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1  Using Arduino / Sensors / Re: Question re: Accuracy of Reference re: Accuracy of ADC on: August 13, 2014, 02:58:00 pm
Hi Retrolefty and thanks for the reply!

In other words, unless a reference was more stable (i.e. better than 9PPM/C) or so, then there would be little benefit in employing a high-accuracy external reference like the ADR3412?

That is, the high-accuracy reference would be mostly useful to reduce calibration requirements since its initial accuracy would be much better than that of the voltage reference in the MCP3910?
2  Using Arduino / Sensors / Question re: Accuracy of Reference re: Accuracy of ADC on: August 13, 2014, 04:10:24 am
Hi guys,

I have a general question regarding the accuracy of voltage references as they pertain to ADCs. Specifically, I was under the impression that the accuracy of the reference was a key component in providing accurate and consistent ADC results.

But, a specification for a analog front end from MCP, the MCP3911 or MCP3910 has me a bit confused. On the one hand, MCP claims that the MCP3910 can be used for power measurements that exceed 0.1% in accuracy and that the references that are built into these chips have very low drift (<10PPM/*C) but the actual accuracy of the reference is about +/- 2%.

Is the explanation that while the reference is "only" 2% accurate, it is very stable in terms of its output? It certainly seems very stable re: ambient temperature, for example. Thus, the error of the reference is so consistent that it can be calibrated out of existence?
3  Using Arduino / Networking, Protocols, and Devices / Re: [SOLVED] SPI communication failure... anything obvious with the code? on: August 08, 2014, 09:30:31 am
Hi Ray,

I am surprised that the code would not work for you. Are you getting the right responses after writing and reading various registers? If you are getting a match between the write and the read then it's pretty likely that there is something else going on like the chip is in reset, the reset pin is not high, etc. So I'd look around the chip electrically and make sure that everything that should have power, has power, that the CLKIN/OSC is happy and in compliance, that there is a good signal going into both ADC channels and so on.

Plus, you may need three reads for each channel read, depending on how you configured the chip (24-bit vs. 16 bit data registers, and so on). The first and second byte may be all zeros because the chip isn't measuring anything but noise. With zero input on the ADC, the 'data' may be limited to just the third byte, i.e. 0x02 and 0x05, respectively.

Cheers, Constantin
4  Using Arduino / Microcontrollers / Re: Microchip crystal errata? on: July 11, 2014, 12:41:17 pm
On the flipside, at least they replied. The MCP forums remain silent and MCP would likely do well to cull the dead stuff or the tumbleweeds will a bit too obvious.

Anyhow, he pointed me to the MCP3910, which indeed appears to be an improved version of the MCP3911. The chip model numbering chronology is a bit odd but an extra 0.3 ENOB, better THD, etc. is not bad. I'm  just not a big fan of the whole two-wire/4-wire thing that MCP implemented with the MCP3910, but as long as I don't accidentally activate the two-wire mode, I should be OK.

Curiously, the Errata page for the MCP3911 does not mention the possibility of upgrading to the MCP3910 as an option to enjoy functional MDAT pins. Odd!
5  Using Arduino / Microcontrollers / Re: Microchip crystal errata? on: July 10, 2014, 07:15:36 am
Here is the official reply from Mihai over there:

"The 100nf on the XTAL are a known issue. The error is on the schematic and on the board. So the user needs to replace them manually with lower value caps (~18pF)."

He also advised me to switch to the more modern MCP3910, which is pin compatible (but which looks harder to program).  I can't say I am terribly impressed:
A known issue that is not documented in the manual or the errata pages.
No apparent incentive / desire to fix the issues.
Knowingly selling a defective product.
Expecting a customer to hand-modify a development board (!!!) that allegedly is ready to use.
Technical support that takes 8 days to get back to a customer.
6  Using Arduino / Microcontrollers / Re: Microchip crystal errata? on: July 01, 2014, 06:13:57 pm
 smiley-grin  It took me a moment to realize that you must have meant the designer and checker of the schematic.  Yeah, assuming they're still with the company, etc. there might be a terse e-mail along the lines of 'what were you thinking'? A mechanical engineer finding an error in a reference design? The shame!  smiley-eek-blue

However, I wonder how much the folk at Microchip actually care. The trouble ticket is still 'pending'. I presume there must be a mountain of unanswered support tickets with 'the world is ending' importance rating ahead of mine.  That does not bode well for folk who have real issues with MCP products since unanswered questions = trouble for commercial projects.

