GB2144286A - Digital to analogue converters - Google Patents
Digital to analogue converters Download PDFInfo
- Publication number
- GB2144286A GB2144286A GB08320594A GB8320594A GB2144286A GB 2144286 A GB2144286 A GB 2144286A GB 08320594 A GB08320594 A GB 08320594A GB 8320594 A GB8320594 A GB 8320594A GB 2144286 A GB2144286 A GB 2144286A
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- ofthe
- converter
- amplifier
- output stage
- constituent part
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- 239000000470 constituent Substances 0.000 claims abstract description 47
- 239000004065 semiconductor Substances 0.000 claims description 6
- 230000008878 coupling Effects 0.000 description 3
- 238000010168 coupling process Methods 0.000 description 3
- 238000005859 coupling reaction Methods 0.000 description 3
- 238000010586 diagram Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/66—Digital/analogue converters
- H03M1/72—Sequential conversion in series-connected stages
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- Engineering & Computer Science (AREA)
- Theoretical Computer Science (AREA)
- Analogue/Digital Conversion (AREA)
Abstract
A digital-to-analogue converter has a reference voltage applied across a series of resistors. Each point (12) between the resistors is connected, individually, to an associated one of a plurality of identical constituent parts (T3, T4) of an output stage, there also being provided within the output stage an amplifier having as an input stage an appropriate constituent part, when driven selectively. In response to each digital input signal, switches of an arrangement of current switches (T1) selectively are actuated, a current is supplied thereby to drive said appropriate part of the output stage, causing the potential of the corresponding point of the resistor series, to cause a related analogue voltage at the converter output (26). The converter is fast acting, currents being operated upon within said parts of the output stage, and the arrangement of switches. Conveniently, in the output stage each said part comprises a differential pair of bipolar transistors (T3,T4). <IMAGE>
Description
SPECIFICATION
Digital to analogue converters
This invention relates to digital to analogue converters.
It is an object ofthe present invention to provide a digitalto analogue converterwhich is capable of a fast manner of operation.
According to the present invention a digital to analogue converter has a plurality of resistors in series, an arrangement of current switches coupled to points intermediate between each adjacent pair of resistors, and to a point at one extremity ofthe resistor series, and an output stage, the output stage includes a plurality of identical constituent parts, each said constituent part, when driven selectively, comprising the input stage of an amplifier of the output stage, the amplifier so formed being connected to an associated point ofthe resistor series, via the driven constituent part, each point of the resistor series being connected, individually, to a said constituent part, in the operation ofthe converter, a reference voltage is applied across the resistor series, and in response to each digital input signal supplied individuallytotheconverterthe current switches are actuated selectively, an appropriate said constituent partofthe output stage selectively is driven by a current from a constant current source common to the constituent parts, the current, which is supplied to the appropriate said constituent partvia instantaneously closed switches of the arrangement of current switches, causes the potential of the associated point of the resistor series, connected to said appropriate constituent part, to cause a related analogue output voltage to be provided from the outputofthe amplifier, also comprising the output of the converter, the analogue outputvoltage representing, in a predetermined way, the magnitude represented by the simultaneous digital input signal in accordance with a predetermined code.
Any such arrangementfor a digital to analogue converter is capable of a fast manner of operation, because currents are operated upon within said plurality of identical constituent parts ofthe output stage, and are switched ON and OFF within the arrangement of current switches.
Each said constituent partofthe output stage ofthe converter may include at least one bipolar transistor.
There may be bipolar transistor in the amplifier, other than in an input stage thereof, this bipolar transistor being common to said constituent parts of the output stage of the converter.
The bipolartransistor included within at leastthe output stage of the converter usually comprise NPN transistors. For convenience, in this specification and the accompanying claims, reference will be made only to NPN transistors, but it will be understood that such statements include references to PNP transitors, with appropriate corresponding modifications to the converterwhere required.
Usually the amplifier includes a resistive feedback loop,for example, when the amplifier includes a common bipolartransistor, other than in an input stagethereof,the arrangement being such that the resistive feedback loop is connected between two adjacent electrodes ofthe common bipolartransistor, comprising the base and collector electrodes, and the converter is connected to the emitter electrode, when a said appropriate constituent part ofthe output stage ofthe converter is selected, by being driven by current from the common constant current source, the potential in the resistive feedback loop ofthe amplifier, and the potential at the converter output, are related to the potential of the corresponding point of the resistor series connected to said appropriate constituent part, the otherwise open resistive feedback loop of the amplifier being closed by said appropriate, driven, constituent part.
