TLV2556: Mixed signal read on ADC

Hi Tom,

I agree to use a zero ohm resistor near ADC to short both the grounds. But my concern is then digital noise will interfere the analog ground and it will definitely affects the ADC readings. That is the reason I kept both the grounds isolated using ferrite bead.

Hi Rohit,

Generally speaking, ADC's usually have a clear 'analog' area and a separate 'digital' area.  With the TLV2556, pins 15 through 19 are the digital block and you can think of the rest of the chip as 'analog'.  If you look at the Absolute Maximum Ratings table (section 6.1) all of the input voltages are referred to GND (pin 11) and this also includes the REF+ and REF- pin.  If you have long traces separating DGND from AGND, you'll get some voltage drops across the various analog inputs.  Try replacing the bead(s) with zero ohms and take some measurements.

First, how fast are you running the TLV2556?  Are you scanning the inputs one by one?  Can you share some of you conversion results?  Maybe a table with the expected Vin, expected conversion result and the actual conversion result?

Hi Tom,

TLV 2556 is running on a clock of 10Mhz.

following are some reading on ADC pin and hexadecimal value read by controller

0.765V -> 4E1

0.772V -> 4F8

0.764V -> 4F0

0.809V -> 4E7

Regards,

Rohit Arora

Hi Rohit,

With your 4.096V reference and the available 4096 codes (12-bit converter), your LSB size is 1mV.  So, for 765mV in, you should get ~765 codes out or 0x2FD.  For each case above, you are getting ~1.6 times the correct result.  Is this consistent with your findings?

Hi Tom,

Sorry for confusion. This same ADC I am using with 2.5V reference also. These readings are taken from that ADC. I have used this same ADC at 4 places in the same board. It looks like by shorting both the grounds nearby ADC gives correct readings.

Hi Tom,

I have one more doubt, in ADC we have analog ground pin to handle Reference voltage and one digital ground pin to handle SPI interface. But in this ADC we have only one ground pin. Should I connect analog ground to Ref- and digital ground to GND pin?

okay, thank you

Hi Tom,

If I have to redesign the board to solve this problem, Will that connecting REF- to analog ground and digital ground to GND  solve the problem or I need to add some jumper nearby ADC for ground shorting.

I have shorted the both grounds in current design board, it helps to improve the noise. But what all changes needs to be considered for redesigning the board?

Regards,

Rohit

All of your 'ground' references have to be common to the GND pin of the TLV2556.  This includes your digital power supplies, your analog power supplies and the digital controller interface.  If you want to share your schematic and layout, we'll look over it for you.

Hi Tom,

I can share the screenshot of schematics and layout, but not possible to share the whole file.

Is it possible to have a meeting with you?

So I can show you the whole schematic and layout.

Regards,

Rohit Arora

I suppose we can do that.  Send me a note and we can try to schedule a meeting.

Great. Is it possible to have it right now?

rohit2.arora@capgemini.com this is my email id to join the meeting.

I'm not available before 10:30 US Central time.

Can we have at 11:30 US Central time?