What is the jelly bean DAC du jour?

[Joerg]

Hello Folks,

Haven't used DACs in a while, did it with PWM all the time. Now I need
to design something that requires lots of low noise DC levels to be set.

What is the common jelly-bean 8-bit multi-DAC with a serial bus these days?

The requirements would be the usual. Multi-sourced if possible, lots of
channels, under 50c/channel or at least under $1/channel, 8 bits or
more, serial bus with two or at the most three wires, speed can be in
the low kHz range. A chip select would be nice but that could also be
handled by gating logic.

I have seen some nice octal 8-bitters like the TLC5628 which
unfortunately needs an extra load command line (but that would be ok).
Don't know whether that one is a mainstream part and I want to avoid
settling for a boutique part, which is why I am asking.

Then there'll be the challenge to bus the data to several dozen modules
with the DACs on them but that's a whole 'nother matter. Got to wait
until the guys tell me what the bus is going to be. Hopefully nothing
that needs lots of arbitration.

 

[[email protected]]

Hello Folks,

Haven't used DACs in a while, did it with PWM all the time. Now I need
to design something that requires lots of low noise DC levels to be set.

What is the common jelly-bean 8-bit multi-DAC with a serial bus these days?

LTC1090. 2 wire I2C bus, 8 ch.

 

[[email protected]]

LTC1090. 2 wire I2C bus, 8 ch.

Wow, those are $12-$25 each from Digikey.

 

[Joerg]

LTC1090. 2 wire I2C bus, 8 ch.

Thanks. It is a bit on the pricey side, probably because it's 10-bit.

 

[Joerg]

Hello Spehro,

You might do better price-wise with a digital pot than a conventional
DAC.

Good point. I am even pondering the use of a uC, cramming numerous PWM
in there. Some will have to be SW because there are only so many CCR and
only one long timer, usually. That way I'd have a neat little UART at my
disposal that could take care of the host communication.

 

[Spehro Pefhany]

Hello Folks,

Haven't used DACs in a while, did it with PWM all the time. Now I need
to design something that requires lots of low noise DC levels to be set.

What is the common jelly-bean 8-bit multi-DAC with a serial bus these days?

The requirements would be the usual. Multi-sourced if possible, lots of
channels, under 50c/channel or at least under $1/channel, 8 bits or
more, serial bus with two or at the most three wires, speed can be in
the low kHz range. A chip select would be nice but that could also be
handled by gating logic.

I have seen some nice octal 8-bitters like the TLC5628 which
unfortunately needs an extra load command line (but that would be ok).
Don't know whether that one is a mainstream part and I want to avoid
settling for a boutique part, which is why I am asking.

Then there'll be the challenge to bus the data to several dozen modules
with the DACs on them but that's a whole 'nother matter. Got to wait
until the guys tell me what the bus is going to be. Hopefully nothing
that needs lots of arbitration.

You might do better price-wise with a digital pot than a conventional
DAC.


Best regards,
Spehro Pefhany

 

[John Larkin]

Hello Folks,

Haven't used DACs in a while, did it with PWM all the time. Now I need
to design something that requires lots of low noise DC levels to be set.

What is the common jelly-bean 8-bit multi-DAC with a serial bus these days?

The requirements would be the usual. Multi-sourced if possible, lots of
channels, under 50c/channel or at least under $1/channel, 8 bits or
more, serial bus with two or at the most three wires, speed can be in
the low kHz range. A chip select would be nice but that could also be
handled by gating logic.

I have seen some nice octal 8-bitters like the TLC5628 which
unfortunately needs an extra load command line (but that would be ok).
Don't know whether that one is a mainstream part and I want to avoid
settling for a boutique part, which is why I am asking.

Then there'll be the challenge to bus the data to several dozen modules
with the DACs on them but that's a whole 'nother matter. Got to wait
until the guys tell me what the bus is going to be. Hopefully nothing
that needs lots of arbitration.

Hi, Joerg,

Take a look at AD8801. Octal 8-bit, $3.83 at 100. Has the standard
nonstandard ghastly serial command set, of course.

John

 

[Joerg]

Hello John,

Take a look at AD8801. Octal 8-bit, $3.83 at 100. Has the standard
nonstandard ghastly serial command set, of course.

Thanks. That is a nice chip and AD has so far never let me down in terms
of obsoleting stuff out of the blue. The street price seems to be around
$5 at 100 but that would be fine in this application.

 

[xray]

What is the common jelly-bean 8-bit multi-DAC with a serial bus these days?

I don't know how it compares, but I was looking for a cheap DAC a while
ago and settled on the former Burr Brown 7611:

http://focus.ti.com/docs/prod/folders/print/dac7611.html

Was one of the cheaper ones in the Digikey catalog when I was looking.

Worked for me.

 

[Arlet]

Hello Folks,

Haven't used DACs in a while, did it with PWM all the time. Now I need
to design something that requires lots of low noise DC levels to be set.

What is the common jelly-bean 8-bit multi-DAC with a serial bus these days?

The requirements would be the usual. Multi-sourced if possible, lots of
channels, under 50c/channel or at least under $1/channel, 8 bits or
more, serial bus with two or at the most three wires, speed can be in
the low kHz range. A chip select would be nice but that could also be
handled by gating logic.

Will a fast delta-sigma modulator on a CPLD or small FPGA be able to
match your requirements ? You can fairly easily roll your own, and get
plenty of channels out of a single part. You'll have to see whether you
can run it fast enough to get the noise level at an acceptable level,
though, but it's easy to outperform a MCU solution.