Each said constituent part of the output stage ofthe converter may comprise, and when the amplifier of the output stage is provided with a resistive feedback loop, the amplifier comprising a unity gain amplifier, each said constituent part of the output stage of the converter is required to comprise a differential circuit configuration, for example, a differential pair of bipolar transistors, with their emitters connected together, and connected to the arrangement of current switches, one ofthe bipolar transistors, comprising said one bipolartransistorofthe constituent part of the output stage referred to above, has its base connected to the associated point of the resistor series, and the base and the collector ofthe other bipolartransistor are coupled to the remainder of the amplifier, for example, its collector is connected to the resistive feedback loop of the amplifier, when provided, and said other bipolartransistor comprises means coupling said one bipolartransistorto the remainder ofthe amplifier. When the amplifier includes a common bipolar transistor, other than in an input stage thereof, the common transistor comprises an emitterfollower. If a resistivefeedbackloop is provided in the amplifier, connected between the base and the collector of the common bipolartransistor, and each said constituent part ofthe output stage of the converter comprises a differential pair of bipolar transistors, for each differential pair of bipolartransistors, the collector of said other bipolartransistor is connected to the feedback loop at a point between the base ofthe common bipolartransistor of the amplifier, and the feedback resistor, and the base of said other bipolar transistor is connected to the emitter of the common bipolartransistor of the amplifier. Such an arrangement is advantageous because only a small resistance is required in the feedback loop, and hence the time constant of the feedback loop is small.
The current switches may have any convenient form, and inherently are capable of a fast manner of operation. Each current switch may comprise a bipolartransistor.
Each analogue output voltage from the converter may represent the magnitude represented by the simultaneous digital input signal in any convenient predetermined way. For example, the analogue out putvoltage may be directly proportionaltothe magnitude, when each digital input signal has n
constituent bits, there being 2", at least substantially,
identical resistors connected in series with each other, and across which resistor series the reference voltage is applied. However, it is not required that the plurality of at least substantially identical resistors are closely matched with each other in orderto obtain a linear relationship between each analogue output voltage and the corresponding magnitudes represented by the digital inputsignals.Ifthe resistors are not at least substantially identical there is a predetermined nonlinear relationship between each analogue output voltage and the corresponding magnitudes represented by the digital input signals.
The predetermined code in accordance with which each digital input signal represents a magnitude may have any convenient form, for example, comprising the binary code, an off-set binary code, orthe Grey code. A code converter may be provided between the inputs ofthe converterand the arrangement of current switches, in which case the arrangement of current switches is partially determined by the predetermined code associated with the digital input signals. Otherwise the arrangement of current switches is deter minedwholly by the predetermined code. Usually, when each digital input signal has n constituent bits, there is a hierarchical arrangement of n levels of current switches.
According to another aspect the present invention comprises a semiconductor device, having a digital to analogue converter of any one ofthe differentforms referred to above embodied within a monolithic semiconductor body, in any convenient way.
When the amplifierincludes a common bipolar transistor, with a resistive feedback loop connected between the base and collector thereof, for such a semiconductor device the speed of operation ofthe amplifier, comprising the only part of the converter to operate upon voltage, is determined by the product of the effective stray capacitance associated with the common bipolartransistor ofthe amplifier, and the resistance of the resistor ofthe feedback loop of the amplifier.Whilstthe effective stray capacitance may be large, including also any stray capacitance associated with the means coupling said one bipolar transistors of the constituent parts of the output stage to the remainder ofthe amplifier, as stated above the resistance ofthe resistor ofthe feedback loop may be small, so thatthe speed of operation of the amplifier may be fast.
The present invention will now be described by way of example with reference to the accompanying drawing, comprising a circuit diagram of one embodimentor a digital to analogue converterin accordance with the present invention.
The illustrated digital to analogue converter, to receive 4bit digital input signals in accordance with the binary code, has a series of sixteen at least substantially identical resistor R, with a reference voltage V applied across the resistor series. Conveniently, the lower potential end 10 ofthe resistor series is indicated as being at zero potential, and the other end 11 of the resistor series is indicated as being at the potential V. Typically each constituent resistor R is50 ohms.
Points 12 between each adjacent pair of resistors R ofthe series, and at the lower potential end 10 of the resistor series, are coupled to a hierarchical arrange mentofcurrentswitchesTi.
