your Password”,EDN, pp. 47-58, May 7, 1998.

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