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AM6012D Datasheet(PDF) 4 Page - NXP Semiconductors |
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AM6012D Datasheet(HTML) 4 Page - NXP Semiconductors |
4 / 11 page Philips Semiconductors Linear Products Product specification AM6012 12-Bit multiplying D/A converter August 31, 1994 779 CIRCUIT DESCRIPTION The AM6012 is a 12-bit DAC which uses diffused resistors and requires no trimming to guarantee monotonicity over the temperature range. A segmented DAC design guarantees a more uniform step size over the temperature range than is normally available with trimmed 12-bit converters. The converter features differential high compliance current outputs, wide supply range, and a multiplying reference input. In many converter applications, uniform step size is more important than conformance to an ideal straight line. Many 12-bit converters are used for high resolution rather than high linearity, since few transducers are more linear than ±0.1%. All classic binarily weighted converters require ±1/2LSB (±0.012%) linearity in order to guarantee monotonicity, which requires very tight resistor matching and tracking. The AM6012 uses conventional bipolar processing to achieve high differential linearity and monotonicity without requiring correspondingly high linearity, or conformance to an ideal straight line. One design approach which provides monotonicity without requiring high linearity is the MOS switch-resistor string. This circuit is actually a full complement to a current-switched R-2R DAC since it is slower, has a voltage output, and, if implemented at the 12-bit level, would use 4096 low tolerance resistors rather than a minimum number of high tolerance resistors as in the R-2R network. Its lack of speed and density for 12 bits are its drawbacks. With the segmented DAC approach, the 4096 required output levels are composed of 8 groups of 512 steps each. Each step group is generated by a 9-bit DAC, and each of the segment slopes is determined by one of 8 equal current sources. The resistors which determine monotonicity are in the 9-bit DAC. The major carry of the 9-bit DAC is repeated in each of the 8 segments, and requires eight times lower initial resistor accuracy and tracking to maintain a given differential nonlinearity over temperature. The operation of the segmented DAC may be visualized by assuming an input code of all zeroes. The first segment current IO is divided into 512 levels by the 9-bit multiplying DAC and fed to the output, IOUT. As the input code increases, a new segment current is selected for each 512 counts. The previous segment is fed to output IOUT where the new step group is added to it, thus ensuring monotonicity independent of segment resistor values. All higher order segments feed IOUT. With the segmented DAC approach, the precision of the 8 main resistors determines linearity only. The influence of each of these resistors on linearity is four times lower than that of the MSB resistor in an R-2R DAC. Hence, assuming the same resistor tolerances for both, the linearity of the segmented approach would actually be higher than that of an R-2R design. The step generator or 9-bit DAC is composed of a master and a slave ladder. The slave ladder generates the four least significant bits from the remainder of the master ladder by active current splitting utilizing scaled emitters. This saves ladder resistors and greatly reduces the range of emitter scaling required in the 9-bit DAC. All current switches in the step generator are high-speed fully-differential switches which are capable of switching low currents at high speed. This allows the use of a binary scaled network all the way to the least significant bit which saves power and simplifies the circuitry. Diffused resistors have advantages over thin film resistors beyond simple economy and bipolar process compatibility. The resistors are fabricated in single crystal rather than amorphous material which gives them better long term stability and tracking and much higher moisture resistance. They are diffused at 1000 °C and so are resistant to changes in value due to thermal and chemical causes. Also, no burn-in is required for stability. The contact resistance between aluminum and silicon is more predictable than between aluminum and an amorphous thin film, and no sandwich metals are required to enhance or protect the contact or limit alloying. The initial match between two diffused resistors is similar to that of thin film since both are defined by photomasks and chemical etching. Since the resistors are not trimmed or altered after fabrication, their tracking and long-term characteristics are not degraded. DIFFERENTIAL VS INTEGRAL NONLINEARITY Integral nonlinearity, for the purposes of the discussion, refers to the “straightness” of the line drawn through the individual response points of a data converter. Differential nonlinearity, on the other hand, refers to the deviation of the spacing of the adjacent points from a 1 LSB ideal spacing. Both may be expressed as either a percentage of full-scale output or as fractional LSBs or both. The graphs in Figure 1 define the manner in which these parameters are specified. The left graph shows a portion of the transfer curve of a DAC with 1/2LSB INL and the (implied) DNL spec of 1 LSB. Below this is a graphic representation of the way this would appear on a CRT screen where the AM6012 is used as a display driver. On the right is a portion of the transfer curve of a DAC specified for 1/2LSB INL with LSB DNL specified and the graphic display below it. One of the characteristics of an R-2R DAC in standard form is that any transition which causes a zero LSB change (i.e., the same output for two different codes) will exhibit the same output each time that transition occurs. The same holds true for transitions causing a 2 LSB change. These two problem transitions are allowable for the standard definition of monotonicity and also allow the device to be specified very tightly for INL. The major problem arising from this error type is in A/D converter implementations. Inputs producing the same output are now represented by ambiguous output codes for an identical input. Also, two LSB gaps can cause large errors at those input levels (assuming 1/2LSB quantizing levels). It can be seen from the two figures that the DNL-specified D/A converter will yield much finer grained data than the INL-specified part, thus improving the ability of the A/D to resolve changes in the analog input. |
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