The hierarchical arrangement has 4 levels. Each current switch T1 comprises an NPN bipolartransistor. Each level ofthe hierarchical arrangement is split into two halves, a lead 14from the bases of each transistorT1 of an associated half is connected individually to an input 15 ofthe converter. Thus, there are two leads 14 associated with each level ofthe hierarchical arrangement. The emitters of one transistorT1 of both halves of a level, lowerthanthe first level, are connected to the collectorofa common associated transistor of a higher level. The first level of the hierarchical arrangement hastwotransistorTi, and the lowest level has two halves each with eight transistorT1 .The emitters of the two transistrsof the first level are connected to a common constant current source, indicated generally at 16, and the collectors of the lowest level of the hierarchical arrangementare coupled to the series of resistors R. At any instant, if the potential on a lead 14 is high, the transistors T1 the bases of which are connected thereto are switched
ON, considered as closing the associated current switches; and if the potential on a lead 14 is low, the transistorsT1 the bases ofwhich are connected thereto are switched OFF, considered as opening the associated current switches. In response to each binary digital inputsignaltotheconverterthe potential on one ofthe two leads 14 connected to a level ofthe hierarchical arrangement is high, and the potential on the other lead 14 is low.Thus, when a constituent binary bit of thedigital inputsignal to one pair of inputs 15 is "0", the potential on the associated lead 14connectedto one half ofthe bipolartransistors Ti ofthe associated level ofthe hierarchical arrangement is high, and the potential on the other lead is low; and when the constituent bit is "1"the potentials on the two leads are reversed.
The common constant current source 16 includes an
NPN bipolar transistorT2, with its collector connected to the emitters ofthe two current switches T1 of the first level ofthe hierarchical arrangement. The emitter ofthe bipolartransistor T2 is connected to a rail 17 maintained at a potential of -5 volts. A diode D is provided between the rail 17 and'the base ofthe transistorT2, and the base is also connected, viaa resistor Ri,to a point 19 maintained atzero potential.
Between the hierarchical arrangement of current switches and the resistor series is provided an output stage of the converter. The output stage includes sixteen identical parts, indicated generally at20. Each part 20 is connected both, individually, to the points 12 ofthe resistor series, and individually to the collectors ofthe bipola r transistors T1 of the lowest level ofthe hierarchical arrangement of current switches. A part 20 is connected to each of the points 12 of the resistor series, and to each of the bipolar transistors T1 of the lowest level of the hierarchical arrangement of current switches.
Each part20comprisesa pairofNPN bipolar transistorsT3 and T4, in a differential circuit configuration, with their emitters connected together. The commoned emitters of each part are connected individually to the collector of an associated current switch T1 ofthe lowest level ofthe hierarchical arrangement. The base of each bipolartransistorT3 is connected individually to a point 12 ofthe resistor series.
In the operation of the digital to analogue converter, each binary digital input signal is supplied individually to the inputs 15, causing a selected current switch or current switches T1 of each level ofthe hierarchical arrangementto be closed. Thus, a current from the common constant current source 16 is supplied to an approriate selected, part 20, and, in particular, to the commoned emitters of the differential pairs of bipolar transistors T3 and T4 ofthe selected part. This current drives the bipolartransistors T3 and T4 of the part. The output stage ofthe converter also includes an amplifier 22, having as an input stage the appropriate constituent part 20 when driven selectively, by being connected to the common constant current source 16.
The amplifier 22 includes a NPN bipolartransistorT5, common to each part 20 ofthe output stage ofthe converter. The amplifier 22 also has a resistive feedback loop connected between its base and collector, and the amplifier has unity gain. The resistor in the feedback loop is indicated at R2, and typically is 100 ohms. A point 24 between the resistor R2 and the collector ofthe common bipolartransistorT5 is connected to a source providing an output of +5 volts. The output 26 ofthe amplifier22, also comprising the output ofthe converter, is connected to the emitter of the common bipolartransitorT5; which emitter is also connected to the base of the bipolar transistorT4 of each part 20. The base of the common bipolartransistorT5 of the amplifier 22 is also connected to the collector of the bipolartransistorT4 of each part20.The collector of the common bipolar transistorT5 ofthe amplifier 22 is also connected to the collector of the bipolartransistorT3 of each part 20. Thus, the common bipolartransistorT5 of the amplifier 22 comprises an emitter follower. The bipolartransistorT4 of each part20 comprises means coupling the associated bipolartransistorT3 ofthe part 20 to the remainder ofthe amplifier 22.
