JPS61196622A - Digital-analog converter - Google Patents

Abstract

PURPOSE:To enhance the signal accuracy in the linking part, to improve a high accuracy, to decrease the number of bits at the electric current dividing system side and to lower the electric power source voltage by providing an electric current correcting means at the linking part in the DA converter to assemble the electric current dividing system and an R-2R RADA system. CONSTITUTION:An R-2R RADA system digital analog converting part 2 which goes to be the converting part of lower order side bits D0-D4, and an electric current dividing system digital analog converting part 1 which goes to be the electric current source of the converting part 2 and the converting part of higher order side bits D5-D8 are linked by a connection P. To the linking part, an electric current source ISE for correction, an electric current dividing device 18 and a transistor TrA for correction are added as an electric current corresponding means, and out of electric currents to flow from an R-2R RADA system DA converting part 2 to an electric current dividing system DA converting part 1, an electric current I0 of the value equal to the sum of the base electric current of respective transistors of the converting part 2 is pulled out as a correcting electric current. Thus, the electric current accuracy of the connecting point is enhanced. Namely, the electric current to flow to the electric current dividing system DA converting part, goes to be the value very close to the output electric current value of all bits of the R-2R RADA system DA converter.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a digital-to-analog converter, and more particularly to a digital-to-analog converter using a combination of a current division method and an R-2R ladder resistance method. .

[Conventional technology]

Digital-to-analog converters (hereinafter referred to as DA converters) have been developed into LSIs in recent years, making it easier to incorporate them into electronic devices such as digital audio devices. In this case, as a general R-A converter, R-2f
The t-ladder resistance type is well known, but with this type, the error in the upper bits with a large load greatly affects the overall error, so as the number of bits increases, it is necessary to trim the resistor. It has the disadvantage of being expensive. On the other hand, as an A conversion method that requires a highly accurate resistor, there is a current division method in which multiple stages of current dividers are stacked to obtain an output signal for each bit. Current dividers used in this current division method include those that use a current mirror circuit to obtain a bit current of a predetermined ratio, and those that use a switching circuit to sequentially switch multi-t currents W to obtain an averaged output current. (direct element matching method, etc.). In this case, the current division type ODA converter has the drawback that as the number of bits increases, the number of stacked current dividers increases, resulting in an increase in the power supply voltage.

Therefore, a DA converter has been developed in which the upper bits are configured using a current division method and the lower bits are configured using an R-2R ladder resistance type, thereby compensating for the drawbacks of both. and,
In this combination type ODA converter, it is essential to improve the signal precision at the joint portion of the current division method and the R-2R ladder method in order to promote high precision.

[Problem that the invention seeks to solve]

However, the conventional combination-type DAi converter has a drawback in that the signal accuracy of the bit combining portion is poor, resulting in poor accuracy. In addition, if the signal accuracy at the coupling point is poor, the number of focuses on the R-2R ladder resistance method side cannot be increased, and as a result, the number of bits on the current division method side increases and the power supply voltage increases. This caused an inconvenience.

This invention was made in view of the above-mentioned circumstances, and in a DA converter that combines a current division method and an R-2R ladder method, it is possible to improve the signal accuracy at the coupling part and achieve high precision. Another object of the present invention is to provide a DA converter that can lower the power supply voltage by reducing the number of bits on the current division side.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention includes an R-2R ladder resistance group, a current source transistor provided for each bit of the R-2R ladder resistance group, and a bit switch transistor. An R-2R ladder type digital-to-analog conversion unit serves as a conversion unit for the side bits, and a current division type digital-to-analog conversion unit serves as a current source for the R-2R ladder type digital-to-analog conversion unit and serves as a conversion unit for the upper bits. an analog converter and the R-2R
Current correction means extracts a current equal to the sum of the base currents of the respective transistors as a correction current from among the currents flowing from the ladder card digital-to-analog conversion section to the current division-type digital-to-analog conversion section. .

