ADC TLC3544 Data Output Issues

Richard Stewart

Other Parts Discussed in Thread: TLC3544

Hi,

I am a student working with the TLC3544 and am having some issues when sampling.

The 14-bit ADC is sampling up to values of 4032, but then the value jumps to 12288. Sampling data is accurate between 0-4032 and 12288-16383, But there is the jump between these ranges, the output will jump from 4032-12288.

The datasheet says the SDO output format is as follows: [14-bit Data][2-bit Don't Care] . With MSB on the left here, so the output is 000000000000XX.

Using an oscilloscope I have looked at the SDO output. The first 8 bits can be seen to jump between 00111111 and 11000000 when input voltage is varied carefully around this level.

I would appreciate if someone could assist me with diagnosing the problem, any questions please ask.

over 13 years ago

0 Tom Hendrick over 13 years ago

TI__Guru**** 174226 points

Hi Richard,

Can you tell us how you have the registers and control lines (FS, /CS, CONVST, etc) configured and let us know what voltage input you have on the part when this jump change happens? Would it also be possible for you to send along an oscilloscope shot showing your SCLK, SDI and SDO? Including the EOC/INT would be nice if you can manage that.

0 Richard Stewart over 13 years ago in reply to Tom Hendrick

Prodigy 50 points

Hello,

Sorry for taking a long time to get back to you.

The jump change happens at 2.5V with a 5V Reference. I have discovered that the ADC samples properly when I am probing SCLK pin on the PIC! When I remove the probe from SCLK, the problem comes back.

If i remove the ground clip from the probe, the problem still exists when the probe is touching.

I do not have any pull up/down resistors in the circuit.

Many thanks for your help, if you require scope plots I can provide these soon.

0 Richard Stewart over 13 years ago in reply to Richard Stewart

Prodigy 50 points

I have figured out the issue!

SPI mode was configured wrongly for this device. When the probe was connected to SCLK the capacitance was fixing the timing issue.

Many thanks for your help.

0 Tom Hendrick over 13 years ago in reply to Richard Stewart

TI__Guru**** 174226 points

Great news! Thanks for the follow up!

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ADC TLC3544 Data Output Issues