PGA855: PGA855 Input supply V

David Ridgway

Part Number: PGA855
Other Parts Discussed in Thread: ADS127L21, PGA854

Hello, the PGA855 has a recommended Vs min of 8V. How would the part function if the Vs was 5V? My input signal is AC coupled to remove the DC offset from my sensor.

Thank you,

David

5 months ago

0 Caro Gomez 5 months ago

TI__Mastermind 24722 points

Hi David,

Could tell me what the DC offset from the sensor is? The PGA855 is a instrumentation amplifier and therefore removes the common-mode voltage.

I think the supply voltage at such a low level would introduce a myriad of problems including certain nodes of the silicon not turning on. I can check in lab for you if you would like, but I would need more details about the input signal.

All the best,
Carolina

0 David Ridgway 5 months ago

Prodigy 40 points

Carolina, the open circuit (no sensor connected) is around 30V. with the sensor, the DC bias drops to 15V. The sensor is a single ended output but has been used in the past as a differential output, just that the neg leg of the sensor is tied to the system return.

0 Caro Gomez 5 months ago in reply to David Ridgway

TI__Mastermind 24722 points

Hi David,

Thanks for the quick reply, what is the amplitude of the signal introduced into PGA855 from the sensor?

All the best,
Carolina

0 David Ridgway 5 months ago in reply to Caro Gomez

Prodigy 40 points

The AC signal can be between 1uV to 2.4V.

0 Caro Gomez 5 months ago in reply to David Ridgway

TI__Mastermind 24722 points

Hi David,

What is the expected next step in the signal chain? an ADC/MCU?

All the best,
Carolina

0 David Ridgway 5 months ago in reply to Caro Gomez

Prodigy 40 points

Carolina, the next step is an ADS127L21 adc.

0 Ashley Kang 5 months ago in reply to David Ridgway

TI__Mastermind 19660 points

Hi David,

Are you trying to scale the entire input signal range of 1uV to 2.4V to the ADC? What are you expecting the full input range of your ADC to be?

Best regards,
Ashley

0 David Ridgway 5 months ago in reply to Ashley Kang

Prodigy 40 points

I would rather not have to go through my signal chain on this design. I would like to know more about what happens at the input of the PGA855 if it has less than 8V of input voltage. The max signal of 2.4V is less than the FSR of the ADC and that would be using G=1 of the PGA. What is the 8V minimum based on?

0 Caro Gomez 5 months ago in reply to David Ridgway

TI__Mastermind 24722 points

Hi David,

Understood, I will test it and ask the designers if there are expected failures if operating inside abs max but outside of recommended conditions.

All the best,
Carolina

0 Caro Gomez 5 months ago in reply to Caro Gomez

TI__Mastermind 24722 points

Hi David,

Sorry for the delay here, I had to figure out how to add such a large common-mode voltage to the input signal in lab and it took me a long time to figure it out.

My recommendation is to either power the input power supply of the PGA with a single-ended 36V or a ±5V. The output power supply can be powered by 5V single-ended to protect the ADC.

I show all the options in this PowerPoint: PGA855_30V_CM.pptx

I also show how a single-ended input power supply of 5V will not work as the AC coupled signal of 2.4V differential will be below the power supply and outside the linear range of the PGA, which is essentially nothing if VS+ is 5V and VS- is GND:

Please let me know if would like me to test this with the PGA854 (in case a higher dynamic gain is needed in the 1uV case described earlier).

All the best,
Carolina

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PGA855: PGA855 Input supply V