impedence of voltage reference in shutdown - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: July 14, 2017, 05:27:53 pm
content: I am looking at this voltage reference:http://www.farnell.com/datasheets/2245224.pdf?_ga=2.157224760.2064864297.1500014695-145563674.1500014695I want a way of fitting this reference and using a 50/50 voltage divider from the 5V supply, if I turn the reference off and it goes high impedance can I run a voltage divider across it's output. The idea is that sometimes I need an absolute and accurate reference but often a ratio-metric reference to the supply is actually better. I could put a diode on it's output and put the feedback pin after the diode.

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 06:30:48 pm
content: The datasheet, does not seem to easily give the information, needed to do it.E.g. Can the feedback and output pins (which are suppose to be shorted together), cope with a diode drop, between them ?Also you would be putting about 2.5 Volts on the feedback pin, while the shutdown pin is set to switch the device off/inhibit. That may or may not be ok, depending on what exactly is inside that chip. I.e. it may cause some unwanted effects.e.g. it may draw or supply (sink or source) some current, throwing off the 50% : 50% dividers accuracy.I tried to find an applications note about the chip, but could not find one. It may have given us the answers.If it is a one off and very unimportant, you might get away with just trying it. But any form of production, would worry me, without knowing for sure, how the chip will handle this situation.

author: Simon
author_rank: Unknown
timestamp: July 14, 2017, 06:40:18 pm
content: Well the feedback pin can be used if there is a voltage drop from the output to the desired location on the PCB but yes what I am thinking could stress it and this is meant for production so I think I'll avoid it. I'll probably have build options for one or the other or will have to properly switch them on and off.

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 06:55:35 pm
content: Quote from: Simon on July 14, 2017, 06:40:18 pmOn looking at the datasheet further. It does say the output is tri-stated, when in shutdown, and it does mention the feedback pin, being resistively (IR) at a lower voltage. (as you already said).So it should have a tendency to work as you desire.But playing safe is probably best. As a diode drop may still be too big, and although the output is tri-stated, there can still be some small current flow (leakage).I'm assuming that in order to get a very low power consumption, your resistive dividers, would be very high values. So even small leakage currents (tri-state), could affect the reading.tl;drI have moved it more into a grey area now. Where it *might* work. Sorry!!!

author: Simon
author_rank: Unknown
timestamp: July 14, 2017, 06:57:08 pm
content: No being automotive power is not a problem, I'm using a 100+100 ohm divider so a high impedance is no problem.

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 07:01:15 pm
content: Quote from: Simon on July 14, 2017, 06:57:08 pmSorry, I'm confused.I assumed you were shutting down the device to save a tiny amount of power. Hence were using tiny batteries as the only power source, which needed to last a very long time.If it is automotive power, why shut it down ?Where I am getting confused, is, why are you shutting it down and yet then using an external 50% : 50% divider ?EDIT:OK, I think I understand now.It is NOT to save power, it is to sometimes get better accuracy.

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 07:11:00 pm
content: What about if you used another A/D pin, with the 50% : 50% signal on it. Then the software could use the second reference, and some mathematics, to correct the A/D readings.I don't know enough about your exact requirements/circuitry etc, to be able to exactly specify it.

author: Simon
author_rank: Unknown
timestamp: July 14, 2017, 07:21:08 pm
content: I want the option of using either a resistor divider off the uC supply as a voltage reference (referenced to the supply) or an accurate reference that is independent of the not very accurate supply. I can turn the reference on and off from the uC, if it's off the resistor divider will provide around 2.5V based on the more or less 5V supply (this will be the same 5V supply powering a resistor divider that a thermistor is part of and I am measuring so as the reference goes up or down so does the measurement proportionality). But if I turn on the reference that can sink and source current it will drag the centre of the potential divider to a more precise 2.5V that can be used to measure any external voltage.

author: Simon
author_rank: Unknown
timestamp: July 14, 2017, 07:21:38 pm
content: Quote from: MK14 on July 14, 2017, 07:11:00 pmYes that can be done

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 07:44:33 pm
content: If I can think out loud (and in public).If you had a 50% : 50% resistor pair, on the 5V rail, connected to the output/feedback, WITHOUT any diode or anything.Then when the device is on, it should easily be able to over-ride the 10K resistors. (But your diode isolated idea, could be better).When the device is in standby (inhibited), the tri-stated output, should then let the 10K resistors, pull the output back to the 50% : 50% level.Obviously, you may use a different value to 10K (but be careful, as you don't want to load the output too much).But the $64,000,000 question is, does the datasheet really let you do this ?(Hence the whole point of this thread).I understand your problem, because for example, some sensors, are connected to the 5V rail, so a reading relative to the specific 5V voltage, can actually be more accurate, than the precise voltage reading. I.e. it is ratiometric etc.

author: Simon
author_rank: Unknown
timestamp: July 14, 2017, 07:59:51 pm
content: Well the reference and voltage divider should be within 200mV of each other so it should not be a problem for the reference to hold it. I think all in all I best just feed the accurate reference into an analogue channel and do some math if I need to use it.

author: MK14
author_rank: Super Contributor
timestamp: July 14, 2017, 08:11:35 pm
content: Quote from: Simon on July 14, 2017, 07:59:51 pmYes, that makes a lot of sense. I agree.By and large, straying too far from the datasheets recommended way of doing things (intentions), can lead to problems, somewhere down the line. So if there is a more straightforward way of doing it, it is probably better, in the long term.My gut feeling is, the reference should be fixed to one of the things (i.e. the voltage reference, or a percentage of the 5V supply). Then use extra A/D pin(s) to the other "reference source", if necessary and software calculations, if more sophistication is needed.Which is EXACTLY what you have just said.