OPA991: Drive capabilities

Mark Philip Felipe

Part Number: OPA991
Other Parts Discussed in Thread: OPA4991, TINA-TI

Hi Team, seeking your assistance.

The opamp being used is an OPA4991, but I would like to refer to the single OPA991 to make my question easier.

The first stage on the whole circuit will help me understand how I can proceed further on and it's a simple non inverting amplifier gain stage. I want to know if, when powered at 9v, can the opamp drive the load created by the input resistor value of 270R and the feedback resistor being 735R? I want to know if this is too low for this opamp or if it will affect the sound in any less desirable way?

A second question is, I am using 10k resistors to form a Voltage Reference. Would it be safe to use smaller value resistor here too to reduce Johnson's noise? If so, how low can I go using this opamp?

Thank you.

-Mark

over 2 years ago

0 kai klaas69 over 2 years ago

Guru 140200 points

Hi Mark,

figures 6-21 and 6-22 of datasheet can help to answer the first question. See also figure 6-14 and 6-15 and the "open-loop voltage gain" and "common mode rejection" specifications in the "Electrical Characteristics".

The second question can only be answered when seeing the schematic.

Kai

0 Robert Clifton56 over 2 years ago in reply to kai klaas69

TI__Mastermind 20770 points

Hi Mark,

For the second part of the question, like Kai said, it would be difficult to answer without seeing the schematic. It's also difficult to answer without knowing what the system requirements are for the noise levels!

While THD+N isn't something Tina-TI or Pspice, it can simulate johnson's noise. I recommend building the circuit in one of our recommended spice programs to adjust the values and simulate the results.

If you need any assistance with how to run noise simulations or getting started with Tina-TI, I recommended taking a look at the TI Precision Lab videos! Link to one of the noise simulation are here: 8.7 TI Precision Labs - Op Amps: Noise - Measuring system noise.

Let us know if you need any additional help!

Best Regards,

Robert Clifton

0 Nuno Maio over 2 years ago

Prodigy 90 points

Part Number: OPA991

I have tried this before but here we go again.

Regarding the OPA991 and a circuit I've been working on which is an onboard bass preamp. The first stage is a non inverting amplifier with a gain of about x3.8 and it's what I'd like to focus on.

At the moment this stage has 2k as input resistor and 5.6k as feedback resistor. The rest of the preamp, which consists of a full Tonestack, has been adjusted to as low as I can get due to parts availability and therefore I'm not including it.

I'm not very good at interpreting the diagrams and figures shown on the datasheet, therefore I'm hoping someone can help me understand If my intention is helpful for reducing thermal noise or not at all.

I want to get less thermal noise from the overall circuit and I'm thinking of reducing, as much as possible, the value of the resistors.

So, my specific question is, could the OPA991 be able to properly drive 270R as input resistor plus 735R as feedback resistor on this specific opamp? Would this help with reducing the Johnson's noise of the circuit?

I have looked at the datasheet and I'm not sure I understand what I'm reading. My inittial interpretation is that it's actually noisier with the values I'm proposing. I must be wrong!!?

0 kai klaas69 over 2 years ago in reply to Nuno Maio

Guru 140200 points

Hi Nuno,

yes, this would decrease the noise. But when the resistances are becoming too low, the distortion can increase. See figure 6-21 of datasheet. Usually, a decrease of resistances only makes sense, if the input signal is very low. What is the amplitude of your input signal?

Don't forget to mount an isolation resistor at the output of OPAmp. See section 7.3.4 of datasheet. An oscillating OPAmp can appear to be very noisy.

Kai

0 Caro Walter over 2 years ago in reply to kai klaas69

TI__Genius 17487 points

Hey Nuno,

We also have a calculator available on ti.com:

https://www.ti.com/tool/ANALOG-ENGINEER-CALC?keyMatch=ANALOGENGINEERCALCULATOR

All the best,
Carolina

0 Mark Philip Felipe over 2 years ago in reply to Robert Clifton56

TI__Genius 9705 points

Hi Robert, attached is the schematic. Thank you.

