Multiplying DAC: Current Out

Unless extremely high speed is an issue, the normal DACs are serial load. A octal DAC is available in a TSSOP16 case. Eg the LTC1665 for 3.5$ or so.

Rene

The dac8408 (or dac-8408) is an ancient PMI part, introduced in 1990 or before, portions of which Analog Devices discontinued in 2003. It might be unwise to use it in a new production design.

You mention size, but do you also have a price point you have to meet? I notice the ad7568 gives you eight dacs in an SMD package, but 12-bit parts do cost more. It uses serial load. Requirements with lots of qualifiers can be hard to meet. :-)

I suppose you considered and rejected octal 8-bit voltage-output 4-quadrant multiplying dacs?

Thank you, Win,

The dacs are needed to control gain of low frequency amplifiers and corner frequency of analog filters. So I do need the current output. Price is not a big deal: This is a low volume industrial product. Serial load is not an absolute no-no either, but I do need to load all dacs every 250 or 500 microseconds (It is a multiplexed application), and the overhead of doing this would severely burden the onboard (8-bit) microcontroller. Of course I could use an FPGA to handle the loading, but that would negate the purpose of space-saving due to the large footprint.

AD maintain the 8408 is still an active part, although the fact that it doesnt appear upfront indicates it is on the way out. As and when they kill it (hope its not anytime soon..) perhaps I'll have to make do with the 7568 or 7564..

If Maxim / Linear etc dont have an equivalent, I wonder what niche was the

Uday

Alternatively, there are R-2R network ladders, which together with a latch such as the 74HC573 form a multiplying DAC. Multiplying in the sense that the supply for the latch is positive within the family limits, which can be 1.8 to 5.5V for a more moderm one.

Rene

If you use a filter configuration that employs summing junctions, you can use voltage-output dacs with output resistors into the summing junctions to get the dac-variable current gains you want.

We did one product that used four 16-bit LTC mdacs. It turned out that they had way too much personality, and alternate mdacs are rare, so we just programmed the multiplies in an fpga and used simple bipolar dacs. At the 16-bit level, nobody can tell the difference. So, can you use some 12 or so bit dacs, and multiply numerically?

John

Oops, I see now that you're controlling analog signal paths. Never mind.

John