[Effect]

The base current of each transistor of the R-2R ladder type DAi switching section is extracted by the current correction means, thereby increasing the current accuracy at the junction point. That is, the current flowing into the current division type DA converter has a value extremely close to the output current value of all bits of the R-2R ladder type DA converter.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of a first embodiment of the present invention. In the figure, reference numeral 1 denotes a current division type DA converter, which is connected to an R-2R ladder type DA converter 2 by a reed and a connection pK. In this case, the current division type DA converter 1 has D6 to D8 bits (25 to 28 bits), that is,
It constitutes the upper side bint i++, and R-2
The R ladder type DA converter 2 has Do to D4 (20 to 24
In other words, it constitutes a conversion unit for the lower bits.

Next, SW1 to SW4 are each a focus switch,
When the bit signal supplied to the control input terminal is 11#,
When terminals a and c are in a conductive state and the bit signal is applied, terminals a and c are in a conductive state. 3 and 4 are current dividers, respectively, and the current flowing into the three input terminals is
Divide into -27=7. This current divider is, for example, a current mirror circuit configured to obtain a predetermined current ratio, or a plurality of current sources (with the same current value) are cyclically switched by switching to average the current value. It is constituted by a circuit (dynamic element matching method) that combines these averaged currents to obtain a predetermined current ratio.

In the above configuration, the current value of the top pin) Ds is set to IM8
If B, the lower bit is D? , D6°D6 bit output current values are IM8B/2°IM8B/4 . I
Msn/g and zru.

Next, SW5 to SW9 are bit switches each consisting of a pair of transistors, and when the signal supplied to the D4 to Do bits is '1'', the transistor on the side whose collector is connected to the current output Ml2 is Turn on, D4~D
When the signal supplied to the o bit is '0#, the transistor whose collector is grounded is turned on. T
r5~'l' rlO are R-2R ladder resistance group 1, respectively.
The emitters of the transistors are connected to each of the three bits, and their bases are commonly connected to the bias terminal 14. Further, the collectors of the transistors Tr5 to Tr9 are connected to bit switches SW5 to SW5.
It is connected to the common emitter of the transistor pair constituting SW9, and the collector of the transistor TrlO is grounded.

The above-mentioned configuration is exactly the same as that of a conventional combination type DA converter], and in this embodiment, a correction current source 15E1 current divider 18 and a correction transistor TrA are added to the above configuration. The structure is as follows. Next, these components will be explained.

First, the correction current source SE is composed of a transistor Q8 and an emitter resistor Re, and a constant voltage Vref is applied to the base of the transistor Qe. In this case, the voltage VrefO value is set so that the value of the collector current of the transistor Qe becomes ■MsB/2.

Next, the current divider 18 divides the current flowing into the correction current source ISg, and if the value of the output current of the D5 bit (that is, the current flowing through the connection p) is In, one input end current is 2In, other input terminal current Y (IM8B
/2)-2In. Transistor 'l'r-A
has an emitter connected to one input end of the current divider 18, a base connected to the connection p, and a collector connected to the bias terminal 20. This correction transistor Tr
-A is the correction 11th stream I from the connection point P1, as will be described later.
The hfe has the same function as each transistor constituting the R-2R ladder type DAfi converter 2.

4. Next, the precision of the current at the connection point between the current division type DA converter 1 and the R-2R ladder type DA converter 2 in the above-described circuit will be explained.

First, in order to compare the current accuracy in this embodiment with the conventional current accuracy, we will explain the case where there is no correction current □ source ISK current divider 8 and correction transistor Tr-A, that is, when the correction current 0 is If not, explain.

(When there is no correction current Io) First, set the number of bits of the current division type DA converter 1 to n, 1R.
-If the number of bits of the -2R ladder type DA converter 2 is ml and the output current of the most significant bit is ml, it is known that in a combination type DA converter, the following relational expression must be satisfied. .

Here, assuming that the current amplification factors hfe of each of the transistors Tr5 to Trto are all equal, the current value of the R-2R ladder resistor group 13 becomes //2 every time it moves to the lower 7 bits, so the transistor Tr5 *'rrs, Tr
The emitter currents of the transistors T r 5 , T r6 , T r? ...
The base currents of each are as follows (3). Therefore, the transistor Tr flowing through the connection p
The sum of the base currents of 5 to Tr1o, ■Bl, is as follows.

