Cool Bananas! - Page 1
Forum section: electronics / projects

author: moffy
author_rank: Super Contributor
timestamp: December 11, 2015, 10:48:30 pm
content: I was reading the data sheet of:http://www.ti.com/product/amc1304l25isolated Delta/Sigma modulator and what caught my attention is that it outputs a raw single bit data stream based on the clock input it is fed. How neat or Cool Bananas! All you need to do is pass it through a low pass filter and you have your analogue signal back. You don't even need a micro just a simple clock input, and of course power supplies.I thought that just by powering a CMOS gate off a precision supply you could probably get quite good accuracy.

author: dom0
author_rank: Super Contributor
timestamp: December 12, 2015, 12:01:40 pm
content: Delta-sigma modulation is still a largely under-appreciated concept in DIY electronics. It is a very powerful concept that allows to do many things with much simpler circuits than the "old way", because it can not only be used for A/A, A/D, D/A, D/D conversion, but also for various control tasks, for example DSMs are perfect for controlling mains-synchronized heater control and such things.

author: tooki
author_rank: Super Contributor
timestamp: December 12, 2015, 06:04:13 pm
content: I'd love if you guys could elaborate on this some more.Am I wrong, or is this the same thing as, or extremely similar to, pulse density modulation? I've actually been meaning to ask about this, because it seems to me that in many applications, pulse density modulation would produce cleaner output than pulse width modulation. (It's precisely equivalent to the difference between the very regular halftone screen used in black and white laser printers vs the stochastic "random" dithering used in inkjets. Stochastic dithering produces far smoother halftones than screening.) I noticed this in the context of LED dimming, where PWM can cause flicker. (To illustrate, suppose you have a pin capable of 1000Hz. If you wanted an LED at 50%, you might use PWM at 50Hz, to give 20 different levels. At 50%, this means the pin is high for 10ms, low 10ms, etc., i.e. 50Hz, which flickers visibly. Yet by using pulse density modulation, where within the same 20ms cycle, you allowed it to go high for 1ms, low for 1ms, etc, you'd end up with 500Hz, which would not appear flickery.)I've tried searching, but have found exceedingly little info on pulse density modulation, nor any info on why it's not used as often as PWM.Thanks for any info/feedback you have!

author: Jeroen3
author_rank: Super Contributor
timestamp: December 12, 2015, 07:10:24 pm
content: There are DM163 led chips from some Chinese company that have non-consecutive pwm to lower flickering at lower brightness.But how would one build a passive filter with these Adc's?I do know of amc1200 which are isolated diff amps based on sigma delta.

author: dom0
author_rank: Super Contributor
timestamp: December 12, 2015, 07:39:42 pm
content: Quote from: tooki on December 12, 2015, 06:04:13 pmA delta-sigma modulator is the usual way of creating a pulse density modulated pulse train. I don't think there is any really clear definition of the exact difference, but I'd say, that PDM always means a two-level/binary pulse train, while that's not always the case in a SDM.Quote from: tooki on December 12, 2015, 06:04:13 pmIndeed. Delta sigma modulators generally produce less low-frequency content for the same output compared to PWM. For example I own a ceramic oven, it has about 3.6 kW (maximum single phase power). The oven controller controls power with a very low frequency PWM, about 0.3 Hz (switching a heavy duty Siemens relay... makes nice and loud KLUNK and KLOK noises). This causes small parts in the oven to instantly overheat due to the strong IR radiation of the heaters. Initially I thought about replacing the entire controller of it, and even began to do so, but I dropped the project after a while, mainly because for most stuff the old controller worked just fine. Later I had the idea to use a MCU to re-digitize the PWM and feed this value as the input to a SDM running at 100 Hz (clocked by zero crossings) whose output controls a triac. And that works perfectly. Because the "instantaneous" measurement of the PWM duty cycle by the MCU (instead of an analog filter) and the SDM only introduce a very small delay the controller has no stability issues or increased overshoot at all. And small parts don't overheat anymore.Quote from: tooki on December 12, 2015, 06:04:13 pmPWM is often not an issue anymore because newer LED drivers/SMPS allow quite high PWM frequencies (~1 kHz - so the PWM counter would run at ~250 kHz, not a problem). A different issue is interference between multiple LEDs, but it's exceedingly rare, because the microcontrollers in LEDs run off their internal RC oscillators, so their frequencies are very inaccurate and not very stable.Another issue is the limited resolution of PWM, at 8 bits you will easily notice steps when a LED lamp fades, especially at low brightnesses (PWM => output intensity is linear, but we perceive intensity log-like - so the effective resolution of the PWM at low levels is very low). This can be overcome by dithering the PWM value at some fraction of the PWM output frequency (e.g. 100 Hz for a 1 kHz PWM).Why is PWM used more commonly than SDM? I think the answer is, that a very flexible PWM unit is very easily implemented in hardware in microcontrollers, while a hardware SDM that would achieve the same flexibility is not as easy to do implement ; there is more design freedom even in a pure digital/digital SDM than in PWM.