All that said, the MCP3911 seems to be an odd duck in the MCP lineup. The MDAT pins in revision B of the chip were acknowledged as useless, noting a future fix. However, the Revision C of the MCP3911 datasheet (see Appendix A) mentions no MDAT fix in the silicon. So it's not clear to me whether the chips have been fixed or not. I won't worry about it, seeing that I don't use the MDAT pins, but this sort of sloppiness perhaps explains why competitors like Analog Devices enjoy such high market shares.
7  Using Arduino / Microcontrollers / Re: Microchip crystal errata? on: July 01, 2014, 11:23:24 am
I left a message on their forum, but the crickets are deafening. While searching through other forum postings re: the MCP3911, I literally found dozens of messages that were left unanswered for weeks on end. Seems like that part of the web site has not managed to attain any kind of critical mass to make the community useful.

Today, I submitted an official trouble ticket with the heading "documentation error" and "moderate" importance. Let's see how quickly the official channels respond, but I am not holding my breath.
8  Using Arduino / Microcontrollers / Re: Microchip crystal errata? on: June 30, 2014, 11:22:20 am
Hi Luis,  and thanks for the reply.

I posted the issue over in the MCP forums, have yet to get a reply. However, in another post , a user complained that the MCP3911 evaluation board was not producing the data he was looking for. I wonder if the MCP3911 on it was set up to run on the crystal-based clock vs. the MCU-based clock - it's a simple pin header with a jumper... and if the caps are all wrong, who knows if the crystal could even start oscillating?

FWIW, other reference schematics published by MCP (like the one for the tamper resistant power meter) show the kinds of pF load capacitors one would expect for a crystal. But, as with many cost-conscious designs, the power meter eschews the use of a crystal for the MCP3911 itself, using an IO line from the PIC MCU as a clock ticker instead.  I suppose that also has the advantage of you being able to vary the clock speed and hence the performance / power consumption of the MCP3911.

Anyhow, for now I am chalking up the crystal load capacitor specification in ADR00398 as erroneous unless someone has a good idea what a 0.1uF capacitor has to do with loading a tiny clock crystal!  smiley-lol

Thanks again for the reply. Constantin
9  Using Arduino / Microcontrollers / Microchip crystal errata? on: June 29, 2014, 07:14:59 am
Hi guys,

Having built a couple of boards over the years, I came across a (to me strange) configuration from a respected vendor. Specifically, if you look up the schematic of the MCP3911 ADC Evaluation Board (ADM00398), it features standard 0.1uF decoupling capacitors on the crystal for the MCP3911.

In my limited experience, the capacitors used with crystals are usually in the picoFarad range.  The crystal (X1) specified in the BOM is a standard Abracon 10MHz unit with a 18pF load capacitance rating, suggesting capacitors more in the 22-30pF range, not orders of magnitude higher...

So do you think MCPs specification of decoupling caps  for use with a crystal like this a likely error?
10  Using Arduino / General Electronics / Re: Conceptual Help With TMP36 (Temperature Sensor) on: May 10, 2014, 12:04:41 am
I think you're mistaken on #2. The point of using a zener diode is that, like a voltage regulator, its 'regulated output' stays relatively constant, as long as the input voltage is higher than the breakdown voltage of the zener diode. Such diodes are frequently used in voltage reference circuits. Some comparators also include voltage references to make them easier to use and less prone to the very issue  you raise.

So, creating a stable power supply is not as much an issue of whether it can be done, but at what cost (in terms of component cost vs. power consumption, vs. board space vs. temperature stability, etc.). So a zener approach might work well, but I'd seriously consider a comparator with an internal reference first - the power consumption is likely lower. Simply head over to digikey, search for a linear comparator, and filter for units with built-in voltage references. The least expensive (SOIC) single channel unit is around $0.50 (TSM931-TSM934). THrough hole versions (DIP) that work well in breadboards start around $2.75.

11  Using Arduino / General Electronics / Re: Decoupling power supply on: May 05, 2014, 08:08:22 pm
Lots of info, thanks. My power source is USB from laptop, so, what's the best choice?

Again, it depends heavily on the application. That 2A power supply I mentioned above runs off a USB connection w/o issues. In that particular instance, the designers are throwing large capacitors at the problem to ensure that the computer never has to put out more than the 500mA limit that the USB folk imposed by default on the USB power supply.  Granted, many laptops can deliver more, but 500mA is the usual design limit without various enumeration schemes, IIRC.

