AD7546: Glitch perfomance

Q We have been using your AD7546KN D-A convertor on our circuit boards. As this has been obsolete for some considerable time, and as a change in cicumstances has dictated that we now need to support this product again, we have decided to redesign the board with a later D-A IC. The original IC was a transparent (no microprocessor interface) 16 bit monotonic D-A. I have looked at two likely candidates from your range of 16 bit D-As. One is the DAC16, which has no interface, and the other is one that we already use, which is the AD569. Although the latter device does have a microprocessor interface, my understanding from the data sheet is that it can be made transparent by holding inputs. Can you comment on glitch problems with either solution as compared to the the AD7546? (bearing in mind the way the AD569 switches its ladder resistor chain). Could you comment on the support for both devices? Are there any other devices suitable? A A couple of general points on glitch performance. ? Using a DAC with a Latch will generally produce better glitch performance that driving a DC in transparent mode. When you use a DAC in transparent mode you rely on all the parallel input signals arriving at the DAC input at the same time, differences in PCB trace and capacitance can increase the settling time as the DAC first slews to one code level and then another. ? When using a current output DAC, the settling time and glitch performance is usually dependant purely on the amplifier used for current to voltage conversion. ? A DAC which uses a segmented voltage divider will generally have low glitch as only two switches are required to operate. Conversely a DAC which uses an R2R divider circuit will generally have higher glitch and code dependant glitch as multiple switches are required to switch on and off for each transition. ? A DAC which uses one current source for each code level can be shown to have small and code independent glitch. ? The glitch due to switching MSBs is usually far worse than the glitch due to switching LSBs ? High resolution DAC will usually use a segmented architecture, employing a different scheme for generating the MSB and LSB. The MSBs will be generated using a low glitch topology such as the voltage divider or "one current source per code" scheme. The LSBSs will be generated using a topology which requires less resistors but may have poorer glitch performance. DACs to consider in this application are DAC16, AD569, AD7846, AD7849. If you want to use a single supply serial programmable DAC consider also the AD5541 and AD5542. In terms of a voltage output DAC, I would expect that the AD569 would show lowest glitch performance. For lowest glitch possible, use a current output DAC and an amplifier with low input capacitance and low input bias/ high input impedance. All the above parts are available from AD and there are no plans to obsolete them. For your reference, We cannot guarantee the lifetime of any of our components. However, components will only be obsoleted when there is little or no demand for the product or when the product is superceded by superior design. When a product is about to be obsoleted, we will give one year's advance notice and offer all existing customers the opportunity of a last time buy.

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