Multiplying DAC: Current Out

[Uday Godbole]

Hi,

I have a board that needs eighty 8-bit MDACs with parallel load and current
out, 4-quadrant multiply with individual Vref and feedback resistor. Analog
Devices' DAC8408 (quad) fits the bill, but I am not happy with the
footprint. (28-pin SOIC / PLCC). Twenty chips take up way too much space.
Looking for something much smaller. Google throws up thousands of pages,
but most of them point to the 8408. (Interestingly, DAC8408 doesn't even
appear on Analog Devices parametric search results, even with "List all
DACs" option!)

I'd appreciate any pointers.

 

[Rene Tschaggelar]

Hi,

I have a board that needs eighty 8-bit MDACs with parallel load and current
out, 4-quadrant multiply with individual Vref and feedback resistor. Analog
Devices' DAC8408 (quad) fits the bill, but I am not happy with the
footprint. (28-pin SOIC / PLCC). Twenty chips take up way too much space.
Looking for something much smaller. Google throws up thousands of pages,
but most of them point to the 8408. (Interestingly, DAC8408 doesn't even
appear on Analog Devices parametric search results, even with "List all
DACs" option!)

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

 

[Winfield Hill]

Uday Godbole wrote...

I have a board that needs eighty 8-bit MDACs with parallel load and
current out, 4-quadrant multiply with individual Vref and feedback
resistor. Analog Devices' DAC8408 (quad) fits the bill, but I am
not happy with the footprint. (28-pin SOIC / PLCC). Twenty chips
take up way too much space. Looking for something much smaller.
Google throws up thousands of pages, but most of them point to the
8408. (Interestingly, DAC8408 doesn't even appear on Analog Devices
parametric search results, even with "List all DACs" option!)

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?

 

[Uday Godbole]

Uday Godbole wrote...

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
8408 designed to address..

Uday

 

[Rene Tschaggelar]

Hi,

I have a board that needs eighty 8-bit MDACs with parallel load and current
out, 4-quadrant multiply with individual Vref and feedback resistor. Analog
Devices' DAC8408 (quad) fits the bill, but I am not happy with the
footprint. (28-pin SOIC / PLCC). Twenty chips take up way too much space.
Looking for something much smaller. Google throws up thousands of pages,
but most of them point to the 8408. (Interestingly, DAC8408 doesn't even
appear on Analog Devices parametric search results, even with "List all
DACs" option!)

I'd appreciate any pointers.

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

 

[Winfield Hill]

Uday Godbole wrote...

The dacs are needed to control gain of low frequency amplifiers and
corner frequency of analog filters. So I do need the current output.

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.

 

[John Larkin]

Hi,

I have a board that needs eighty 8-bit MDACs with parallel load and current
out, 4-quadrant multiply with individual Vref and feedback resistor. Analog
Devices' DAC8408 (quad) fits the bill, but I am not happy with the
footprint. (28-pin SOIC / PLCC). Twenty chips take up way too much space.
Looking for something much smaller. Google throws up thousands of pages,
but most of them point to the 8408. (Interestingly, DAC8408 doesn't even
appear on Analog Devices parametric search results, even with "List all
DACs" option!)

I'd appreciate any pointers.

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

 

[John Larkin]

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