You don't want to shift your analog reference for the circuit shown because the maximum voltage is still close to 5VDC. Thus, if you were to use the above circuit with the internal reference set at 2.56VDC, you'd potentially damage the ADC. Remember, the AREF needs to be higher than the maximum voltage the ADC is likely to encounter. Even then, it's a good idea to put in a tolerance for overshoot (i.e. inductive kickback from a motor being disconnected, for example) and to pay close attention to the maximum currents developed under those conditions (so that the protection diodes in the Atmel can perhaps save your bacon if your calculation was off a bit).

The reason to shift AREF to a lower voltage such as 2.56VDC is to accomodate lower input voltages on the ADC with better resolution. The AREF can be set internally (with limited voltages, dependent on the processor) and externally (with no such limit) within a range of about 1-5VDC on a 8-bit Atmel MCU. Other processors like the ARM on the DUE have a maximum AREF of approximately 3.3VDC.

Using an external voltage reference for AREF can bring several benefits, such as better voltage stability vs. temperature and other electrical issues and hence lead to more accurate ADC measurements. However, the gains of putting on a dedicated and accurate external reference on a 10-bit ADC like the one found in the 8-bit Arduinos are usually pretty small. IMO, it makes more sense to just use a voltage divider and smoothing capacitor based on VDD for AREF as a starting point, then move on to an external voltage source when you see a big potential benefit.

I have used such references for 16-bit ADCs, for example, where even a little ripple on the reference pin can mak a big difference regarding the stability of your measurements. However, the cost of high-stability series references like the LM4132A, for example, can reach several dollars, so I reserve them for 16+ bit applications. Many external AREF applications can also be well-served with relatively inexpensive but precise zener diodes. So, yes, by all means changing the AREF can have benefits but try your best make sure that the ADC never sees a voltage above AREF or you may smoke the ADC.

Good to know... thank you!

OK, time for my Friday afternoon tale of woe...

Over the last couple of days, I have been getting strange pieces of mail from the USPS... a donation letter from a Maine museum in July 2012, our home mortgage tax documents, etc. - all in unsigned envelopes from the USPS. I thought it was just plain weird until I got a cheerful phone call from a dentist in Medford, MA who asked me if I wanted my tax documents back... it appears that the USPS not only shredded the envelope that my tax documents were traveling in, they also appear to have added these documents willy nilly to the packages / envelopes of other people who were similarly inconvenienced.

Long story short: even if your past experience with the USPS has been stellar, now is the time to rethink that stance. I am kicking myself for not sending the letter registered mail, because then there would be a USPS employee who would be personally responsible. Instead, I just sent it with a delivery confirmation request. It's just great to know that everything a identity thief would need to set up fake accounts is now freely available to them... SSNs, domicile data, etc. Just great. 

Hi Magician,

Thanks for the clarification. I appreciate that and I didn't express myself well. I am not worried about the overall excitation voltage because I'm using a 0.05% accurate voltage source (series reference LM4132A @ 2.048V). Rather, I was hoping that any changes in excitation to the thermistor wires due to EMI, etc. would be canceled. However, I now realize that accounting for line losses to and from the thermistor is going to require some careful rethinking. Thanks again.

You do need to watch the ESR requirements and what type of capacitor you are using.  There are Aluminum and Tantalum available now with a conductive-polymer counter-electrode which gives them orders of magnitude less ESR than traditional wet-aluminum or MnO2 Tantalum.

Very good point. It may make sense to move exclusively to 'ceramic-allowed' LDO's and then use the recommended ceramic capacitor to be sure and call it a day.

You also have to be careful when using high-C ceramics.  Their voltage coefficient tends to be so strong you aren't getting anything near the expected capacitance anyway.

This is new to me, will have to research but thanks for pointing it out! On the other hand, if a OEM recommends a particular capacitance and a ceramic to be used, presumably that would be covered, or not?

Are you picking that value based on measurements and simulations?  If not, you need to do so.

Um... no, not simulating enough, I guess.  I suppose there are no rules of thumb, like 'don't bother more than doubling the recommended output capacitance unless...'

FWIW, I have yet to encounter a ringing LDO, or trip the auto-shutdown on a Vreg... but I also do not want to push my luck. Last but not least... on a small board, does it make much a difference where the storage caps are located? That is, other than the minimum for the LDO, close to the LDO...

