OPA683 Saturation in Simulation

Samuel Jaffe

Mastermind 25590 points

Other Parts Discussed in Thread: OPA683, OPA684

Hi E2E,

I have a customer who's testing the OPA683 in TINA with the following setup:

+V rail = 5V
- V rail = GND
+ input = Sine wave, 200kHz, 200mVpp, 2.5VDC offset
Gain of 200
Disable is tied to 5V

He is expecting to see saturation but instead sees a 1.89VDC output.

The op amp should saturate with these conditions. Would the simulation model saturate with these conditions? Is the model trustworthy in this setup?

Thanks,

-Sam

over 7 years ago

0 Samir Cherian over 7 years ago

TI__Genius 12631 points

Hello Samuel,
I ran some tests and it looks like the combination of high gain and low supply voltage seems to mess up the model. I will bring this up with the modeling team and see if they can work on fixing this.

-Samir

0 Samuel Jaffe over 7 years ago in reply to Samir Cherian

TI__Mastermind 25590 points

Samir,

Thanks for looking into this. Please keep me posted on any developments.

Thanks,

-Sam

0 John Miller - Sensing over 7 years ago

TI__Mastermind 34405 points

Sam,

To resolve this we really need the customer's schematic.
Please upload it to this thread when it is available..

Regards,
John

0 Samuel Jaffe over 7 years ago in reply to John Miller - Sensing

TI__Mastermind 25590 points

John,

The customer is not able to provide a schematic. How can we proceed in support from here?

-Sam

0 John Miller - Sensing over 7 years ago in reply to Samuel Jaffe

TI__Mastermind 34405 points

Understood Samir.
Is the gain positive as shown in the thread or could it be negative?
John

0 Samuel Jaffe over 7 years ago in reply to John Miller - Sensing

TI__Mastermind 25590 points

All I know is that the input is applied to the positive terminal through an AC coupling capacitor and supply voltage is 5V.

The customer swapped the OPA684 for the OPA683 and it works! Would swapping the op amps truly fix the output or is it an issue with the OPA683 model?

Note: When he clicks on the model, he is not able to change the parameters. Maybe it's just HIS model. Thoughts?

Thanks,
-Sam

0 Samuel Jaffe over 7 years ago in reply to Samuel Jaffe

TI__Mastermind 25590 points

Thoughts?

Thanks,
-Sam

0 John Miller - Sensing over 7 years ago in reply to Samuel Jaffe

TI__Mastermind 34405 points

Samuel,

Without more information we can only speculate about what might be going on.

Given that, a OPA683 TINA reference circuit was modified to have G = +200, Vin = 200mVpp, VinDC = 2.5V, Vsupply = +5V, f = 200kHz.
Rf = 1.2K, Rg = 6 ohms.

It seems to behave as expected; it biases up to the correct DC voltages and the output is saturated as it should be.

The key was to bias one end of the 6 ohm gain-setting resistor to 2.5V, instead of having it grounded.
When it was grounded, the output was saturated as the customer has reported.
When it is biased to 2.5V the circuit seems to work fine.

The circuit and the output are shown below. The modified OPA683 TINA reference circuit is attached.

John

OPA683_ModRefCkt.TSC

0 Samuel Jaffe over 7 years ago in reply to John Miller - Sensing

TI__Mastermind 25590 points

John,

Looks good. The customer thinks it might be a bandwidth issue so he's moved on to another op amp. The circuit is working now. Thank you for your support and your thorough answers.

-Sam

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OPA683 Saturation in Simulation