Further, in relation to the operation of the digital to analogue converter, each binary digital input signal, causing the appropriate part 20 of the output stage to be selected, by being driven by the common constant current source 16, causes the resistive feedback loop of the amplifier, which otherwise has been open, to close,to includethe selected differential pairof bipolartransistors T3 and T4. This causesthe potential ofthe base ofthe bipolartransistorT3, connected to the associated point 12 of the series of resistors R, to cause a related rise in the analogue voltage in the resistive feedback loop connected to the base ofthe common bipolartransistorT5, and in the analogue voltage in the emitter circuit of the common bipolar transistorT5, connnected to the output26 ofthe converter.Different parts 20 when so selected cause different rises in the analogue voltage, at least, atthe converter output 26, because ofthe different potentials ofthe different points 12 of the resistor series. The arrangement is such thatthe analogue converter output voltage corresponds to the number of resistors oftheseries between the appropriate associated point 12 and the lower potential end 10 ofthe series, and corresponds to the magnitude represented by the simultaneous digital input signal.
Theonlyvoltageswhich are operated upon within the converterarethose associated with the amplifier 22, the remainder of the converter operating upon currents. Hence, inherently, each part 20 of the output stage is fast in operation, as also is the hierarchical arrangement of current switches. The amplifier, conveniently, can be arranged to have a fast manner of operation. Thus, the digital to analogue converter is fast in operation.
Further, the constituent, identical resistors R, of the series, do not have to be closely matched with each other.
The converter may be embodied within a monolithic semiconductor body, in any convenient way. For such a semiconductor device, the speed of operation ofthe amplifier, comprising the only part ofthe converterto operate upon voltages, is determined by the time constant associated with the resistive feedback loop of the amplifier, comprising the product of the effective stray capacitance associated with the common bipolar transistorT5 ofthe amplifier22, andthe resistance of the resistor R2 of the feedback loop of the amplifier.
Whilst the effective stray capacitance may be large, including stray capacitances associated with the bipolartransistorsT4ofthe parts 20 ofthe output stage connected thereto, the resistance ofthe resistor R2 is small. Thus, the speed of operation of the amplifier 22 is fast.
The digital to analogue converter described above may be modified in various different ways.
The current switches ofthe hierarchical arrangement; and/orthe different circuit configurations of the parts 20 ofthe output stage; may comprise MOS field effecttransistors.
A point 12, connected to a constituent part 20 ofthe output stage, may be provided at the high potential end ofthe series of resistors, instead of at the low potential end.
Theamplifierofthe outputstage may have any convenientform, and may not be provided with a resistive feedback loop.
Each ofthe constituent parts 20 ofthe output stage ofthe converter, and comprising input stages forthe amplifier, also may have any convenient form.
However, ifthe amplifier is provided with a resistive feedback loop it is essential that the constituent parts 20 ofthe output stage each comprise a differential circuit configuration. In any arrangement, each part 20 may include at least one bipolartransistorcapable of being coupled to the remainder of the amplifier.
Each analogue outputvoltage from the converter may represent the magnitude represented by the simultaneous digital input signal in any convenient predetermined way. Instead of the analogue output voltage being directly proportional to the magnitude, there may be a predetermined non-linear relationship therebetween, the resistors connected in series with each other not being at least substantially identical.
The predetermined code in accordance with which each digital input signal represents a magnitude may have any convenient form, and instead of comprising the binary code, may comprise, for example, an offset
binary code, or the Grey code. A code converter may
be provided between the inputs ofthe converter and the arrangement of current switches, in which case the arrangement of current switches is partially determined by the predetermined code associated with the digital inputsignals. Otherwisethe arrangement of the current switches is wholly determined by the predetermined code, as described above with reference to the accompanying drawing.
Claims (12)
1. A digital to analogue converter having a plurality of resistors connected in series, an arrangement of current switches coupledto points intermediate between each adjacent pair of resistors, and to a point at one extremity of the resistor series, and an output stage, the output stage including a plurality of identical constituent parts, each said constituent part, when driven selectively, comprising the input stage of an amplifierofthe output stage, the amplifier so formed being connected to an associated point ofthe resistor series, via the driven constituent part, each point ofthe resistor series being connected, indi vidually,to a said constituent part, in the operation of the converter a reference voltage being applied across the resistor series, and in response to each digital input signal supplied individuallytotheconverterthe current switches being actuated selectively, an appropriate said constituent part of the output stage selectively being driven buy a currentfrom a constant current source common to the constituent parts, the current, which is supplied to the appropriate said constituent part via instantaneously closed switches ofthe arrangementof currentswitches, causing the potential ofthe associated point of the resistor series, connected to said appropriate constituent part, to cause a related analgoue output voltage to be provided from the output ofthe amplifier, also comprising the output ofthe converter, the analogue outputvoltage representing, in a predetermined way, the magnitude represented by the simultaneous digital input signal in accordance with a predetermined code.