author: jsmith45
author_rank: Newbie
timestamp: December 12, 2015, 08:14:04 pm
content: Quote from: tooki on December 12, 2015, 06:04:13 pmQuote from: tooki on December 12, 2015, 06:04:13 pmAssuming we are talking about a two level signal in both cases, they are basically identical in terms of what you see on the wire. To some extent they are a difference of preferred terminology between different manufacturers.One could also argue that they are different things when transmitted on a wire, where delta sigma is referenced to a clock and being treated as a digital signal, so that noise and jitter on the encoded lines do not cause loss of data (the losses being only the accuracy of the modulator, and the precision limit of the chosen clock rate). On the other hand, pulse density modulation is a more direct alternative to PCM, and would normally be considered an analog signal with a lot of undesired high frequency components.Those are really more viewpoint differences, rather than anything fundamental.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 12, 2015, 08:19:12 pm
content: The signalpathblog has an excellent tutorial about sigma delta modulation.http://youtu.be/z9u-QTDAeaM

author: moffy
author_rank: Super Contributor
timestamp: December 13, 2015, 10:41:33 pm
content: Nice to see the interest. Does anyone have any info or experimentation with using a CMOS logic gate as a single bit DAC. It seems they should give good performance as the designs I've looked at for 1 bit DAC's are just two transmission gates i.e. switches, connected to 2 voltage references, one positive the other negative.

author: mark03
author_rank: Frequent Contributor
timestamp: December 14, 2015, 07:02:22 pm
content: I was just reading about this in the context of a new peripheral in the STM32L4 series MCUs (and maybe others? not sure). The "DFSDM" performs all of the sinc^N, otherwise known as CIC--cascaded integrator comb, filtering in hardware, yielding 16-bit samples at the desired output rate:http://www.st.com/st-web-ui/static/active/en/resource/training/technical/product_training/STM32L4_System_DFSDM.pdfThey seem to have designed it with digital MEMS microphones in mind, which have a PDM output from an on-chip sigma-delta modulator (although no one publishes the order--frustrating). But it makes a lot of sense for general A/D applications too, to quarantine the sensitive analog stuff away from the noisy MCU and reduce the interconnect complexity.I hope this is something the chip makers will run with. I was a bit disappointed not to find any standalone SD modulators except for the specialty isolation parts. That's great if it's what you need, but it would be nice if someone sold, e.g., the SDM portion of an AD7175 or LTC2440 as a standalone, low-power part.

author: moffy
author_rank: Super Contributor
timestamp: December 16, 2015, 12:52:49 am
content: Quote from: mark03 on December 14, 2015, 07:02:22 pmI agree, they would be very flexible and easy to use, perhaps another 555 type chip for popularity and versatility.

author: coppice
author_rank: Super Contributor
timestamp: December 16, 2015, 01:39:11 am
content: Quote from: mark03 on December 14, 2015, 07:02:22 pmThey will make what there is a clearly identified high volume market for. In the early 90s there were a number of tiny sigma-delta modulator chips made as part of a chipset with a pure digital chip that contained the digital back end of the sigma-delta converter. The market for these went away as the cost of mixed signal chips (in particular mixed signal test costs) reduced.

author: mark03
author_rank: Frequent Contributor
timestamp: December 16, 2015, 04:35:08 pm
content: Quote from: coppice on December 16, 2015, 01:39:11 amYes, and as few MCUs so far have that dedicated digital hardware, this may remain the case. Still, you save at least one wire (only data and clock needed, with configurable parameters being mostly on the digital side, so no remote registers). Might be worth something. I could see the digital microphone concept morphing into a general digital-sensor interface standard. After all, the same SDM/PDM scheme would make sense for other sensors--pressure, accel, gyro--not just audio. So there may be room for unification, and resulting cost reduction. Maybe this is what ST has in mind?

author: coppice
author_rank: Super Contributor
timestamp: December 16, 2015, 04:44:43 pm
content: Quote from: mark03 on December 16, 2015, 04:35:08 pmThere are some MCUs, like the MSP430F6779, which have sigma delta converters, where the sigma delta modulator can be disconnected and disabled. A bit stream from an isolated sigma delta modulator, or from a MEMS device with a build in modulator, can then be fed directly into the ADC back end in the MCU.

author: moffy
author_rank: Super Contributor
timestamp: December 16, 2015, 11:00:08 pm
content: https://www.maximintegrated.com/en/products/digital/microcontrollers/DS8102.html#popuppdfSeems there is the odd Sigma/Delta modulator floating around. It is Manchester encoded so the bit stream doesn't have a DC component. Very nice application for 3 phase metering.

author: coppice
author_rank: Super Contributor
timestamp: December 17, 2015, 03:58:31 am
content: Quote from: moffy on December 16, 2015, 11:00:08 pmI think that is part of Maxim's failed attempts at the metering market before they bought Teridian. The parts from Teridian, and what they have produced since buying Teridian, are far more interesting if you are looking for split isolated measurements. Teridian took a very interesting approach to isolating power, clock and data through a single path.

author: moffy
author_rank: Super Contributor
timestamp: December 17, 2015, 11:15:55 pm
content: Thanks for the info.