PPM Help

[Ranger14]

I am working on building a 2.5v Ref as a little project.. I have found 2 chips I would like to use but one is 5ppm and the other is 11ppm... So does it really make a difference which one I use ?
Also they are the same tolerance too..


Thanks 

[timb]

That depends on how accurate you want the reference to be. If they're the same tolerance, the 5ppm unit will be 6ppm more accurate than the 11ppm one.

[Signal32]

0ppm means  that if you heat up the reference by 10C the voltage will not change at all, this would be ideal.
5ppm means that if you heat up the reference by 10C the voltage will change by 0.125mV.
11 ppm will be more than twice as bad.

What is your circuit and how will you be calibrating it ?
Generally the initial error is somewhere around 500-1000 ppm so the tempco might not matter that much.

[MosherIV]

Sorry signal32 but the OP has not actually said what the ppm is for : thermal drift or stability.

Thermal drift is as signal32 has said.

Stability is how much the device will drift over time.

Both should be quoted for the device(s)

The other thing to consider with Vref devices is initial accuracy, ie how far off the device will be from the stated output. Some of the very best Vref devices can have poor inital accuracy but then have incredible thermal and drift stability.

[Signal32]

Yes there are many ppm associated with voltage references (temperature, long term drift, hysteresis, noise, initial error, line regulation, load regulation etc).
Since the OP didn't mention which one he is very likely referring to the temperature since that's how manufacturers advertise the voltage references, if you see "2.5v 5ppm reference" you can be certain that the 5ppm is the tempco.

[BU508A]

Hi,

I have found 2 chips I would like to use

it would be helpful if you could provide us what exactly the chips are you have found. (Type, link to the datasheet, etc.)

Andreas

[suicidaleggroll]

You have to establish your requirements for the system before you or anybody else will know which specs are "good enough" and which aren't.

[Ranger14]

Sorry about no answer I completely forgot about this...

The plan was to use the chip as a Vref and have no adjustment pot since no meter I have is calibrated or enough digits for it..

I was looking at
Max6126
http://datasheets.maximintegrated.com/en/ds/MAX6126.pdf
or
LM368H-2.5
http://pdf.datasheetcatalog.com/datasheets/135/50057_DS.pdf


Also up for suggestion but just trying the price of chip low but with Great tolerance.

[MosherIV]

Hi

Well, as was stated before the device with the lower tempco is the one to go for.
According to the spec sheets, it is the Maxim device.

The plan was to use the chip as a Vref and have no adjustment pot since no meter I have is calibrated or enough digits for it

Exactly what I did, just used the Vref device without any adjustment, for me it was because the quality of any adjustment pot would have added more inaccuracy and drift than the original device and I can live with the initial error.

FYI, with most of these precision references, it is a good idea to leave them on for months because some of them take that long to settle. I re-purposed a linear PSU into a project box and put some 4mm jacks onthe outside. Here it is driving my 2 high precision meters