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Generation of Two Least Significant Bits in a Resistive Digital-To-Analog Converter

IP.com Disclosure Number: IPCOM000040232D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Related People

Verhaeghe, M: AUTHOR

Abstract

This article describes how a 10-bit digital-to-analog converter (DAC) may be designed from a 8-bit digital-to-analog resistive converter without accuracy degradation and with a limited additional area on the chip into which the converter is integrated. Fig. 1 shows schematically an 8-bit resistive DAC. It comprises 256 elementary resistors Ro in series with resistors Rx and Ry. The output voltage follower provides the analog output voltage, the value of which depends upon the bits of the word to be converted. The power supplies of the follower limit the input range of the follower. Resistors Rx and Ry allow the resistor bridge to fit the follower input range.

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Generation of Two Least Significant Bits in a Resistive Digital-To-Analog Converter

This article describes how a 10-bit digital-to-analog converter (DAC) may be designed from a 8-bit digital-to-analog resistive converter without accuracy degradation and with a limited additional area on the chip into which the converter is integrated. Fig. 1 shows schematically an 8-bit resistive DAC. It comprises 256 elementary resistors Ro in series with resistors Rx and Ry. The output voltage follower provides the analog output voltage, the value of which depends upon the bits of the word to be converted. The power supplies of the follower limit the input range of the follower. Resistors Rx and Ry allow the resistor bridge to fit the follower input range. The resistors Rx and Ry are adjusted according to the values of the least significant bits (LSBs) so that Ry = Ryo - n Ro and Rx = Rxo + n Ro __ __ 4 4 with n = 0, 1, 2, 3, according to the value of the two LSB bits. Thus IREF = VREF/(Rxo + Ryo + 256 Ro) = constant. As Rx is varying according to the two LSB bits, Ry must vary so that the sum Rx + Ry is constant. This is achieved by two similar circuits, as shown in Fig. 2. By choosing adequately the values of resistors Ro, RA, RB, R1, R2 and R3, and activating one of the N switches and one of the P switches detailed in Fig. 2, it is possible to keep Rx + Ry = constant whatever may be the values of A1 and A2 digital inputs. The activated switches and the corresponding Rx and Ry values are given as a function of the values of the digital inputs A1 and A2 in the following table. ____...