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From: Steve on 28 Jan 2010 20:19
On Thu, 28 Jan 2010 16:01:13 -0800 (PST), Bill Sloman
<bill.sloman(a)ieee.org> wrote:

>A 24-bit sigma-delta D/A converter from Burr-Brown (now part of TI)
>turned out to be pretty attractive; for the slow rates I was planning

looks like 20 bits is the best they do now (DAC1220)
From: Steve on 29 Jan 2010 17:33
Yes, it does seem that digital would be best for me. This design will
also be measuring current during the voltage scan using a computer
data acquisition card, so a card with a dac could be used to generate
the ramp. But the less expensive 16 bit dac cards do not allow their
reference and offset voltages to be set externally, so their range is
restricted to +/- 10 V which is 0.3 mV lsb. In practice, that may be
ok.

On Fri, 29 Jan 2010 06:36:38 -0800 (PST), George Herold
<ggherold(a)gmail.com> wrote:

>Cool, Sounds like a digital solution, as others have suggested, would
>be best. But I have built an analog triangle wave generator (constant
>current into a cap.) with a maximum 1000 second period. (This is with
>a 100uF tantalum cap.) I could try sticking a few mF aluminum
>electrolytic on it and see what happens. Is the circuit going to live
>in a nice temperature stable enviornment? How critical is the exact
>timing?
>(+/- 20%?)

From: John Fields on 29 Jan 2010 18:22
On Fri, 29 Jan 2010 17:33:44 -0500, Steve <steve(a)nomail.net> wrote:

>Yes, it does seem that digital would be best for me. This design will
>also be measuring current during the voltage scan using a computer
>data acquisition card, so a card with a dac could be used to generate
>the ramp. But the less expensive 16 bit dac cards do not allow their
>reference and offset voltages to be set externally, so their range is
>restricted to +/- 10 V which is 0.3 mV lsb. In practice, that may be
>ok.

---
Please bottom post (or inline post where appropriate) in order to keep
the thread chronologically coherent.

Thanks. :-)

I've already posted a digital scheme which will allow you to accomplish
what you said you wanted to do, but without the constraints you've
introduced which the "less expensive" DAC cards will place on you.

can you tell us what, exactly, you want to do and how much money you've
got to be able to do it with, please?

JF
From: Paul Keinanen on 30 Jan 2010 00:27
On Fri, 29 Jan 2010 06:37:05 -0800 (PST), MooseFET
<kensmith(a)rahul.net> wrote:

>On Jan 28, 2:43�pm, Steve <st...(a)nomail.net> wrote:
>> I'm looking for a design (analog ?) for a triangle ramp generator with
>> an adjustable slope around 100 microvolts / sec. �Its output voltage
>> ramp limits need to be independtly adjustable. The typical range is
>> 2.0 to 4.8 volts which results in a total period of 15.6 hours. �It
>> would need to be able to be reset or held at one of its limits (its
>> lower voltage) and started upon an external signal (relay contact
>> closure, digital logic state change, etc). � Unipolar positive output
>> voltage range is fine.
>
>The micros from Silabs have built in DACs. They are only 12 bits but
>that may be all you really need. You can follow the DAC with a low
>pass filter and dither the LSB to make the ramp much smoother. Since
>you can stuff numbers into the DAC at about 100KHz and your output
>doesn't have much of a bandwidth the filter can be a very serious low
>pass.

Apparently these devices use R/2R type DACs.

While in principle a high speed low resolution DAC could be used as a
slow speed high resolution DAC by oversampling, the linearity errors
could be a problem with such slow ramps.

While an ideal 12 bit converter would generate a clean step from say
7FF.00 to 800.00, the actual analog step could be 7FF.ff to 800.00 and
the device would still considered monotonic :-).

At least a quite large (several LSB) dither noise amplitude in the
digital domain needs to be added, to get rid of the worst linearity
errors. Some RC filtering on the analog side will then remove the
dither noise.

>If you get the more "up market" ones, the micro has a fairly accurate
>oscillator built in. This may save you from needing a crystal.

On delta/sigma etc. type converters the fluctuation of the clock would
alter the output value, thus an oscillator with low phase noise is
required and the oscillator should also be free of microphonics,
unless the RC filter cut-off would be below 1 Hz, however, such
filters would either have a very high output impedance or would
require a huge non-electrolytic capacitor.

Thus, high quality timing is requiring, so that the analog RC filter
would only have to remove oversampling noise, thus operating at a high
(100 Hz - 10 kHz) cut-off frequency.

From: John Fields on 30 Jan 2010 15:21
On Thu, 28 Jan 2010 16:01:13 -0800 (PST), Bill Sloman
<bill.sloman(a)ieee.org> wrote:

>On Jan 29, 12:23�am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
>> Steve wrote:
>> > I'm looking for a design (analog ?) for a triangle ramp generator with
>> > an adjustable slope around 100 microvolts / sec. �Its output voltage
>> > ramp limits need to be independtly adjustable. The typical range is
>> > 2.0 to 4.8 volts which results in a total period of 15.6 hours. �It
>> > would need to be able to be reset or held at one of its limits (its
>> > lower voltage) and started upon an external signal (relay contact
>> > closure, digital logic state change, etc). � Unipolar positive output
>> > voltage range is fine.
>>
>> Analog ? Something like a huge tank slowly filled with water... with
>> some kind of conversion of water level to voltage (capacitive or
>> resistive sensor, strain gauge, potentiometer attached to a float,
>> whatever).
>>
>> This kind of timing asks for digital solution; the design of the DAC
>> with required precision could be tricky although it is feasible. The
>> quantization steps of the ramp could be smoothed by post-DAC analog
>> filtering.
>
>Sure. I looked into something like this once for a low-temperature
>magnetic refrigerator - as in starting off with liquid helium and
>going down.
>
>A 24-bit sigma-delta D/A converter from Burr-Brown (now part of TI)
>turned out to be pretty attractive; for the slow rates I was planning
>on mark-to-space modulating the least signficant bit, which put the
>residual noise at a frequency high enough the the filter capacitors
>weren't too big.
>
>Audio D/A converters have rotten-to-nonexistent specifications at DC;
>the Burr-Brown part was also aimed at instrumentation, and did
>guarantee the DC levels.
>
>Bill Sloman, Nijmegen
>
>Never did get to build it.

---
Always a bridesmaid, never a bride...

JF
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