In addition, the base current flowing from the transistor bare forming the bit switches SW5 to SW9 to the connection p is caused by the fact that hfe of each transistor pair is connected to the transistors Tr5 to Trto.
If it is equal to , then each will be the same. Therefore, the sum of the base currents flowing from bit switches SW5 to SW9,
Since In2 is the sum of the sequence shown in equation (5), it becomes as follows. Therefore, the total base current I flowing into connection p
B8 is expressed as follows by applying the well-known approximation formula to equation (7).Then, this base current Ins is calculated as As a result, the output signal of the R-2R ladder type DA converter 2 becomes smaller by the amount of the base current IBH.

In addition, the output current m1 of the top bin (D4) of the R-2R ladder ODA converter 2 is In
Engineering n 1 + h f e・・・・・・
... It will be a loss. Here, if hfe=100, then the maximum value of m will be 5. In other words, the number of bits of the R-2R ladder type DA converter cannot be increased to 5 or more,
As a result, when the number of bits in the entire DA converter increases, the number of focuses that the current division method side has to bear increases. Therefore, a problem arises in that the power supply voltage cannot be kept low.

(If there is a correction current Io) Next, a case where there is a correction current Io will be explained.

First, the current amplification factor hfe of the correction transistor Tr-A
When set to the same value as the transistors Trs to Trlo, the current flowing to the base of the transistor Tr-A (that is, the correction current Io) is as follows. As a result, R-2R
The current work n' output from the ladder method is the current work. The current is the sum of the correction current Io, In' = In+Io...
・It becomes (to). Therefore, in this case, the current 1111t of the most significant bit of the R-2R ladder type DA converter 2
can be obtained by replacing In in the above-mentioned formula (9) with n'. In other words, it becomes . Then, by substituting the α4 formula into the above-mentioned α) formula, we get: Here, if hfe=100, then n. That is, the value of m can be set to 10 or more. Therefore, even if the number of bits in the entire DA converter increases by 4, it is possible to increase the number of bits on the R-2R ladder method side and reduce the number of bits on the current division method side, which reduces the power supply voltage. It can be small.

, - In this case, since it is extremely easy to set hfe to /θθ, the value of m can be made even larger by increasing the value of hfe. Therefore, for example, a 16-bit DA converter with n=6 and m=10 can be easily created. Next, FIG. 2 is a circuit diagram showing the configuration of the first embodiment of this invention. It is. In the figure, the same reference numerals are given to the parts corresponding to those in FIG. 1, and the explanation thereof will be omitted.

In the figure, each of ISA to ISD is a current source, and consists of transistors Qd whose bases are commonly connected, and emitter resistors of these transistors Qa to Qd. These current sources ISA to ISD are each connected to the output end of the current combining section 3a, and the output current of each current source ISA to ISD flows out to the bias terminal 5. The output current values of the current sources Is-SD are each set to IM8B/2.

Next, the current combination unit 3a appropriately combines the output currents of the current sources ISA to ISD to create a shunt ratio of 2:1:1. The current combination unit 3a supplies, for example, the output currents of the current sources ISA and ISB to the switch SW1,
It may be a simple connection that supplies the output current of the current source ISc to the switch SW2 and supplies the output currents of the currents W and ISD to the current divider 4, or the above connection relationship may be sequentially and cyclically shifted. It may also be a switching circuit. In this case, the current combination unit 3a and the current source ISA
~ISD constitutes a current divider 3. Next, the correction current source INK consists of a transistor Qe and an emitter resistor Re, and the base of the transistor Qe constitutes a current source 15A-ISD. The emitter is connected to the bias terminal 5 via a resistor R, and the collector is connected to the output terminal of the current divider 18. In this case, the characteristics of the transistor Qe are set to be the same as those of each transistor constituting the current source I5A-ISD, and
The value and characteristics of the resistor Re are also set to be the same as the value and characteristics of the emitter resistor constituting the current source 15A-ISD. Therefore, the value of the current flowing through the correction current source ISE is equal to the value of the current flowing through each of the current sources 15A-ISD (IM8B/2).

