Very low frequency 100 microvolt/sec triangle ramp with adjustable limits and slope

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.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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.

-- Bill Sloman, Nijmegen

looks like 20 bits is the best they do now (DAC1220)

Can you use the timer of a uC and PWM a reference that is accurate enough for your purpose? Maybe even the old TL431 suffices :-)

Then lowpass it via RC.

--
Regards, Joerg

http://www.analogconsultants.com/

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Oops. I was relying on my memory, and it was ten years ago. The DAC1220 does indeed seem to be the part I was thinking of using, and it does offer only 20-bits.

Thanks for the correction.

-- Bill Sloman, Nijmegen

That would be some significant time constant for the RC filter.

Steve schrieb:

Hello,

I would prefer a digital solution for such long periods. With Direct Digital Synthesis (DDS) it is no problem to generate periodic signals with frequencies of Millihertz, Mikrohertz and even Pikohertz.

But for

the minimum clock frequency given in the diagrams is 10 MHz, maximum 50 MHz and the accumulator is 32 Bits long. Both limits the minimum possible frequency. But with a DDS Chip for slower clocks and with a larger accumulator very small frequencies should be possible. When only very small frequencies are needed, the DDS may be programmed using a suitable mikroprozessor. An internal bit width of 32 or 64 bits would be helpful, but support for very long integers spread over multiple words would do it too. Don't try using double precision floating point values for DDS, long integers are needed. Some counters for the generation of a slow clock for the DDS algorithm are also needed.

Bye

Joerg schrieb:

Hello,

the necessary very large time constants for the lowpass filter might be a problem when a period time of 15,6 hours is needed.

Bye

Since the required output is only a triangle, no sine, a CPLD could be used as timing circuit.

Rene

Steve schrieb:

Hello,

a DAC with 16 bit resolution allows steps of less than 100 microvolt for a unipolar maximum voltage of 5 V. 18 or 20 bits will be even better.

The triangle ramps with adjustable slopes togehter with reset, hold and start may be programmed using a mikroprozessor. Care should be taken when implementing the slow slopes, the use of single precision floating point values is not sufficient for very small slopes and frequent updates several times a second. If you add to the single precision floating point representation of 4 volts less than 59 nanovolts, the result is still 4 volts. The full implentation of DDS is not necessary if the resolution of the frequency dividers by the timers of the mikroprozessor is small enough.

An analog solution would be very difficult, the time constants of several hours realized with resistors and capacitors will not be stable.

Bye

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%?)

George H.

Oh can you make a higher voltage triangle wave and then scale it at the output?

G.

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.

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.

--- Here's how I see the system from a functional POV: (View in Courier)

PHI>--/n/--------+ | PLOW>-/n/-+ | | MUX | +-+------+-+ | A B | | _| | Y A/B|----------------------+ +----+-----+ | | | [VCO]--[COUNT]AOUT | | | | LOGIC | | +----------|+\ +------+ | | | | >--| |--+ | LOWSET>--[n]-|--[DAC]---|-/ | | | | | | | START>-------|----------------| |----+ | | | +----------|+\ | | | >--| | HISET>--[n]-----[DAC]---|-/ +------+

Any chance to work up a full design for you for $?

Email me if you're interested. :-)

JF

"Uwe Hercksen" schrieb im Newsbeitrag news: snipped-for-privacy@mid.dfncis.de...

If you want to do this in analogue, you might do your own D/A converter with a charge amplifier with a big capacitor say 10uF and a low bias current opamp say AD845. You can then transfer little bits of charge with a 100p cap and switches driven from a sqarewave generator. Each time the output will rise in the proportion of the caps times the voltage you charged the small cap. So each step is i.e. 5V/100000= 50uV. The switch is critical, LT has something like a flying cap switch (forgot the number), since its dual the charge injection is cancelled. The principle is shown below. To subtract you use a neg reference or another pair of switches.

.----. 10u || | | -----o--------||----. | | | | || | === o /o | | GND / | | / | |\ | o '----|-\ | | | >------' 100p | .--|+/ --- | |/ --- === | GND o \ Vref \ o o \o | | | '-----' === GND (created by AACircuit v1.28.6 beta 04/19/05

ciao Ban Apricale, Italy

Well, you don't have to run the PWM at "turtle speed" :-)

All you need to do is have the PWM run at the max frequency the timer can do while still being long enough for the accuracy, and then lowpass enough to muffle the resulting ripple so you get under 20bits or whatever is needed. It has nothing to do with the 15.6h total, all you are building is a "poor man's DAC".

Of course the reference might need to be rather accurate but Steve hasn't said anything about his requirements there. That would go for any solution though, something needs to provide a baseline reference.

--
Regards, Joerg

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Have you thought about the leakage currents of the 10uF cap and the board? You'd almost have to run this in a vacuum. Or pack it all into a thick layer of lacquer.

--
Regards, Joerg

http://www.analogconsultants.com/

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Of course you need a good film cap, I didn't look up data sheets for caps, what is the typical leakeage there? Somebody in this group measured caps some years ago and they were keeping the charge very well. If you use pdip ICs you can bend up the sensitive pins, also this node is on virtual earth. I responded here, because nobody was encouraging an analog solution. Also the OP didn't specify any specs, so why not? Ban

snipped-for-privacy@mid.dfncis.de...

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Ohh that's neat. If the switch leakage is a problem could you use pulses of light into a LED used as photodiode. (I was going to suggest a PIN photodiode, but the leakage current will be too large.) I'm not sure how to discharge the capacitor.... I'll have to think about it.

George H.

Ohh that's neat. If the switch leakage is a problem could you use pulses of light into a LED used as photodiode. (I was going to suggest a PIN photodiode, but the leakage current will be too large.) I'm not sure how to discharge the capacitor.... I'll have to think about it.

I just found the datasheet. The switch is called LTC1043, it even has an internal oscillator and with a few tricks the charge injection can be nulled out. The leakage with +10V across the switch is typ. 6pA, with only 1V or

400mV reference it will be much less. The opamp has only 45fA leakeage. This is all at 25°C. So probably the drift will be below 1uV/s. If that is acceptable I cannot say. ciao Ban