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> Ltc1867, Maximum analog voltage on REFCOMP
johnfulgor
Posted: May 30, 2017 10:10 am
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Good morning,
after reading the LTC1867 ADC datasheet:

Datasheet

I'm not 100% of the maximum analog voltages that can be applied to analog inputs and REFCOMP pin in normal working conditions. The datasheet writes about a 4.096S "typical" value. but I need to use 5V as reference and maximum input voltage (the same level of the IC digital supply). Is this possible?
Have a nice day.


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gremlinsa
Posted: May 30, 2017 05:27 pm
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5V on the input pins is correct...
However, REFCOMP is an output pin and 4.096V is the "typical" reading you can expect to get off it...


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Sch3mat1c
Posted: May 30, 2017 09:10 pm
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Buh. They show VREF supplied from internal ref via resistor, but they don't say that you can overdrive VREF, instead they say, pull it to GND which disables the buffer and then overdrive REFCOMP with your reference.

But they way they draw it implies there's a buffer that you can overdrive the input of, which apparently isn't true. It wouldn't be a buffer anyway, because VREF is 2.5V, and REFCOMP is 4.096V, for which I have a rant laying around somewhere, but anyway that means there's gain between the two, yet they don't specify the gain, if it operates as an amplifier at all (despite, seemingly very intentionally, drawing it that way).

Anyway, the voltage limits are VDD+0.3, so you can tie that to VDD just fine. It's not obvious if that will upset anything else inside, like if they need that 0.906V of headroom for the comparators and amps inside to work correctly.

Curiously, they don't say VREF and REFCOMP have diodes to VDD, which implies they could go much higher than 0.3V above VDD, or that their voltage limit isn't actually dependent on VDD. It would be ... interesting, to find out why they are specified this way.

This doesn't seem to be one of the better written LT datasheets. YMMV.

Tim


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johnfulgor
Posted: May 31, 2017 02:47 pm
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Hello,
CAREFULLY reading the datasheet I've found an explicit reference to "5V external REFCOMP" at page 6. So it means that the IC can work in this condition with no problem.
Thank you, anyway!
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Sch3mat1c
Posted: June 01, 2017 11:19 pm
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Ah, cool. There you go, then. smile.gif


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