As is clear from the figure, the operation of this embodiment is exactly the same as that of the first embodiment described above, but the characteristics of the correction current source ISK are set exactly the same as those of the current sources ISA to ISD. Therefore, even if the vBE and 'hFE of the transistors constituting the current sources ISA to ISn vary depending on the terminal or the wafer, a correction current suitable for the variation can be obtained.

Incidentally, an example of the voltage relationship of each part in the above-mentioned eighth first embodiment is as follows.

First, as for the input digital signal, the '1' signal is about 5V,
'O# signal is set to about -1.5V, and terminal 15.1
4,2(1) Voltage values are -IV and -2V respectively.

It is set to about θ~5v. In addition, the terminal 19 is connected to the CI
Since it is sufficient that a current of M8n/2-2In) flows, the voltage value can be set arbitrarily. Note that the above set values are merely examples, and it goes without saying that appropriate values can be set depending on the circuit used.

Furthermore, when the current division type DA converter 1 is configured by the dynamic element matching type and the current dividers are provided in multiple stages, the current IM8B of the most significant bit flows through the correction current source ISi. It is better to install it in parallel with multiple current sources.

Furthermore, in each of the above embodiments, the correction transistor T
If the characteristics of r-A and the characteristics (temperature characteristics, etc.) of each transistor constituting the R-2R ladder type DA converter are set equal, the hfe of all transistors will be the same even if the temperature changes etc. This is extremely suitable because the accuracy does not deteriorate.

〔Effect of the invention〕

As explained above, according to the present invention, an R-2R ladder resistance group, a current source transistor provided for each bit of the R-2R ladder resistance group, and a bit switch transistor are provided, and the lower side R, which is the bit conversion part
- A correction current source (current correction means) serving as a current source for the 2R ladder type digital/analog switching unit and the R-2R ladder type digital/analog conversion unit.

In addition, a current division type digital-to-analog conversion section serving as a conversion section for the upper bit, and each of the transistors among the current flowing from the R-2R ladder type digital-to-analog conversion section to the current division type digital-to-analog conversion section. and current correction means for drawing out a current equal to the sum of the base currents as a correction current.
The signal accuracy in the coupling portion is extremely high, and as a result, high accuracy can be achieved, and the number of bits on the current division method side can be reduced to lower the power supply voltage.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of a first embodiment of the invention, and FIG. 1 is a circuit diagram showing the configuration of a second embodiment of the invention. 1...Current division type DA converter (current division type DA converter), 2...R-2R ladder type DA
Converter (R-2R ladder type DA converter), 18...
...Current divider (current correction means), Tr-A...
...Correction transistor (current correction means), ISE...
····of

Claims (3)

[Claims] (1) an R-2R ladder resistance group, and a current source transistor provided for each bit of the R-2R ladder resistance group;
an R-2R ladder type digital-to-analog converter, which is equipped with a bit switch transistor and serves as a converter for the lower bit; and a current source for the R-2R ladder type digital-to-analog converter, and serves as a converter for the upper bit a current division type digital-to-analog conversion section serving as a conversion section; and a sum of the base currents of each of the transistors among the currents flowing from the R-2R ladder type digital-to-analog conversion section to the current division type digital-to-analog conversion section. A digital-to-analog converter comprising: current correction means for extracting a current having a value equal to , as a correction current. (2) The current correction means includes a correction transistor having a current amplification factor equal to that of each transistor, and causes an emitter current corresponding to the correction current to flow through the emitter of the correction transistor, so that the base current of the correction transistor is obtained from a connection point between the R-2R ladder type digital-analog converter and the current division type digital-analog converter. . (3) A correction current source transistor is provided in parallel with the current source transistor that generates the most significant bit current of the current division type digital-to-analog converter, and the output current of this correction current source transistor is divided. 3. The digital-to-analog converter according to claim 2, wherein the digital-to-analog converter generates an emitter current of a transistor for use.