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0 Robert Clifton56 over 2 years ago in reply to Mark Philip Felipe

TI__Mastermind 20770 points

Hi Mark,

Just to clarify, when you mark the capacitors as .1 that's 0.1uF?

Best Regards,

Robert Clifton

0 kai klaas69 over 2 years ago in reply to Robert Clifton56

Guru 140200 points

Hi Robert,

seems to have to do with this thread:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1090427/opa991-non-inverting-amplifier-resistors-help-less-thermal-noise

Kai

0 kai klaas69 over 2 years ago in reply to Nuno Maio

Guru 140200 points

Caution, double thread!

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1088243/opa991-drive-capabilities

Kai

0 Robert Clifton56 over 2 years ago in reply to kai klaas69

TI__Mastermind 20770 points

Thank you Kai!

Hi Mark,

It looks like we have a double thread going on here. We have several tools that could help you to determine the system noise and the noise that each of the resistors contribute.

For spice simulations, I recommend taking a look at this video: 8.5 TI Precision Labs - Op Amps: Noise Spice Simulation. Also to help you out I have attached the simulation circuit so that you or the customer can make adjustments to the resistor values to see the overall affect! E2E - OPA991 Drive Capabilities.TSC

We also have a calculator that can do integrated noise and thermal noise of resistors. It can be found here. Please note that it requires to be downloaded.

https://www.ti.com/tool/ANALOG-ENGINEER-CALC?keyMatch=ANALOGENGINEERCALCULATOR

Let us know if you have any other questions!

Best Regards,

Robert Clifton

0 Nuno Maio over 1 year ago in reply to kai klaas69

Prodigy 90 points

The input signal will vary, but they will be guitar/bass magnetic pickups.

0 Nuno Maio over 1 year ago in reply to Robert Clifton56

Prodigy 90 points

Thank you a lot for the help guys, and also, sorry for the double thread. Now, i have downloaded the calculator but I'm having some trouble opening the simulator circuit labelled E2E - OPA991 Drive Capabilities.TSC. When I click on it a TSC file gets downloaded but I cannot open it and I do not know what to do with it. AM I missing something pretty basic?

0 Nuno Maio over 1 year ago in reply to Robert Clifton56

Prodigy 90 points

Yes, 0.1uf/ 100nf

0 kai klaas69 over 1 year ago in reply to Nuno Maio

Guru 140200 points

Hi Nuno,

when having an electric guitar connected at the input of OPAmp the winding resistance of pick-up will dominate the noise. A winding resistance of 7k will give a thermal noise (also called Johnson noise) of 11nV/SQRT(Hz). If you set the resistor at the -input of OPAmp to signal ground to 2k2, this will give an additional noise of 6nV/SQRT(Hz), resulting in a noise sum of 12.5nV/SQRT(Hz), according to

SQRT (11^2 + 6^2) = 12.5

This gives an increase in noise of 20 x log (12.5/11) = 1dB which is not detectable with your ear.

If you additionally take into the account the 10nV/SQRT(Hz) noise of OPAmp, the noise increase due to the 2k2 resistor becomes even only 0.7dB.

So, there's no need to take a 270R. A 2k2 resistor will fully do. This will also save battery current.

Kai

0 kai klaas69 over 1 year ago in reply to Nuno Maio

Guru 140200 points

You would need to install TINA-TI first:

https://www.ti.com/tool/TINA-TI

Kai

0 Robert Clifton56 over 1 year ago in reply to kai klaas69

TI__Mastermind 20770 points

Hi Nuno,

I'm sorry I didn't mention that you will need to have Tina-TI installed before you can read .TSC files. The link Kai generously provided is where you can download Tina-TI.

Best Regards,

Robert Clifton

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OPA991: Drive capabilities