Am I stupid, or does this chip just not work like its suppose to?  - Page 1
Forum section: electronics / general

author: TopherTheME
author_rank: Regular Contributor
timestamp: June 28, 2012, 02:46:53 am
content: Its been a long time since I've posted here but I'm at about my wits end and could really use some help.I've run into a dilly of a pickle using the MAX5494 Digital Potentiometer from Maxim. Its a simple two pot per chip IC which has a rated full resistance value of 10k with 1024 steps, or so the datasheet says. The relationship between wiper to ground resistance (Rwl) is suppose to follow this formula,Rwl = D/1023*Rtot + Rzwhere D is the digital value being sent, Rtot is the total resistance (10k), and Rz is an error offset. So I have this thing integrated into a project for work and much to my surprise it doesn't seem to work! The pot only seems to be linear in the first 1k to 4k region and after that its resistance goes all over the place. For example, sending it a value of 512 should give a resistance of around 5.5k but its actually about 6.5k. A value of 750 gives a resistance of 16k. A value of 900 gives 11k (way over 10k!), but a value of 1023 gives 7.5k.I thought it might be code being screwy but I've scoped the SPI bus and double counted the bits and my code is solid. Double checked the schematic and pinouts and its all good as well. Voltages are all within working levels. I just can't figure out what the problem is and I'm suppose to be testing this thing tomorrow. Any idea what it could be? Do you think the manufacturer just may have screwed up a batch of chips (I've got several, they all do it but with random values).

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 28, 2012, 04:22:29 am
content: Is the signal voltage within recommended operating conditions?

author: Bored@Work
author_rank: Super Contributor
timestamp: June 28, 2012, 05:12:01 am
content: From the datasheetQuoteI.e. you are using an IC for something it is not intended for. The bold sentence is even in bold in the datasheet. Don't you think it means something if the manufacturer sees the need to tell you that in bold letters? You also might want to consider Figure 4 and Figure 5.And you are using an equation which is for another IC type:Quote

author: free_electron
author_rank: Super Contributor
timestamp: June 28, 2012, 12:18:49 pm
content: In other words : they only guarantee the division ratio... Not the absolute resistance.If you program it for 1/2 then it may be 10k and 10k... If you go to 1/4 you 'think" it should be 5k and 15k but it may be 10k and 30k.. The ratio is still correct. The absolute resistances did change...Page 12 of the datasheet has the plots of what the resistance really does in function of code ...You can olso not send any current through the wiper in these things... So don't troy to make something like a variable gain amplifier with this and an opamp. You need to buffer the output voltage coming from the wiper.... The resistor from the ladder to the output pin is not a fixed value ! And any current through that resistor will cause a voltage drop that may be large enough to switch off an internal mosfet.These thing are not simply 1024 resistors in series with a 1024 channel multiplexer behind it... They are a sequencer that switches resistor chains in parallel and series. The 'block schematic' is NOT how it is done internally....

author: SeanB
author_rank: Super Contributor
timestamp: June 28, 2012, 03:56:54 pm
content: You been bitten by them as well FE?

author: free_electron
author_rank: Super Contributor
timestamp: June 28, 2012, 05:55:40 pm
content: no, not been bitten. i just know how these things work.if you look in the appnotes they are always carefull connecting the output of that thing to an opamp input..... that says enough.there are TRUE potmeters out there but they are 64 steps or 128 steps and that's it. no more than that. it's very hard to make a monotone ladder in ic's...Microchip has a good one. analog devices too.