Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
1999-05-05
2001-07-24
Pierre, Peguy Jean (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C327S207000
Reexamination Certificate
active
06266001
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to digital to analog converters. More particularly, the invention relates to voltage level translation for controlling switches within digital to analog converters.
BACKGROUND OF THE INVENTION
The functional operation of a digital to analog converter (DAC) is well known. Generally, a DAC accepts a digital input signal and converts it into an analog output signal. The digital input signal has a range of digital codes which are converted into a continuous range of analog signal levels of the analog output signal. DACs are useful to interface digital systems to analog systems. Applications of DACs include video or graphic display drivers, audio systems, digital signal processing, function generators, digital attenuators, precision instruments and data acquisition systems including automated test equipment.
There are a variety of DACs available for converting digital input signals into analog output signals depending upon the desired conversion functionality. The variations in the DACs available may have different predetermined resolutions of a digital input signal, receive different encoded digital input signals, have different ranges of analog output signals using a fixed reference or a multiplied reference, and provide different types of analog output signals. Additionally there are a number of DAC performance factors to consider such as settling time, full scale transition time, accuracy or linearity, and a factor previously mentioned, resolution.
The digital input signal is a number of bits wide that defines the resolution, the number of output levels or quantization levels, and the total number of digital codes that are acceptable. If the digital input signal is m-bits wide, there are 2
m
output levels and 2
m−1
steps between levels. The digital input signals may be encoded in straight binary, two's complement, offset binary, grey scale code, binary coded decimal or other digital coding. The range of analog output signal values usually depend upon an analog reference. The analog reference may be internally generated but is usually externally provided for precision. The analog output signal range may be proportional to the digital input signal over a fixed analog reference level as in a fixed reference DAC. Alternatively, the analog output signal may be the product of a varying input analog reference level and the digital code of the digital input signal as in multiplying DACs. The analog output signal may be unipolar ranging in either positive values or negative values or it may be bipolar ranging between both positive and negative output values. The analog output signal may be an analog voltage signal or an analog current signal.
Additionally, the type of electronic circuitry used to form a DAC varies as well. Bipolar junction transistor (BJT) technology, metal oxide semiconductor (MOS) technology or a combination thereof are used to construct DACs. BJT technology may be PNP technology with PNP transistors or NPN with NPN transistors or both, while MOS technology may be PMOS with P-channel field effect transistors (PFET), NMOS with N-channel field effect transistors (PFET) or CMOS technology having both PFETs and NFETs. Generally, BJT technology is preferable for constructing precision amplifiers because it has higher transconductance than CMOS technology. This results in lower offset for differential amplifiers. CMOS technology is preferable for constructing switches because it has nearly zero offset when used as a switch. This is so because PFETs and NFETs have virtually no gate current as compared to base current for PNP and NPN transistors of BJT technology. Integrated circuits or wafer fab manufacturing processes combining both BJT and CMOS technology, referred to as BICMOS circuits or processes, are used to provide BJT technology for amplifiers and CMOS technology for switches on the same integrated circuit.
Referring now to
FIG. 1
, a block diagram of a DAC
100
has a digital input signal DIN
101
, a positive analog supply voltage level, AVref+
104
, and a negative analog supply voltage level, AVref−
105
, in order to generate an analog voltage output signal, AVout
110
. Alternatively DAC
100
can generate an analog current output signal with minor changes to its circuit configuration. To generate either type of analog output, the DAC
100
includes digital and analog power supplies. The digital power supply provided to DAC
100
is input across the positive digital supply terminal, VCC
102
, and the negative digital supply terminal, DGND
103
. The positive analog power supply provided to DAC
100
is input across the positive analog supply terminal, VDD
106
, and the analog ground terminal, AGND
107
. The negative analog power supply is input across the negative analog supply terminal, VSS
108
, and the analog ground terminal, AGND
107
.
For simplicity in discussion consider DAC
100
to be a fixed reference DAC such that the output voltage range of AVout
110
is a function of DIN
101
and the range of voltage is defined by the predetermined voltage levels of AVref+
104
and AVref−
105
. DIN
101
is m bits wide. The predetermined value of m represents the range of decimal numbers that DIN
101
will represent. The selected circuitry for DAC
100
varies depending upon a number of factors including power supply inputs and desired parameters of input and output signals. As illustrated in
FIG. 1
, DAC
100
includes a signal converter
112
and an amplifier or buffer
114
. Some forms of DACs, specifically current output DACs, may not include the buffer
114
and require external amplification. Signal converter
112
converts DIN
101
into a form of analog signal VLADR
120
which is input to buffer
114
. Buffer
114
buffers the analog signal VLADR
120
generated by the signal converter
112
from a load that may be coupled to AVout
110
. The signal converter
112
includes a switched R-2R ladder
116
and a switch controller
118
. Switch controller
118
controls switches within the switched R-2R ladder
116
to cause it to convert the value of DIN
101
into an analog signal.
As previously discussed, there are a number of DAC performance factors to consider including a DAC's accuracy or linearity. In measuring linearity of a DAC, the analog output AVout
110
from a DAC may be bipolar output voltages, positive unipolar output voltages or negative unipolar output voltages over the range of the digital input signal DIN
101
. If a change in an analog voltage reference level is required to establish a zero point or a midpoint of the conversion range, it is referred to as an offset voltage. Differential linearity is the linearity between code transitions measuring the monotonicity of a DAC. If increasing code values of DIN results in increasing values of AVout, the DAC is monotonic, and if not, the DAC has a conversion error and is not monotonic. The linearity of a DAC is very important for accurate conversions and is usually specified in units of least significant bits (LSB) of DIN
101
. Linearity of a DAC can vary over temperature, voltages, and from circuit to circuit. Additionally, DAC linearity becomes more important as the predetermined DAC resolution is increased, where the value of m is larger, and additional digital codes are desired to be converted. Furthermore, as the analog voltage reference level range between AVref+
104
and AVref−
105
may be increased to accommodate additional resolution, it is desirable to maintain linearity in a DAC.
Referring now to
FIG. 2
, a prior art switched R-2R ladder
116
is illustrated. The switched R-2R ladder
116
is a 4 bit inverted R-2R ladder to provide an analog voltage output signal but may be easily expanded to m-bits with the addition of other intermediate R-2R switch legs and additional switch control lines. Alternatively, a non-inverted R-2R ladder could be used to provide an analog current output signal. Signals DBn/DBp
201
, the switch control lines, are selectively controlled
Castaneda David
Fang Gary G.
Rahim Chowdhury F.
Blakely , Sokoloff, Taylor & Zafman LLP
Jean Pierre Peguy
Maxim Integrated Products Inc.
LandOfFree
Method and apparatus for switching low voltage CMOS switches... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for switching low voltage CMOS switches..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for switching low voltage CMOS switches... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2558739