A RESISTANCE LADDER NETWORK

Abstract: A plurality of resistors 5 are connected in a ladder format and disposed in parallel with each other. Each of the resistors has a resistance part 5A and 5B and two connection ends connected to the resistance part. The resistance part of each resistor 5 includes two generally parallel limbs 5A and 5B. The connection ends are all located at one side of the network. This configuration enables wiring lines to overlie the resistance part of the ladder network without the use of two layer aluminium wiring since wiring running in the same direction as the resistors 5 is avoided.

Description: The present invention relates to a resistance ladder network, more particularly to an R-2R type or 2 n -R type resistance ladder network. Resistance ladder networks are commonly used in digital-to-analog (D/A) converters. The resistors of a resistance ladder network are normally diffusion resistors and thin film resistors. The diffusion resistors are formed by a number of short generally parallel strips. In the layout of the converter there are portions of the wiring which run parallel to the strips forming the resistors. Therefore, it is not possible to form another wiring layer extending over the diffused strips and generally perpendicular to the axis of the strips. If an insulation layer is formed above the wiring extending parallel to the strips, it is possible to form wiring on top of the insulation layer but this is not preferred since a two-layer aluminium wiring process is required and this increases the number of masks required for the manufacturing process. Thus, the yield rate decreases and the manufacturing process becomes more complex. According to this invention, a resistance ladder network in an input portion of a D/A converter formed as an LSI circuit, comprises a plurality of resistors connected in a ladder format and arranged generally parallel to one another and transverse to a longitudinal direction of the network, each of the resistors having a resistance part, and two connection ends connected to the resistance part, and is characterised in that each resistor includes two side-by-side limbs, in that both the connection ends of each resistor are located on the one lateral side of the network, and in that conductor wirings inter-connecting the resistors are formed on a different substrate level from them and run generally lengthwise of the ladder network so that a space is provided on the same substrate level for other conductors superimposed on the resistors. With the present invention, it is possible to use the resistance part of an R-2R type resistance ladder network and a 2 n -R resistance ladder network as a passage for other wiring on the same level as the wiring forming the interconnections of the resistors. This is useful for improving the degree of integration of large-scale integrated circuits (LSI's). Particular examples of ladder networks in accordance with this invention will now be described and contrasted with the prior art with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a R-2R type D/A converter; Figure 2 is a plan of the layout of a conventional R-2R type resistance ladder network; Figure 3 is a plan of a R-2R resistance ladder forming a first example of the present invention; Figures 4A to 4F are plans of various shapes of resistors; Figure 5 is a circuit diagram of a 2 n -R type D/A converter; and Figure 6 is a plan of a 2 n -R resistance ladder forming a second example of the present invention. Before describing the preferred embodiments of the present invention, a detailed description will be given of the prior art for reference purposes. An R-2R type D/A converter which uses an R-2R type resistance ladder network is, as shown in the example schematically illustrated in Figure 1, comprised of an R-2R type resistance ladder network RD, switch group SWG, and operational amplifier OP for converting the output current to voltage. This example is a current-driven type. There is also a voltage-driven type. The resistance ladder network RD includes five resistors R₁ to R₅ connected in series. One of the ends of the five resistors R₁ to R₅ are grounded; the other of the ends P₁ are connected to a switch S₀ via two resistors R₆ and R₇. A junction point P₂ is connected to a switch 5, via two resistors R₈ and R₉, P₃ to a switch S₂ via two resistors R₁₀ and R₁₁, and P₄ to a switch S₃ via two resistors R₁₂ and R₁₃. The resistance value of each resistor R₁ to R₁₄ is the same. In this circuit, the resistance value of the two resistors R₄ and R₅ connected to the ground is 2R. The resistance value of the two resistors R₁₂ and R₁₃ connected to the switch S₃ is also 2R. Since these two-resistor arms are parallel, the composite resistance value seen from point P₄ is R. Since these two-resistor arms are connected in series with the resistor R₃, the composite resistance value is 2R. Since this resistor circuit connected in series is connected in parallel with the two resistors R₁₀ and R₁₁, the composite resistance seen from the point P₃ is R. In the same way, in the circuit, seen from any point P₁, P₂, P₃, and P₄, two resistors are parallel, therefore, the composite resistance is R. The current flowing to each two resistors R₆ and R₇, R₈ and R₉, R₁₀ and R₁₁, and R₁₂ and R₁₃ is 8i, 4i, 2i, and i respectively. The switches S₀ to S₄ are switched to the left or the right in response to a digital input of 4 bits. As shown in Figure 1, when switched to the left side, connection is made to the inverting input-of the operational amplifier OP. When switched to the right side, connection is made to the noninverting input+side. The value of the current flowing to these inverting and noninverting inputs from R ref is determined by the digital input in accordance with the state of the switch. The analog output A out converting this to a voltage value becomes a D/A conversion value of the above digital input. For the resistors of the R-2R type resistance ladder network RD, diffusion resistors or thin film resistors are normally used. Figure 2 is one example of this layout of the prior art, wherein the 1's are the diffusion resistors of resistance value R. The diffusion resistors 1 are formed in a short strip form and so that all are fairly parallel. Used individually, the resistance is R. Used with two connected in series, the resistance becomes a resistance value 2R. Reference numeral 2 is wiring for forming the 2R resistors (for example, aluminium), 3 is wiring for connecting R and 2R, and 4 is wiring for successively connecting between R₁ and the junction point of R₂ and R₈. The defect of this layout is that there must be a portion of wiring 4 parallel to the diffusion resistors 1, in other words a portion running from one end to the other of the diffusion resistor 3. Therefore, for example, it is not convenient to form another wiring layer horizontally traversing the diffusion layer in the arrow direction A. Figure 3 is a plan pattern diagram showing an embodiment of the present invention. This embodiment shows an R-2R type resistance ladder network. In the figure, 5 is a rectangular U-shaped diffusion resistor. This diffusion resistor 5 has a direct resistance value R comprising the total of one side 5A and the other side 5B and also the connection side 5C joining the ends of the two sides 5A and 5B. The connection side 5C may be an aluminium or other wiring layer of low resistance as shown in Figure 4F. In such a case, the resistance values of the sides 5A and 5B are each R/2. By making the basic diffusion resistor R of the R-2R type resistance ladder a rectangular U-shape, it is possible to gather all mutual connection ends on the same side. Reference numerals 2 to 4 are wiring as in Figure 2. Both are on one side of the rectangular U-shaped diffusion resistor. The mutual connection ends are connected at the same side without having to run the wiring parallel to the sides 5A and 5B. In this way, it is possible to form another wiring 6 horizontally traversing the sides 5A and 5B on the same level as the wiring 2 to 4, thereby enabling effective utilization of the chip area and improvement of the degree of integration. Further, while it was said that the connection side 5C could be a low resistance wiring layer, if sides 5A and 5B are each R/2, it is possible to make the two slanted and joined in a V-shape and therefore eliminate the side SC, as shown in Figure 4B. In another embodiment, the diffusion resistor may be U-shaped as shown in Figure 4A, the connection side 5C may be constructed by a V-shape diffusion layer as shown in Figure 4C. In the above embodiments, the lengths of the sides 5A and 5B of the diffusion resistor 5 are the same as each other. However, in another embodiment, the side 5A may be shorter than the side 5B, as shown in Figure 4D, or longer than the side 5B, as shown in Figure 4E. Regarding the overall area of the ladder network RD, in the case, for example, where R is set to 25 kiloohm in the layout of figure 2, if the sheet resistance of the diffusion layer is made ps = 1 kiloohm/square 10 µm × 1 kiloohm per 10 µm, the width W of the diffusion layer is 10 µm and the length is 250 µm. Therefore, when forming an 8-bit D/A converter, since the number of diffusion layers reaches 25, the resistance covers about 400 µm in the horizontal direction, assuming a distance between resistances of 6 µm, and a considerable area (250 µm × 400 µm) is taken up by the ladder network. The area of the switch group SWG in this case is, with a complementary metal oxide semiconductor (CMOS), about 100 µm × 400 µm. Consequently if it is possible to pass other wiring through the ladder network RD, having a large area of twice or more of the same, a considerable improvement in the degree of integration can be expected. In the case of Figure 3 too, the same is true regarding area as with Figure 2. The present invention can also be applied to the 2 n R type D/A converter shown in Figure 5. In Figure 5, 2R, 4R,.... 2 n R are resistors, 5 is their switching circuit, V ref is the reference voltage, OP is an operational amplifier, and R is its feedback resistor. Figure 6 is a plan pattern diagram showing another embodiment of the present invention. In the figure, each rectangular U-shaped diffusion resistor has the same constitution as that in Figure 3. The resistor 4R is composed by connecting in series two rectangular U-shaped diffusion resistors. The resistor 8R is composed by connecting in series four rectangular U-shaped diffusion resistors. The diffusion resistor shape may also be modified in this embodiment as shown in Figures 4A to 4F.

