OPA657 current noise in TINA-TI v9 seems to be wrong

Hello alberto,
Thanks for bringing this to our attention. Our modeling team is in the process of verifying our SPICE models for errors .The OPA657 is also on the list and we will add the OPA657 to the bug list and ensure it is corrected. The current projection is to have the OPA657 fixed by end of May.

One minor thing I should add is that the OPA657 is a "decompensated amplifier" so is not stable in unity gain. Not a big deal here but once you start running dc/ac/transient simulations you should keep this in mind.
-Samir

Dear Samir,

thank you for your reply..

Please, let me know as soon as you have the corrected model.

Also, I would be grateful if you could suggest me an op-amp with similar

specs and with a  fully working TINA-TI noise model, in order to

start my noise analysis as soon as possibile. Does OPA656 have a fully qualified

TINA noise model (both current and voltage) ? Any other suggestions ?

The voltage follower configuration you have seen in my TINA schematics

has only be used to check the noise model. I am aware that OPA657 is

unstable for A>7.

Kind Regards,

Alberto

Alberto,

  The OPA656 will also suffer from the same issue. Can you provide me with a schematic of your circuit and I will look to see how I can back calculate the correct total system noise.

-Samir

Dear Samir,

thank you for your help.

Please find attached the TINA schematic of the circuit. It is a TIA based on the

BurrBrown's classic embedded gain architecture. The TIA is intended to

amplify the 1/f current noise of MOS transistors. The TIA output goes to a

signal analyzer giving the noise power spectral density (nV/sqrt(Hz)) in the

range: 0->100 KHz.

The Rsource (see schematic) is > 1 MEG, I guess OPA657 is the best

candidate. Any advice is very very welcome.

I would like to estimate current and voltage noise Referred-to-Input.

I look forward to hearing from you.

Kind Regards,

AlbertoTIA_noise_bench.TSC

Alberto,
The closed loop bandwidth of your 1st stage seems to be only 250kHz. At that point the OPA657 noise is around 1fA/rtHz which is very close to what is specified in the datasheet. Even with your 20MOhm resistor the output noise due to current noise is 20nV/rTHz. The thermal noise due to the resistor is around 500nV/rTHz! I don't think you need to worry about the current noise error in your particular case. Do you agree?

Also, when I look at the OPA657 + THS4001 circuit it looks like you have 2 cascaded amps in negative feedback which is positive feedback. I think you need to reverse the polarity of the THS4001 amp.

The current noise of a FET is extremely difficult to measure and challenging....what is the magnitude of the current you are expecting to measure? One thought is rather than working with a transimpedance amplifier which is harder to compensate can you shunt the MOSFET with a large resistor, say 1MOhm and configure your amplifier as a voltage gain amplifier?

-Samir

Dear Samir,

The closed loop bandwidth of your 1st stage seems to be only 250kHz.

-> you are right:.However, the TIA has an overall intended bandwidth of 100kHz, so that's not a problem

The thermal noise due to the resistor is around 500nV/rTHz! I don't think you need to worry about the current noise error in your particular case. Do you agree?

-> yes, I agree. However, my concern is the 1/f contribution at VERY low frequency, say 0.1-1 Hz. Could it be larger of resistor

thermal noise, such to cover my signal ?

Also, when I look at the OPA657 + THS4001 circuit it looks like you have 2 cascaded amps in negative feedback which is positive feedback. I think you need to reverse the polarity of the THS4001 amp.

-> 657 is configured as inverting , 4001 as NON-inverting, so the composite amplifier is inverting, isn'it ?

The current noise of a FET is extremely difficult to measure and challenging....what is the magnitude of the current you are expecting to measure?

-> the current noise of the device we are going to test is in the range 10^-13 - 10^-11 A/sqrt(Hz), see attached file:

One thought is rather than working with a transimpedance amplifier which is harder to compensate can you shunt the MOSFET with a large resistor, say 1MOhm and configure your amplifier as a voltage gain amplifier?

-> thank you for your advise. I will look into it.

