OPA4991: OPA4991 IDR TIA Non Linearity and Saturation

Hi Krishna, 

Please take a look at this reference design circuit for a TIA circuit: https://www.ti.com/tool/CIRCUIT0020 

We recommended adding a feedback capacitor to the design in order for stability and bandwidth purposes and the calculations for the appropriate values can be seen in the design guidelines. This may be why the circuit is saturating. 

Please let me know if you have further questions.
Thank you!

Best Regards,
Ashley

Ashley

We did put a Capacitor 22pF Initially, it did not solve my problem

However we also checked with 120pF Capacitor as suggested by you  in Feedback Path and found 2 Channels 1, 2 are not linear and Saturating.

Channel 3,4 is fine except 2 readings each and can we know how to resolve this

Please note ours is a pure dc application.

Attached Excel Sheet with Plots

Thanks n regards

Krishna.P.N./cfs-file/__key/communityserver-discussions-components-files/14/TIA_5F00_OPA4991IDR_5F00_19072023_5F00_Feedback_5F00_Cap.xlsx

2772.TIA_OPA4991IDR_19072023_Feedback_Cap.xlsx

Hey Krishna, 

Could you please share your schematic?
I don't understand how CH1 and CH3 have the same input conditions but very different outputs, similarly CH2 and CH4.

All the best,
Carolina 

Carolina

Attached Schematics as well as repeatability results of 25/07/2023 report.

Simulation of TINA also shows linearity but there is a saturation at 4V not swing up to +5V. Have sent this to you by email for TLC2272IAR Report OPA499ID_25072023.pdf

Thanks 

Krishna.P.N.

Hey Krishna, 

Thank you for the additional resource, the two conflict each other but I think the correct one is the one I built in TINA, see conflicts here: 

Could you please confirm that all 4 channels on the op amp have the exactly what the TINA schematic shows? 

TIA_4991.TSC

Carolina

The 1 Mega Resistor in the document is a typo error and we have used all 4 channels same Rf=1K Feedback resistor.

Thanks

Krishna.P.N.

Hey Krishna, 

Understood, since the customer schematic matches the simulation but the result is different, I will test in lab to see if there is a mismatch between simulation and real-world. Please allow a few days. 

All the best,
Carolina 

Please do let us know the results. I believe this is of interest to multiple interested parties.

Hey Andrew, 

Thanks for the ping and the patience.
Lab measurements are a bit more time consuming than simulations, I have to build the circuits, wait on samples from ti.com, etc. 

I will update on this thread when I have the results and my recommended next steps. 

All the best,
Carolina 

Carolina

Any updates on this, our Customer is following up with us

Thanks

Krishna.P.N.

Hey Krishna, 

I have done some preliminary testing in the lab, so far the results look normal (matching the simulation results). 
I will share more in depth lab results tomorrow. 

All the best,
Caro

Carolina

Please do share as we are curious and is there any change to schematics, hopefully not as board design is complete

Thanks

Krishna.P.N.

Hey Krishna and Andrew, 

The lab measurements matched simulation results. Please see attached. 

OPA4991 TIA.pptx

All the best,
Carolina 

Carolina/Andrew

Your results are surprising due to following 

1)We have used 1K, 120pF capacitor for Each Channel and Untested Channels are configured with similar components.

1a)Only tested channel current inputs are provided and outputs are monitored with untested channels open.

2)SMU- Keithley 2450 and 6.5 Digit DMM GWINSTEK 8261A

Samples from TI-USA

3)Can we get 3-4 Parts of the Batch that you have tested so that if solves the problem & we can close this issue??

4)Can you please send the same to TI-India and then we can collect the same through Courier as there is Customs Clearance issues here if we get it shipped through TI-China or TI-Malaysia

Please respond to this

Thanks n regards

Krishna.P.N.

Thank you, Carolina! We will continue using the OPA4991 as a TIA in our design.

Hey Krishna, 

I believe the issue seen in testing is due to setup not the parts. 
Could you please confirm that this is not the wiring: 

In this scenario, the source current will split unevenly across the op amps (as current always chooses the path of least resistance). 
I ask because in my preliminary testing, I set it up like this and got unexpected results (similar to those seen in the test report). 

All the best,
Carolina