Claims: 1. A resistance ladder network (RD) in an input portion of a D/A converter formed as an LSI circuit, the resistance ladder (RD) comprising a plurality of resistors (5) connected in a ladder format and arranged generally parallel to one another and transverse to a longitudinal direction of the network, each of the resistors having a resistance part, and two connection ends connected to the resistance part, characterised in that each resistor includes two side-by-side limbs (5A and 5B), in that both the connection ends of each resistor are located on the one lateral side of the network, and in that conductor wirings (2, 3, 4) interconnecting the resistors (5) are formed on a different substrate level from them and run generally lengthwise of the ladder network (RD) so that a space is provided on the same substrate level for other conductors (6) superimposed on the resistors (5). 2. A resistance ladder network according to claim 1, wherein the resistance part of each resistor (5) comprises a continuous resistance element strip (Figure 4A-4E). 3. A resistance ladder network according to claim 1, wherein the resistance part of each resistor (5) comprises two straight resistance element strips and a conductive element (5C) connected between ends of the resistance element strips located at the other side of the network. 4. A resistance ladder network according to claim 2, wherein the continuous strip forming the resistance part of each resistor is U-shaped. 5. A resistance ladder network according to claim 2, wherein the continuous strip forming the resistance part of each resistor is V-shaped (Figure 4B). 6. A resistance ladder network according to any one of the preceding claims, wherein the lengths of the two limbs (5A and 5B) are equal to each other. 7. A resistance ladder network according to any one of claims 1 to 5, wherein the lengths of the two limbs (5A and 5B) are different from one another. 8. A resistance ladder network according to any one of the preceding claims, wherein the resistors (5) are connected in an R-2R ladder format. 9. A resistance ladder network according to any one of claims 1 to 7, wherein the resistors (5) are connected in a 2 n -R ladder format.

Inventors: HINO, YOUZI, OGAWA, HIRONOBU
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: H01L27
IPC codes: H03M1, H03H7, H01L27, H01L21, H01C1