Kind Regards,

Alberto

Hello Alberto,

  Please see my responses below:

However, my concern is the 1/f contribution at VERY low frequency, say 0.1-1 Hz. Could it be larger of resistor thermal noise, such to cover my signal ?

-> The datasheet shows the noise to be flat down to 10Hz so I am not really sure what happens at lower frequencies. It is extremely difficult to isolate and measure the current noise at these levels. Datasheets don't usually publish this for this reason. One thing you can do is contact the manufacturer of the datasheet you attached here and see how they made the measurement for the 100pA level. I would be very interested in knowing this myself.

 657 is configured as inverting , 4001 as NON-inverting, so the composite amplifier is inverting, isn'it ?

-> Incorrect. Irrespective of inverting or noninverting config., the feedback is always negative. To make the feedback positive on the THS4001 you should have OPA657 drive its inverting input and put the feedback from the output to the noninverting input. This would be positive feedback.

-Samir

Dear Samir,

the plot attached to my previous reply has been taken from:

IEEE ELECTRON DEVICE LETTERS, VOL. 36, NO. 4, APRIL 2015
Evidence of Correlated Mobility Fluctuations in
p-Type Organic Thin-Film Transistors
Gino Giusi, Orazio Giordano, Graziella Scandurra, Sabrina Calvi, Guglielmo Fortunato,
Matteo Rapisarda, Luigi Mariucci, and Carmine Ciofi

The Authors are specialist of very low noise measurements. The device under test is a p-typeOrganic Thin-Film Transistor.

We would like to be able to repeat their measures.

Any advice is very appreciated !

Regards,

Alberto

Please, let me know if OPA657 noise model has been updated.

If so, how can I dowloand the new model in TINA-TI ?

Kind Regards,

Alberto

Hi Alberto,
I checked with the modeling team and they ran into some issues during the model fix. They are looking to fix it. They are working on a couple of parts in parallel trying to verify that the fix works on all of them with no issues. Can you please check back in a month or so? In case it gets done earlier I will let you know immediately. Sorry for the inconvenience.
-Samir

Dear Samir,,

t
thank you for your quick reply.

Indeed, this delay is for me quite a problem. I hope the team will make it as soon as possible.

Please, let me know when you have news.

Kind Regards,

Alberto

Hi Samir,

is there any news about OPA657 updated current noise model ?
Please, let me know about this issue.
Could you please alert the team working on TINA noise model ?
Thank you for your time.
Best Regards,

Alberto

Alberto,
Sorry to keep delaying this. I spoke to the modeling team and they have promised that this will be complete by July 18th.

-Samir

Dear Samir,

thank you so much.
I hope they will keep the promise....
Regards,

Alberto

Hi Samir,

I am wondering if the modeling team held his promise. Did they modify the OPA657 TINA model ?
Are the noise specs now correctly modeled ?
Thank you for your help.
Kind Regards,
Alberto

Alberto,
I was promised an updated model this week. Once I get it I will run some quick checks to make sure everything is good and send you a draft copy.

Regards,
Samir

Dear Samir,

this is a very GOOD news !
I look forward to hearing from you.
Kind Regards,

Alberto

Hi Alberto,
I have been testing the model and a lot of minor issues look like they are fixed. With regards to the current noise we can get the noise to be flat at low frequencies but < 10kHz the noise begins to rise. We are now debugging the model to see where the problem could be. In the past we have usually ignored this since people don't care too much about the current noise of FET amplifiers. Can you please remind me if you were interested in the low frequency or high frequency behavior of the current noise?

Samir

Dear Samir,

Yes indeed, I am interested in a model giving reliable results for current noise at very low frequency. I am designing a TIA to measure
1/f noise in organic FET transistor and I need to simulate the opamp behavior at very low freq. Current noise is important to us due to the fact the source impedance of the device-under-test could be quite high.

Thank you for your help. Please let me know about this issue.
Kind Regards,

Alberto

Hello Albertoo,

Here is the updated model.OPA657_20160801.TSC