Odd voltage reference
Odd voltage reference
(OP)
As with all of my designs, I need a solution that's minimal in cost 
I require a voltage reference (actually, I don't need the reference voltage itself, I'm only using the ref's go/no-go logic to determine if the battery level has dropped too much) but do not yet know the specific level... it will be somewhere in the 7-8V range, though. An acceptable piece-to-piece error would be in the neighborhood of 0.05-0.10V (+/-). This is a battery-operated circuit, so minimal current draw is of importance.
I've come up with two potential solutions, but I'd like everyone's feedback on their accuracy and repeatability. The first is to use a resistor in the ground leg of a reference diode (such as National's LMx85 series)... they list a typical operating current of 20uA, so a several hundred k range resistor would raise the operating voltage to the level of interest. My question to this design would be... would this be an acceptable design practice? Using an SOT-23 package, the total solution would cost around $0.40, well within my budget, and current draw is in the 20uA range.
The second design I thought of includes a FET controlling a resistor dividor network and an A/D channel reading the network every once in a while... if the network voltage drops below a certain level, battery is getting low. This circuit has the advantage of being less expensive (in the $0.10 range?), and may be the way I go. I'll have to run the quick calcs, but even 5% tolerance resistors should be accurate enough. It uses up an extra A/D channel, but I have extras, and since it's FET switched, I can turn it off while the processor is in sleep mode.
Design #2 looks like the way to go, but I still wouldn't mind any comments you guys might have...
I require a voltage reference (actually, I don't need the reference voltage itself, I'm only using the ref's go/no-go logic to determine if the battery level has dropped too much) but do not yet know the specific level... it will be somewhere in the 7-8V range, though. An acceptable piece-to-piece error would be in the neighborhood of 0.05-0.10V (+/-). This is a battery-operated circuit, so minimal current draw is of importance.
I've come up with two potential solutions, but I'd like everyone's feedback on their accuracy and repeatability. The first is to use a resistor in the ground leg of a reference diode (such as National's LMx85 series)... they list a typical operating current of 20uA, so a several hundred k range resistor would raise the operating voltage to the level of interest. My question to this design would be... would this be an acceptable design practice? Using an SOT-23 package, the total solution would cost around $0.40, well within my budget, and current draw is in the 20uA range.
The second design I thought of includes a FET controlling a resistor dividor network and an A/D channel reading the network every once in a while... if the network voltage drops below a certain level, battery is getting low. This circuit has the advantage of being less expensive (in the $0.10 range?), and may be the way I go. I'll have to run the quick calcs, but even 5% tolerance resistors should be accurate enough. It uses up an extra A/D channel, but I have extras, and since it's FET switched, I can turn it off while the processor is in sleep mode.
Design #2 looks like the way to go, but I still wouldn't mind any comments you guys might have...





RE: Odd voltage reference
I don’t get how #2 is so cheap, given that you are using a FET to switch the top end of the resistive divider.
RE: Odd voltage reference
RE: Odd voltage reference
When you calculate your pennies, don't forget that the cost of assemly, per part, can be in the ten cent range. More than many passives.
RE: Odd voltage reference
RE: Odd voltage reference
RE: Odd voltage reference
I think 5% is within my level of precision, but even if that turns out to not be the case, switching to 1% isn't going to cost me much more. As far as assembly costs, we're talking a penny a pad.
RE: Odd voltage reference
You might discount this on cost grounds, although in space requirement it is similar to that of the FET idea: use the high resistance divider network proposed by FelixC, and buffer it using a micropower op-amp so the A-D doesn't load the divider. There are a number of micropower op-amps with a shutdown feature that drops the power consumption to virtually nothing when it is not required.
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RE: Odd voltage reference
I suppose you realise that your numbers don’t add up. You said that you were willing to accept ±0.1V tolerance on your approximately 7.5V power rail. That is a tolerance of ±1.3%. Using two equal ±5% resistors will give you a tolerance of ±5%. Thus you should start with ±1% resistors.
What you could also do is divide the signal down less. If the ADC has a full range input of 5V for example you could divide the signal down by say 0.6. This will improve the accuracy (slightly).
RE: Odd voltage reference
Zeners can sometimes be used but you often need lower voltages. Ordinary LEDs have threshold voltages from around 1.2 V (IR diodes) to 2.6 V for blue-green LEDs with 1.6 - 1.8 for red and green LEDs i between.
The green LEDs seem to have the sharpest knees with useful current levels between 10 and 50 microamperes. The voltage is then about 1.5 V. The temperature coefficient may be a problem - but it is worth an investigation.
RE: Odd voltage reference
The voltage drop is 1.70 V at 10 uA and 1.76 V at 40 uA and +20 C. These voltages drop to 1.66 and 1.72 V at about +70 C. No noticeable increase in leakage current when increasing temperature from 20 to 70 C.
The spread between LEDs is neglible - the colour determines energy levels and they determine voltage drop. So it should be quite constant.
RE: Odd voltage reference
Yageo offers 4.99k, 1% 0603 at less than half a penny a piece, and Susumo offers roughly the same at 0.5% if I really need it (I doubt it, the app isn't that critical).
skoggs, Useful idea, I'll keep that one in my back pocket.
scotty, your idea has merit, and I've considered something along those lines. Complexity has stopped me so far as I believe I can get the accuracy/repeatability I want with something more simple. I may change my mind in a day or two, but so far the resistor dividor with the FET is looking pretty good.
RE: Odd voltage reference
TTFN
RE: Odd voltage reference
1% resistors are typically 100ppm/degC or better, so over 15degC the resulting error is negligible compared to the 1% spec. If he uses a pair of equal valued resistors then one would have to suppose that their TCs will track anyway, giving another factor of say 5x improvement.
RE: Odd voltage reference
RE: Odd voltage reference
#1 doesn't work-- you can't use one single serial R for
the current is changing too much with temp. and chip to
chip. Voltage divider must use multiple of the chip's
current unless you buffer it with a voltage follower.
The resistor tolerance can be compensated by using a
cal. factor in the program, the termal variation can't...
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