Thank you all.

James, I just looked at your site and I thought I'd amuse you with my toaster-based reflow oven. It uses the rocket scream shield and an dual-channel SCR for the fan and the quartz tubes. I put a heat sink on the back of the unit for the SCR, as the SCR is mounted on the inside, while the Arduino + shield are housed on the outside, inside a plastic enclosure that I salvaged from a dead Neuton lawn mover.

I modified the program to allow Pb- and Pb-less solder profile selection using one of the shield buttons. I also insulated the unit extensively with 2000*F insulation to limit the thermal mass as much as possible. The unit runs great and the boards come out great also. The only thing is that one has to baby-sit the process a bit, i.e. open the door at the end to ensure a quick but controlled descent from reflow to room temperature.

And yes, that is high-temp RTV holding appliance grade insulation to the glass window. A small window allows me to peek inside but it's really only there to take pictures with.

Hi Magician, and thanks for the reply. Having gone over the section, I am still baffled re: your response.

The most compelling reason I did stumble across (elsewhere) to use a bridge circuit is the possibility of using a three-wire system for the RTD and hence eliminate errors due to line losses. Is this what you were referring to?

More to it than that. Theoretically, all seeds that may be imported have to go through a process of verification / etc. to ensure that the seeds are biologically 'clean' (i.e. do not import diseases unknown in the destination country), are on an approved list of seeds to be imported (i.e. with USDA/etc. approval in the USA), bonded, etc. Vendors can ensure that (they have the scale and means to ensure that seeds are clean, that the paperwork is OK, etc.) while private folk jetting around do not.

For example, the US Agriculture industry is saving billions of $$$ every year because they do not have hoof and claw disease to deal with and hence do not have to vaccinate for it. Similarly, mad cow disease is virtually unknown and also does not have to be tested for regularly, etc. Some countries like NZ are even 'cleaner' than the US and hence preserve that status by having very stringent inspections at border crossings (i.e. no dirt on shoes, disinfecting baths for dirty shoes and all that). It only takes one lugnut to destroy that status and cause widespread costs as herds are innoculated or preventively slaughtered.

So I understand why statutes are what they are. Also consider how often fauna and flora inadvertently finds its way into broader nature, i.e. the Burmese python infestation in the FL everglades, longhorn beetles in the NE of the USA, kudzu in the SE of the USA, the cane toad in Australia, introduced rats throughout the world leading to massive declines in bird populations, etc.

I have a question regarding the measurement of thermistors using a differential ADC like the MCP3421 that is unipolar. Attached are two circuits, one the bridge circuit that you can find in the MCP3421 literature (see page 28), and the one I modeled and used in my application. Try as I might, I cannot figured out what the advantage of the bridge circuit is other than linearizing the output somewhat. In addition, my research suggests that the bridge-based output loses a bit of information due to the voltages that the circuit produces with a unipolar ADC. So why advocate the bridge circuit over the 'direct' approach I show on the right, where both ADC inputs remain positive, any excitation voltages disturbances will affect both inputs equally, and the voltage difference vs. temperature output is just a function of a couple of lookup tables?

My first basic question is regarding the best locations for storage capacitors on a board. I am not concerned with de-coupling caps (i.e. the common 0.1uF approach) for various IC's on the board, rather more about bulk storage. From the reading I have done, it would appear that storing these capacitors near the voltage regulator (the shorter the distance the better) is preferable to having them distributed across the board. On the other hand, I would think that having localized storage (near potential spike loads) would help voltage stability by minimizing the distance that the power has to travel to the point of use. Or should I not be concerned with this and just put the largest storage cap that may be needed near the LDO and call it a day? FWIW, the boards in question are pretty small (i.e. 10x10cm) and the power lines are generally 32 mils wide and usually form a grid across one side of the 2-sided board. 

