Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1997-12-18
2001-05-08
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S540000, C326S062000, C326S080000
Reexamination Certificate
active
06229373
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a level shifter which is to be used in conjunction with a circuit that requires a reference voltage which is also used as a reference for the level shifter.
2. Discussion of the Related Art
FIG. 1
shows a conventional structure of a level shifter used in conjunction with, for example, a resistor-tree digital-to-analog converter (DAC)
10
. A resistor-tree DAC needs a precision reference voltage Vref of a predetermined value, for example 2 volts. This reference voltage is provided at low impedance, for example, as shown, by a band-gap reference source
12
followed by an operational amplifier
14
whose gain is set by two resistors
16
,
17
for obtaining the desired value of the reference voltage Vref.
A DAC is usually supplied with a relatively low voltage, for example 5 volts, and its output voltage Vi is usually amplified and level shifted to adapt it to a circuit supplied at a higher voltage, for example 12 volts.
A level shifter needs an accurate reference voltage for determining the amount by which its input voltage (Vi) should be shifted with respect to a generally inaccurate common-mode voltage Vcm.
The level shifter of
FIG. 1
is essentially a differential amplifier
20
including an operational amplifier
22
. The inverting input of operational amplifier
22
is connected to its output Vo through a resistor
24
and receives a reference voltage qVref through a resistor
25
. The non-inverting input of operational amplifier
22
receives the common-mode voltage Vcm through a resistor
27
and the voltage Vi to be shifted through a resistor
28
. By choosing a normalized value
1
for resistors
25
and
28
, and a normalized value k for resistors
24
and
27
, the gain of the differential amplifier
20
is set to k.
With this arrangement, the output voltage Vo of the level shifter is expressed by:
Vo=Vcm+k
(
Vi−qVref
).
The differential amplifier
20
of
FIG. 1
has relatively low impedance inputs, whereby its input voltages, qVref and Vi, should be provided by low impedance sources.
The input voltage Vi of the differential amplifier
20
is provided from the output voltage of the resistor-tree DAC
10
through a unity gain operational amplifier
30
.
In most cases, the reference voltage Vref needed by the resistor-tree DAC
10
is not adapted to the level shifter and must be either attenuated or amplified. In
FIG. 1
, reference voltage Vref is attenuated by a bridge of two resistors
32
,
33
connected to ground. The values of resistors
32
and
33
are chosen so that the midpoint of this bridge provides the desired reference voltage qVref for the level shifter, where q is the ratio between the reference voltage for the level shifter and reference voltage Vref. The normalized values of resistors
32
and
33
are respectively
1
-q and q. The midpoint of the resistor bridge being at relatively high impedance, its voltage is buffered by a unity gain operational amplifier
35
which provides the final reference voltage for the level shifter.
A drawback of the level shifter of
FIG. 1
is that precision in its reference voltage qVref is lost due to the inherent offset voltage of operational amplifier
35
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a level shifter for at least the above application, which is of particularly simple structure and overcomes the above drawback.
This and other objects is are achieved by a level shifter coupled to a circuit using an accurate reference voltage of a predetermined value, comprising a differential amplifier with a first input coupled to the reference voltage and a second input receiving an intermediate voltage; a low impedance source providing the reference voltage; and a first operational amplifier providing the intermediate voltage from an input voltage. The output of the low impedance source is directly connected to the first input of the differential amplifier and the gain of the first operational amplifier is set to obtain a desired ratio between the reference voltage and the intermediate voltage.
According to an embodiment of the invention, said circuit using the reference voltage is a resistor-tree digital-to-analog converter.
According to an embodiment of the invention, the reference voltage is provided from a band-gap source through an operational amplifier with suitable gain.
According to an embodiment of the invention, the differential amplifier comprises an operational amplifier and a resistor network for setting the gain of the differential amplifier at a desired value.
REFERENCES:
patent: 4498040 (1985-02-01), Tatsushi et al.
patent: 4870262 (1989-09-01), Hasegawa
patent: 5508604 (1996-04-01), Keeth
patent: 5631598 (1997-05-01), Miranda
patent: 5777503 (1998-07-01), Faulk
Callahan Timothy P.
Galanthay Theodore E.
Morris James H.
Nguyen Minh
SGS-Thomson Microelectronics Pte. Ltd.
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