Rail-to-rail DAC drive circuit

Details Rail-to-rail DAC drive circuit Rail-to-rail DAC drive circuit

1999-11-04

2003-04-22

JeanPierre, Peguy (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Nonlinear

C341S154000

Reissue Patent

active

RE038083

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital-to-analog converter (DAC) drive circuits, and particularly to DAC circuits that are connected to output operational amplifiers having a permissible input voltage range less than the circuit's rail-to-rail differential.
2. Description of the Related Art
DACs that are configured to operate in the voltage mode, in which an output analog voltage that corresponds to an input digital signal is produced, typically have their outputs buffered by an operational drive amplifier. This type of design is used, for example, in the PM-7224 8-bit CMOS DAC described in the Analog Devices, Inc. Data Converter Reference Manual, volume 1, 1992, pages 2-267 through 2-278.
The basic circuit is illustrated in
FIG. 1. A
voltage mode converter DAC
1
is shown with its output 2 connected to the non-inverting input of a buffer operational amplifier A
1
. The output
4
of A
1
is tied back to the amplifier's inverting input, thus providing a unity gain drive for a load
6
.
A typical circuit used to implement amplifier A
1
is shown in FIG.
2
. The circuit is supplied by positive and negative voltage supply lines V
dd
and V
ss
, respectively. The voltage supply levels are referred to as the circuit “rails”. With V
ss
tied to analog ground potential, the circuit can be operated with a single power supply V
dd
, typically 12-15 volts. The circuit could alternately be operated from dual power supplies, such as +5 and −5 volts for V
dd
and V
ss
, respectively.
The amplifier's output stage is shown as an NPN bipolar transistor Q
1
that provides a low-impedance, high-output current capability. The emitter of Q
1
is loaded with a current source I
1
, such as a 400 microamp NMOS current source referenced to V
ss
, while its collector is connected to V
dd
. Sinking the I
1
current into V
ss
allows the amplifier's output to go directly to ground.
An input stage consisting of
art

an
NMOS transistor Q
2
has its drain connected to V
dd
, with another current source I
2
sinking current from the source of Q
2
to V
ss
; the Q
2
source is also connected through a resistor R
1
to the base of Q
1
. Transistor Q
2
operates as a source follower, driving the resistor R
1
and output transistor Q
1
.
The converter DAC
1
by itself is a high impedance device; the operational amplifier A
1
provides a buffer function to drive the load. However, proper operation of A
1
generally requires that its input
8
from the DAC be 1 volt or more below the positive voltage supply level V
dd
. This means that the voltage swing at the output of the DAC must be limited to the permissible input voltage range for the amplifier, and consequently also limits the output range for the overall circuit to a similar level. The circuit is thus limited to an output range less than a desired “rail-to-rail” voltage swing.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved DAC drive circuit that is capable of operating in a voltage mode with a full rail-to-rail output range. This goal is accomplished by dividing the DAC output by a factor that places it within the permissible input range for the op amp, and providing the amplifier's output stage with a gain that allows the overall drive circuit to produce a rail-to-rail output if desired.
In a preferred embodiment the divider is implemented with an attenuation network, in the form of m dummy DAC bits, that is impedance matched to the DAC. The DAC, which is connected to receive an n-bit digital signal, thus has n+m bits. The dummy bits are connected as the DAC's m most significant bits, and are always OFF. This produces a downscaling of the input signal range by a factor ½
m
. The op amp is configured to produce a compensating 2
m
amplification, and includes a feedback circuit that is also impedance matched to the DAC. By setting m equal to 1, the DAC's output range is divided by 2, with the op amp doubling the DAC's output to restore a rail-to-rail output swing.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.


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