2. Aconverterasclaimed in claim 1 in which each said constituent part ofthe output stage ofthe converter includes at least one bipolartransistor.
3. A converter as claimed in claim 1 orclaim2in which there is a common bipolartransistor in the amplifier,otherthan in an input stage thereof.
4. Aconverter as claimed in claim 1, or claim 2, or claim 3, in which the amplifier includes a resistive feedback loop.
5. Aconverterascaimed in claim 4, and inwhich there is a common bipolartransistor in the amplifier, otherthan in an inputstagethereof,the arrangement being such that the resistive feedback loop is connected between two adjacent electrodes ofthe common bipolartransistor, comprising the base and collector electrodes, and the converter output is connected to the emitter electrode, when a said appropriate constituent part of the output stage ofthe converter is selected, by being driven by currentfrom the common ccnstant current source, the potential in the resistive feedback loop of the amplifier, and the potential at the converter output, are related to the
potential of the corresponding point of the resistor series connected to said appropriate constituent part, the otherwise open resistive feedback loop ofthe amplifier being closed by said appropriate, driven, constituent part.
6. Aconverteras claimed in anyoneofthe preceding claims in which each said constituent part ofthe output stage of the converter comprises a differential circuit configuration.
7. A converter as claimed in claim 6 in which each said constituent part of the output stage comprises a differential pair of bipolar transistors, with their emitters connected together, and connected to the arrangement of current switches, one ofthe bipolar transistors, comprising said one bipolartransistorof the constituent part of the output stage, has its base connected to the associated point of the resistor series, and the base and the collector of the other bipolartransistor are coupled to the remainder ofthe amplifier.
8. A converter as claimed in claim 7, and in which the amplifier includes a common bipolar transistor, otherthan in an input stage thereof, and has a resistive feedback loop connected between two adjacent electrodes of the bipolartransistor, and the converter output is connected to the third electrode, for each differential pair of bipolartransistors, the collector of said other bipolar transistor is connected to the resistive feedback loop at a point between the base of the common bipolartransistor ofthe amplifier, and the feedback resistor, and the base of said other bipolartransistor is connected to the emitterofthe common bipolar transistor of the amplifier, connected to the converter output.
9. A converter as claimed in any one of the preceding claims in which the arrangementof current switches is determined wholly by the predetermined code.
10. A converter as claimed in claim 9 in which the arrangement is such that each digital inputsignal has n constituent bits, there being a hierarchical arranged of n levels of current switches.
11. Asemiconductordevice having a digital to analogue converter as claimed in anyone of the preceding claims embodied within a monolithic semiconductor body.
12. An analogue to digital converter substantially as described herein with reference to the accompanying drawing.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08320594A GB2144286B (en) | 1983-07-30 | 1983-07-30 | Digital to analogue converters |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08320594A GB2144286B (en) | 1983-07-30 | 1983-07-30 | Digital to analogue converters |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8320594D0 GB8320594D0 (en) | 1983-09-01 |
| GB2144286A true GB2144286A (en) | 1985-02-27 |
| GB2144286B GB2144286B (en) | 1986-10-08 |
Family
ID=10546548
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08320594A Expired GB2144286B (en) | 1983-07-30 | 1983-07-30 | Digital to analogue converters |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2144286B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4633104A (en) * | 1983-09-15 | 1986-12-30 | Ferranti Plc | Bipolar transistor logic circuits |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1427769A (en) * | 1972-12-18 | 1976-03-10 | Cit Alcatel | Decoder |
-
1983
- 1983-07-30 GB GB08320594A patent/GB2144286B/en not_active Expired
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1427769A (en) * | 1972-12-18 | 1976-03-10 | Cit Alcatel | Decoder |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4633104A (en) * | 1983-09-15 | 1986-12-30 | Ferranti Plc | Bipolar transistor logic circuits |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2144286B (en) | 1986-10-08 |
| GB8320594D0 (en) | 1983-09-01 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19950730 |