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[SOLVED] How to make LTC2400 ADC work

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gary36

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Hi

I am having trouble in testing 24 bit LTC2400 ADC. The data sheet specifies free running mode, where ADC generates serial clock along with data if chip select in permanently grounded. I tried to do so and I see no pulses at all.
 

Additional requirement is that SCK must not be pulled down by an external load, in case of doubt apply a stronger pull-up. Not discussed in the datasheet, but I guess FO input should not be floating.
 

Hi FvM

SCK and SDO floating, CS is permanently connected to ground as shown.

LTC2400.png
 

The schematic also shows FO connected either to GND or VCC.

My doubt was, if you "watch" SCK output for pulses, it's probably not floating because you have connected a load, at least an oscilloscope probe. As far as I understood it's essential that SCK can be actually pulled high by the internal weak pull-up during power up.
 

Hi FvM,
FO is connected VCC. SCK and SDO are connected to scope. I tried pulling SCK to high, bit I found no pulses.
I pulsed CS from high to low just after power up. But I find no pulses at output as well. I tried this with 2 LTC2400 ADCs and I guess they are not faulty.
 

Hi,

according post#3:

I don´t think posting a schematic from a datasheet is useful for finding a circuit´s problem. (all we can do is to verify if the datasheet is correct, which in most cases is)
But usually the problem isn´t the datasheet schematic, but the real built circuit.
And if you want somebody to find an issue in your real circuit, then you need to give us this information.

And often the users say "I´ve built it exactly like in the datasheet" .... but the users miss to recognize that additional connections, missing capacitors, or simply a bad PCB layout may cause malfunction.

--> please give the informations (schematic, PCB layout, photo of wiring....) of your circuit.

Klaus
 

Hi KlausST

Attached is the schematic for LTC2400

LTC2400_schm.JPG
 

Hi,

Why do you have "+5V" and "+VCC". What is +VCC?

SCK, SDO and CS connections are not clear.

Especially it seems you treat SCK as an input, but I assume it should be an ouptut in free running mode.

Klaus
 

Hi KlausST,
VCC is +5V, CS is permanently grounded ( not shown in the figure). SCK and SDO are connected to scope
 

Hi,

SCK and SDO are connected to scope
Click to expand...
Then - Why don´t you show use the scope picture?

Without showing us your PCB layout we can not verify this datasheet recommendation:
GND (Pin 4): Ground. Shared pin for analog ground,
digital ground, reference ground and signal ground. Should
be connected directly to a ground plane through a minimum
length trace or it should be the single-point-ground
in a single point grounding system.

Kind remark: Please don´t let us ask for every piece of information (several times). Please try to give all informations at once.

Klaus
 

Hi KlausST

I am will not be able to share complete PCB layout. But the layout is done following the guidelines recommended in LTC2400 eval board. No tracks run in between pins and ground is poured as recommended in eval board

- - - Updated - - -

Hi everybody

I was able to finally get the output. Something subtle in LTC2400 not mentioned in data sheet. SCK must be pulled high through a resistor and CS must be initially high and then it must go low. Any other conditions, ADC does not work.
 

Something subtle in LTC2400 not mentioned in data sheet. SCK must be pulled high through a resistor and CS must be initially high and then it must go low. Any other conditions, ADC does not work.
Click to expand...
The pull-up thing was mentioned before. Question is if the LTC2400 misses any internal SCK pull-up and thus won't work even without any external load. In this case it's possibly a counterfeit IC missing the original specification. I rather guess you have some external circuit connected pulling down SCK during power up. Same thing with CS conditions. It's clearly said to work when tied to GND, there may be additional requirements regarding VCC ramping, e.g. to be monotonous.

In other words, I don't fully believe your conclusion, but good that you found a solution.
 

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