If you current is small and relatively constant, you may not need any additional capacitors. However, installing some capacitors (one electrolytic 10uF @10V or higher at the entrance, and 0.1uF capacitors to decouple every IC component on board) will likely improve board performance. If you notice funny behavior (MCU resets and the like), pull out a good oscilloscope and see if the voltage supply is the culprit. If so, investigate what you can do in terms of on-board storage to deal with transients, etc.

If you have any analog signals to deal with, you need to take an even closer look at board layout, decoupling, and so on. Especially if you're dealing with very sensitive ADCs, op-amps, and the like. I.e. you'll spend as much time troubleshooting the power supply to make sure it delivers clean power as designing the actual circuit that uses said power.

Some folk also like to run their USB power through ferrites to help deal with transients / high frequency noise on the line. 10uH 0805 chips and the like are pretty common for that. Only a good scope though would tell you how much of a difference such chips actually make.
12  Using Arduino / General Electronics / Re: Decoupling power supply on: May 05, 2014, 03:40:22 pm
Small ceramic capacitors are excellent sources of power for digital signals / activity inside a chip. Their respective ends should be placed as close as possible to the power pins of the chip you're using. See "Noise Reduction In Electronic Systems" by Henry Ott, a classic on the subject.

A bigger capacitor is typically placed close to the power supply to help deal with transients. How big a capacitor depends on the power supply, the scale  of the transient relative to the capabilities of the power supply, etc. I'd consult the data sheets on the power supply to be sure. For example, most of my power supplies have bulk output capacitors ranging from as little as 1uF to 10uF. However, a GPRS-chip power supply has 470uF input and output capacitors to help deal with the very high transients that occur during GPRS transmissions (very short bursts of 2A draws).

What type of bulk capacitor to choose is again to be determined via the data sheets of your power supply. Some devices like low-ESR aluminum electrolytic capacitors, some prefer Tantalum, and some insist on ceramic X5R types. It all depends on what you're using to regulate the power supply. If you go with Aluminum Electrolytic or Tantalum Capacitors, most of those are polarized and will be very unhappy if you put them in the wrong way round. Also, it's usually a good idea to choose a rating voltage at least 20% higher than the highest you expect to hit your circuit.
13  Using Arduino / General Electronics / Re: Can i power an arduino uno board from a 2.3V super capacitor on: May 02, 2014, 10:18:44 am
Couple of things,

Most supercaps have very low current draw allowances... on the order of 1-5mA. You can certainly keep an RTC alive on this power supply or preserve the contents of the Arduino RAM while changing batteries, etc. but it's not meant to power a MCU unless you buy a supercap that allows much higher current draws (they tend to be 1F and up) and combine that with a boost switchmode regulator to pull all the available power out of the supercap. Multi-Farad supercaps (and you'd want a dual-layer model, if memory serves) are not cheap and a boost converter is just what the doctor ordered to realize their full value.

Even so, I would expect you would have to put the MCU to sleep most of the time to conserve power. A LiPo battery pack may be a better and cheaper solution.
14  Using Arduino / General Electronics / Re: Which regulator for 30V DC to 5V DC on: May 02, 2014, 09:23:50 am
There goes another Karma point. Cannot agree more. While copying Gerber files, layouts, etc. published by switch mode regulator component manufacturers can yield perhaps the desired results, these types of devices are inherently more difficult to implement than most folk seem to give them credit for. Which brings up a good point, i.e. how confident is anyone re: the layout practices and EMI / EMC compliance of boards sold on Ebay?

While I briefly considered the switch-mode route for a design I've been working on, I ultimately decided to keep it simple, not extract every mWh from the battery pack and simply run my rig straight from a Lipo battery.

Coming back to the request from the OP, I'd buy from a reputable vendor, i.e. one that has put the time and resources in to ensure the switch mode power supply is going to work properly. Or, consider adding an additional power supply that simply takes line power instead. USB chargers are very inexpensive now and reputable brands don't cost an arm and a leg, yet offer 1A+ charge performance at 5VDC.
15  Using Arduino / General Electronics / Re: Which regulator for 30V DC to 5V DC on: May 02, 2014, 06:43:31 am
Given the prices over at Pololu and the fact they they're a respectable merchant, I'd source their DC-DC power supplies over a 'unknown' merchant on eBay. But a lot has to do with living in the same country as Pololu, which the OP does not. Thus, an Ebay merchant with a very high rating (99%+) may be a better bet and closer too (re: shipping).
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