My second question regards the LDO tunnel of death, i.e. the areas of stability and instability that are a function of LDO design, ESR, and input/output capacitance. Depending on the LDO and the manufacturer, capacitances beyond the minimum indicated in the data sheet are encouraged as promoting stability. I also gather that some LDO designs (presumably older ones) need some ESR and hence need to rely on electrolytic or tantalum output capacitors for output smoothing, while other (newer?) designs have eliminated the need for ESR and rely on ceramic capacitors on the output stage instead. However, few manufacturers publish a graph that shows the allowable ESR vs. capacitance curves (aka the tunnel of death) and even when they do, these charts are usually limited to the recommended range of output caps, not for output caps that are larger than the minimum recommended by the data sheet. Usually, the best one can hope for is for LDO manufacturers to publish ESR and capacitance recommendations and little else.

FWIW, I like placing a relatively large capacitor on the output (i.e. 470uF) of my LDOs to minimize the potential for spikes and so on. However, a 470uF cap is well beyond the minimum output caps recommended (and in some cases discouraged) by LDO manufacturers. So what is the downside of a large tantalum or aluminum electrolytic capacitor on the output of an LDO? And how worried should I be if I place the recommended output cap in addition to this storage cap about ringing, etc.? Many thanks for any insights!

Mihun,

Nice job running OSX on a Dell. Must have been an interesting project also!

One of my funnier experiences with the (generally awesome) German postal system was sending a gift package to my god-daughter over there... except it didn't arrive... instead, the mother got a notice to pay up... long story short, the Droid at the customs hall took a look at the contents listed on the customs form, noted "gift", read it in German (in which case you're shipping 'poison') even though the package came from a English-speaking country and items shipped internationally are always declared in English, and decided that a duty applied.

A terse letter from the mother regarding the English-reading skills of the droid in question unlodged the package without the need for duty payments. Naturally, without comment.

Hi all,

Just wondering if any of you have used the MCP3901 before or not? It's an analog front end for energy measurements that looks like a neat intermediary chip between dedicated energy IC's like the AD7753 from Analog Devices and general-purpose dual-channel ADC's like the ADC122 series from National Semi/TI, for example. The device is interesting in multiple ways that I find quite intriguing:

1) It doesn't feature a bazillion internal registers and impossible (for a hobbyist, IMO) calibration routines like the AD7753. Just a few internal control registers that mere mortals can handle.
2) It does feature a PGA for each channel (up to 32x gain), a Voltage Ref In and Out, a phase compensation register, up to 256x oversampling, an external clock, dithering, and a data ready pin (for interrupt-driven data sampling).
3) The analog end runs on 4.5-5.5V, the digital end can handle everything from 2.7-5.5V. This makes interfacing with the chip a lot simpler, regardless of MCU used.
4) It uses SPI, which I dislike, but there is a library available to make configuring / listening to the chip easier.

The data sheet suggests that if you put a 8MHz crystal on this thing, set up the PRE/OSR registers appropriately, and you'll allegedly get 7812 samples per second for both channels with 14 ENOB... and if you can live with 3.9ksps per channel, up to 14.9 ENOB... all in a chip that costs less than $2.5 ea in single quantities. Seems like a pretty awesome solution, wonder why not more people are talking about / using this chip?

Along similar lines, the MCP3911 offers 3.3V-only operation but a much wider latitude re: oversampling, gain calibration, offset calibration, etc.

32 TQFP, but in a 5mm x 5mm format. Leaves 0.5mm pitch per pin and even with a stencil that is a PITA!!!

Reason being that unless you have just the right type and amount of solder paste, there is so little solder mask between pins to prevent shorts. And when you try to clean up accidental solder bridges with anything but a robotic helper and a giant magnifying glass, you have the joy of observing pins wandering from pad to pad as you brush them with your soldering iron when you try to suck up / drag solder the excess away. Thanks but no thanks, I am never touching those types of chips again, if I can help it.

I note that MinuteMan used to make a UPS with ~12V unregulated battery output. However, that model appears to be discontinued and much of the electronica out there that is UPS-protected really wants regulated 12VDC and 5VDC inputs (i.e. like a ATX power plug into a motherboard). In an ideal world, the manufacturers on both sides (i.e. CPU as well as UPS) would come to an agreement for a standardized plug that offers several voltage outputs and a means to communicate between the devices to establish just what needs to be sent, how much can be sent, etc.

I really like the Pico line of products, as they also offer the option of efficiently running all sorts of standard PC motherboards off of unstable power sources like boat, car, and like systems.