TLC2574 unstable

Martin Grinfeld

Other Parts Discussed in Thread: TLC2574

We have a new design using the TLC2574 in single ended mode. We use as reference voltage the ADR434AR with adjustable output, set to 4.0V. We connected several external sources and noticed that the readings are not stable. We even connected the reference voltage (4.0V) as one of the inputs just to test the stability. We read the ADC once a second and get readings changing in the range of 2850 to 2870. We expected changes of 1 LSB (+/- 5 mV) but not that much (equivalent to almost 100 mV). What can be the reason of that unstableness?

Thank you for your help

over 13 years ago

0 Tom Hendrick over 13 years ago

TI__Guru**** 174496 points

Hi Martin,

Welcome to our forum! Its hard to say where the trouble might be with the TLC2574 using only the information given above. Can you tell us how you have your inputs connected to the ADC and how you have the configuration register set? Is it possible for you to provide a schematic?

0 Martin Grinfeld over 13 years ago in reply to Tom Hendrick

Prodigy 90 points

Thank you Tom. The configuration register is set as follows:

#define CFR_DEFAULT 0xA04 // bipolar binary, short sampling, internal osc.
// single ended, one shot, EOC

Attached is a picture with the schematic. Please let me know if you need any further information.

0 Tom Hendrick over 13 years ago in reply to Martin Grinfeld

TI__Guru**** 174496 points

Hi Martin,

How do you have DGND and AGND1/2 strapped on the TLC2574? They should be tied together, preferably near the chip. What is the nature of your analog input? Do you have any buffers there to help drive the input to the ADC? Here are some articles that describe ways to drive the inputs of a successive approximation register (SAR) ADC like the TLC2574 which you might find useful:

Designing SAR ADC drive circuitry, Part I: A detailed look at SAR ADC operation

Designing SAR ADC drive circuitry, Part II: Input behavior of SAR ADCs

Designing SAR ADC drive circuitry, Part III: Designing the optimal input drive circuit for SAR ADCs

Since you are sampling relatively slow, there are some tricks you can use to extend the sampling time of this ADC. First thing is that you can slow down your SCLK. The long sampling option might also give you better results - essentially that doubles your acquisition time, giving the inputs to the TLC2574 more time to settle. Using the CSTART feature extends sampling time as well.

0 Martin Grinfeld over 13 years ago in reply to Tom Hendrick

Prodigy 90 points

Hi,

The DGND and AGND are tied together only in one point next to the chip. Analog inputs come from different sensors (temperature, pressure, etc.), having the signals amplified by op-amps. The matter is that as I mentioned before I used the reference voltage instead of one of the inputs just for tests and still had the same stability problem.

I will try your suggestions and keep you posted. If you have any more ideas please let me know. Thank you.

0 Martin Grinfeld over 13 years ago in reply to Martin Grinfeld

Prodigy 90 points

Hi,

I tried slowing down the SCLK and changing to long sampling but didn't observe significant improvement. CSTART is not an option.

Can the power supply cause those problems? Any ideas?

0 Tom Hendrick over 13 years ago in reply to Martin Grinfeld

TI__Guru**** 174496 points

Hi Martin,

Yes, a noisy power supply and/or reference can cause problems. The layout as well could cause problems. Keeping you analog input traces short and having bypass caps near the ADC pins should help. Another thing you can do is to try and sample the same channel 3-4 times in quick succession, see if the conversion results are more stable. If they are, you might simply throw out the first few sample or average them together.

0 Martin Grinfeld over 13 years ago in reply to Tom Hendrick

Prodigy 90 points

Successive sampling and averaging samples together makes indeed a significant improvement. That and an improvement in the board ground layout might solve the problem. I would appreciate if you can send me an example of a recommended ground and power layout for the TLC2574. Thank you

0 Martin Grinfeld over 13 years ago in reply to Martin Grinfeld

Prodigy 90 points

I will appreciate if you can send me an example of a recommended ground and power layout for the TLC2574. Thank you

0 Tom Hendrick over 13 years ago in reply to Martin Grinfeld

TI__Guru**** 174496 points

Hi Martin,

The Users Guide for the TLC2574 board that we had a few years back is attached. In that document (pages 26-28) there is a depiction of the power and ground planes we used for the board. Hopefully this helps!

8080.slau110.pdf

0 Martin Grinfeld over 13 years ago in reply to Tom Hendrick

Prodigy 90 points

Thank you Tom for your help.

Martin

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TLC2574 unstable