To improve noise, you could even combine 2 output pins through
resistors.

 

[Joerg]

I don't know how it compares, but I was looking for a cheap DAC a while
ago and settled on the former Burr Brown 7611:

http://focus.ti.com/docs/prod/folders/print/dac7611.html

Was one of the cheaper ones in the Digikey catalog when I was looking.

Worked for me.

Thanks. However, that's only one per package so it'll be around $3/ch.

 

[Joerg]

Hello Arlet,

Will a fast delta-sigma modulator on a CPLD or small FPGA be able to
match your requirements ? You can fairly easily roll your own, and get
plenty of channels out of a single part. You'll have to see whether you
can run it fast enough to get the noise level at an acceptable level,
though, but it's easy to outperform a MCU solution.

To improve noise, you could even combine 2 output pins through
resistors.

Yes, that is certainly an alternative. FPGAs are quite expensive though,
and often power-hungry.

 

[Joerg]

Hello Spehro,

There are a fair number of micros around with multiple hardware PWMs,
often aimed at motor control. Philips LPC 8051 series come to mind as
a low-cost alternative, but there are many others including PICs and
MSP430s.

Currently I am looking at the MSP430. There is only one timer on most of
these, with 2-3 CCRs. If it doesn't have to do anything else I could add
more in assembler. The need to listen to some kind of bus for value
changes throws that a curve though. Plus in this case I'd have to filter
them LC because the wee corners of an RC need to be muffled. Has to be
whisper quiet.

 

[Spehro Pefhany]

Hello Arlet,


Yes, that is certainly an alternative. FPGAs are quite expensive though,
and often power-hungry.

There are a fair number of micros around with multiple hardware PWMs,
often aimed at motor control. Philips LPC 8051 series come to mind as
a low-cost alternative, but there are many others including PICs and
MSP430s.


Best regards,
Spehro Pefhany

 

[Nico Coesel]

Hello Spehro,


Currently I am looking at the MSP430. There is only one timer on most of
these, with 2-3 CCRs. If it doesn't have to do anything else I could add
more in assembler. The need to listen to some kind of bus for value
changes throws that a curve though. Plus in this case I'd have to filter
them LC because the wee corners of an RC need to be muffled. Has to be
whisper quiet.

The MSP430F149 has 7 or 8 PWM outputs from one timer.

 

[linnix]

Yes, that is certainly an alternative. FPGAs are quite expensive though,

CPLD are cheap. You can use micro for computations and CPLD for
outputs. For example: a cheap ARM (qty 1: $3 to $4) and XC9572XL (qty
1: $2 to $3) can give you either 4 channels of 8 bits or 3 channels of
10 bits. You can build cascaded chips of CPLDs as cost of less than $1
per channel. It could even meet your 50 cents per channel at higher
volume.

and often power-hungry.

Depends on the drive loads.

 

[Joerg]

Hello Nico,

The MSP430F149 has 7 or 8 PWM outputs from one timer.

That would work. I hadn't looked at their big flagships yet, mostly the
F2xx family because they finally managed to design good BOR capabilities
into those, plus internal oscillators with a decent accuracy.

Do you think the F149 will stick around for a while? One version of the
427 was designed for electricity meters so hopefully that one will have
good longevity.

 

[Joerg]

Hello Linnix,

CPLD are cheap. You can use micro for computations and CPLD for
outputs. For example: a cheap ARM (qty 1: $3 to $4) and XC9572XL (qty
1: $2 to $3) can give you either 4 channels of 8 bits or 3 channels of
10 bits. You can build cascaded chips of CPLDs as cost of less than $1
per channel. It could even meet your 50 cents per channel at higher
volume.

That's another idea although I can get octal DACs for around $4-$5. That
would be a better deal then. Long term DAC availability is another
matter but that can also become a real problem with programmable logic
where the fashion changes much faster.

Depends on the drive loads.

The FPGA I saw where pretty much always hungry. But so am I ))

 

[linnix]

Hello Linnix,



That's another idea although I can get octal DACs for around $4-$5. That
would be a better deal then. Long term DAC availability is another
matter but that can also become a real problem with programmable logic
where the fashion changes much faster.

But the basic functions never change. I can implement the same shift
registers using PLA equations as well as VHDL. In fact, I almost
always use the same serial (micro side) / parallel (i/o side) shift
registers for CPLD. CPLD would get cheaper with higher volume. Below
$1 for the XC9572XL might be possible. I would rather overstock on
CPLD then custom DACs.

If you really need the low power (more expensive), cool runner II need
12uA @1.8V for standy You can run at very low frequency and the only
power would be driving the op-amp integrators. I don't think custom
DACs would be any better.

The FPGA I saw where pretty much always hungry. But so am I ))

CPLDs are not.

 

[Nico Coesel]

Hello Nico,


That would work. I hadn't looked at their big flagships yet, mostly the
F2xx family because they finally managed to design good BOR capabilities
into those, plus internal oscillators with a decent accuracy.

Do you think the F149 will stick around for a while? One version of the
427 was designed for electricity meters so hopefully that one will have
good longevity.

As far as I know the F149 is among the biggest in both flash, ram and
I/O and most 'small quantity sellers' like RS-components and Farnell
seem to keep it in stock for next-day delivery even if you order 15
pieces at a once. So it seems popular enough to keep around for a
while.