Calibrated line driver with digital-to-analog converter

Details Calibrated line driver with digital-to-analog converter Calibrated line driver with digital-to-analog converter

1999-11-22

2001-11-06

Wamsley, Patrick (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Digital to analog conversion

C375S254000

Reexamination Certificate

active

06313776

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to line drivers and, more particularly, to a calibrated line driver.
2. Description of the Related Art
A line driver is a device that drives a signal onto a transmission line, such as a local-area-network or telephone line. Line drivers are typically associated with transmit protocols that define the characteristics of the signal that is driven onto the line.
FIG. 1
shows a schematic diagram that illustrates a conventional line driver
100
. As shown in
FIG. 1
, driver
100
includes a transmit circuit
110
which has a pair of differential outputs OUT
1
+ and OUT
1
−, and a transformer
112
which has a pair of inputs IN+ and IN− that are connected to the outputs OUT
1
+ and OUT
1
−. In addition, transformer
112
also has a pair of outputs OUT
2
+ and OUT
2
− that are connected to a transmission line
114
, such as a CAT-
5
coaxial cable.
Transmit circuit
110
can be implemented as a current-based circuit or as a voltage-based circuit. A current-based circuit can be implemented in a variety of ways, but typically includes a number of resistors, a number of current sources, and a number of switches.
FIG. 2A
shows a schematic diagram that illustrates a first example of a current-based transmit circuit
110
. As shown in
FIG. 2A
, circuit
110
includes a resistor R which is formed across the inputs IN+ and IN− of transformer
112
, a first switch S
1
which is connected between a power supply voltage Vcc and the input IN+, and a first current source
210
which is connected between the input IN− and ground.
In addition, circuit
110
also includes a second switch S
2
which is connected between the power supply voltage Vcc and the input IN−, and a second current source
212
which is connected between the input IN+ and ground. Further, circuit
110
also includes a control circuit
214
that controls the operation of switches S
1
and S
2
.
In operation, when switch S
1
is closed and switch S
2
is open, current source
210
pulls a current I
p
, through resistor R which sets up a positive output voltage V
OD1
across the inputs IN+ and IN− of transformer
112
. On the other hand, when switch S
1
is open and switch S
2
is closed, current source
212
pulls a current I
N
through resistor R which sets up a negative output voltage V
OD2
across the inputs IN+ and IN− of transformer
112
. As shown, the negative output voltage V
OD2
has a polarity opposite to the polarity of voltage V
OD1
.
In addition, when switches S
1
and S
2
are both open, a voltage intermediate to the output voltages V
OD1
and V
OD2
is placed across the inputs IN+ and IN− of transformer
112
. (An intermediate voltage is required by some transmission protocols, such as MLT3.)
FIG. 2B
shows a schematic diagram that illustrates a second example of a current-based transmit circuit
110
. Circuit
110
shown in
FIG. 2B
is similar to circuit
110
shown in
FIG. 2A and
, as a result, utilizes the same reference numbers to designate the structures which are common to both figures.
Circuit
110
of
FIG. 2B
differs from circuit
110
of
FIG. 2A
in that a third switch S
3
and a multiplying digital-to-analog converter (DAC)
216
are used in lieu of current sources
210
and
212
. Third switch S
3
, which is controlled by control circuit
214
, has first and second positions P
1
and P
2
. DAC
216
, in turn, receives a bandgap current I
BG
from a bandgap current source
218
, an n-bit control word CW, and sinks a DAC current I
DAC
which is defined by the bandgap current I
BG
and the control word CW.
Conventionally, switches S
1
, S
2
, and S
3
, DAC
216
, and current source
218
are formed as part of a transmit integrated circuit, while resistor R is externally connected to the transmit integrated circuit. Control circuit
214
, in turn, can be part of the transmit integrated circuit, or part of another integrated circuit that outputs control signals to the transmit integrated circuit.
In operation, when switch S
1
is closed, switch S
2
is open, and switch S
3
is in position P
1
, DAC
216
pulls DAC current I
DAC
through resistor R which sets up the voltage V
OD1
across the inputs IN+ and IN− of transformer
112
.
On the other hand, when switch S
1
is open, switch S
2
is closed, and switch S
3
is in position P
2
, DAC
216
pulls DAC current I
DAC
through resistor R which sets up the voltage V
OD2
across the inputs IN+ and IN− of transformer
112
.
As above, voltage V
OD2
has a polarity which is opposite to the polarity of voltage V
OD1
. In addition, when switches S
1
and S
2
are both open, a voltage intermediate to the output voltages V
OD1
and V
OD2
is placed across the inputs IN+ and IN− of transformer
112
.
FIG. 3A
shows a schematic diagram that illustrates a third example of a current-based transmit circuit
110
. As shown in
FIG. 3A
, circuit
110
includes a first resistor R
1
which is connected between the input IN− and a power supply voltage Vcc, and a second resistor R
2
which is connected between the input IN+ and the power supply voltage Vcc.
As further shown in
FIG. 3A
, circuit
110
includes a first switch S
1
which is connected in parallel with resistor R
2
, and a second switch S
2
which is connected in parallel with resistor R
1
. In addition, circuit
110
further includes a first current source
310
connected between the input IN− and ground, and a second current source
312
connected between the input IN+ and ground. Further, circuit
110
also includes a control circuit
314
that controls the operation of switches S
1
and S
2
.
In operation, when switch S
1
is closed and switch S
2
is open, the power supply voltage Vcc is shorted to the input IN+, while current source
310
pulls a current I
p
through resistor R
1
which sets up a voltage VP on the input IN− which is less than the power supply voltage Vcc−VP is a result, a voltage V
OD1
equal to Vcc−VP is dropped across the inputs IN+ and IN− of transformer
112
.
On the other hand, when switch S
1
is open and switch S
2
is closed, the power supply voltage Vcc is shorted to the input IN−, while current source
312
pulls a current I
N
through resistor R
2
which sets up a voltage VN on the input IN+ which is less than the power supply voltage Vcc.
As a result, a voltage V
OD2
equal to Vcc−VN is dropped across the inputs IN+ and IN− of transformer
112
. In addition, when switches S
1
and S
2
are both open, a voltage intermediate to the output voltages V
OD1
and V
OD2
is placed across the inputs IN+ and IN− of transformer
112
.
FIG. 3B
shows a schematic diagram that illustrates a fourth example of a current-based transmit circuit
110
. Circuit
110
shown in
FIG. 3B
is similar to circuit
110
shown in
FIG. 3A and
, as a result, utilizes the same reference numbers to designate the structures which are common to both figures.
Circuit
110
of
FIG. 3B
differs from circuit
110
of
FIG. 3A
in that a third switch S
3
which has first and second positions P
1
and P
2
, and a multiplying DAC
316
are used in lieu of current sources
310
and
312
. DAC
316
receives a bandgap current I
BG
from a bandgap current source
318
, an n-bit control word CW, and sinks a current I
DAC
which is defined by the bandgap current I
BG
and the control word CW.
Conventionally, switches S
1
, S
2
, and S
3
, DAC
316
, and current source
318
are formed as part of a transmit integrated circuit, while resistors R
1
and R
2
are externally connected to the transmit integrated circuit. Control circuit
314
, in turn, can be part of the transmit integrated circuit, or part of another integrated circuit that outputs control signals to the transmit integrated circuit.
In operation, when switch S
1
is closed, switch S
2
is open, and switch S
3
is in position P
1
, the pow

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