ADS1294: Effect of connecting unused pins to GND or RLDOUT instead of AVDD

user4725373

Part Number: ADS1294
Other Parts Discussed in Thread: ADS1292

Hi,

Our next design uses an ADS1294 (TQFP package). For safety reasons, we need to connect some unused I/Os to RLDOUT or GND and I was wondering if that's acceptable and what would be the effect. Here are the connections we need:

IN4P to RLDOUT
IN4N to GND
WCT to GND (we don't use it)
RLDREF to GND (we use the internal AVDD/2)

It this acceptable? On a similar thread (https://e2e.ti.com/support/data-converters/f/73/p/549242/2005819#2005819

over 5 years ago

0 user4725373 over 5 years ago

Expert 1645 points

It seems I can't edit the original post and my last sentence got cut.

"Is this acceptable? On a similar thread (see linked thread), it was said that this would only affect the power consumption. How would it affect it?"

0 user4725373 over 5 years ago in reply to Alexander Smith

Expert 1645 points

Hi Alex, thanks for the answer

I realized I accidentally wrote ADS1292 in my original post, whereas it's the ADS1294. I fixed it. I'm guessing Figure 19 of the ADS1292 is similar to Figure 25 of the ADS1294.

By accidental short-circuit, you mean sending a wrong configuration to the device?

0 Alexander Smith over 5 years ago in reply to user4725373

TI__Mastermind 48610 points

Happy to help!

You are correct.

Yes, I simply meant to make sure that you're not accidentally grounding another part of the device by incorrectly switching the MUXs.

0 Alexander Smith over 5 years ago in reply to user4725373

TI__Mastermind 48610 points

Hi,

Thanks for your post!

These connections are acceptable as long as a short circuit is not accidentally created. Take a look at Figure 19 in the datasheet for reference. The power consumption will increase due to leakage.

0 user4725373 over 5 years ago in reply to Alexander Smith

Expert 1645 points

Thanks again, Alex!

One last thing. Since we don't use the 4th channel, it will be powered-down (PD4 = 0). The datasheet recommends to set MUX4[2:0] to 0b001 (input short) in that case. By looking at Figure 25, it looks like this will provide (AVDD+AVSS)/2 to the input of the internal opamps, while still disconnecting IN4P/IN4N from the circuit. Is that correct? I don't want to accidentally short RLDOUT to GND.

Thank you,
Fred

0 Alexander Smith over 5 years ago in reply to user4725373

TI__Mastermind 48610 points

Hi Fred,

You are correct!

9.3.1.3.1 Device Noise Measurements
Setting CHnSET[2:0] = 001 sets the common-mode voltage of (AVDD – AVSS) / 2 to both inputs of the channel. Use this setting to test the inherent noise of the device.

0 user4725373 over 5 years ago in reply to Alexander Smith

Expert 1645 points

Thank you!

0 Alexander Smith over 5 years ago in reply to user4725373

TI__Mastermind 48610 points

You're welcome!
Please let us know if you have additional questions and have a good day!

0 user4725373 over 5 years ago in reply to Alexander Smith

Expert 1645 points

Hi Alexander,

I'm now wondering if I can connect TEST_PACE_OUT1/2 to GND instead of AVDD.

From the datasheet, TEST_PACE_OUT1/2 are set as output for Pace Detect, which we don't use, but also for outputting the internal test signals. Since we use internal testing (without external signal), is it wise to connect those pins to GND?

Thank you,
Fred

0 Alexander Smith over 5 years ago in reply to user4725373

TI__Mastermind 48610 points

Hi Fred,

If it is acceptable in your design, I would leave them floating.

A couple of threads that may help:
e2e.ti.com/.../2071024
e2e.ti.com/.../172573

0 user4725373 over 5 years ago in reply to Alexander Smith

Expert 1645 points

Thank you Alex, that explains why we weren't able to make the self-test work.

Have a good day,

Fred

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ADS1294: Effect of connecting unused pins to GND or RLDOUT instead of AVDD