OPA140: There are two different OPA140 tina simulation files and the results are very different

Please run transient mode for 1ms

Hello Aykut,

It looks like the TINA-TI op amp selection under Spice Macros hasn't been updated in quite some time. The one listed there is the original simulation model from 2011. It uses an earlier version of TI's sophisticated behavioral op amp model. SBOM430C.zip is the newer OPA140 model just updated in 2019. It is similar to the 2011, but with further enhancements made to the model. That is the model to use for most accurate results. 

I do observe the differences obtained from the transient analysis with the two OPA140 model versions. Note however, that there is a setup problem with your circuit. The OPA140 is powered by a single +5 V supply, and the common mode voltage is +2.5 V. See from the OPA140 datasheet Electrical Characteristics table that the Common-mode voltage (VCM) range is (V–) – 0.1 to (V+) – 3.5 V, for TA = –40°C to 125°C. If V+ is 5 V, then the upper VCM is (V+) - 3.5 V, or +1.5 V max. The high end VCM is being exceeded a volt. Increasing V+ to a level where the high end VCM is no longer being violated should correct the circuit.

If you need to operate with a single +5 V supply TI has many precision low voltage op amps that have a VCM range from (V-) - 0.1 V to (V+) + 0.1 V. We can help you select the most appropriate low voltage op amp if needed.

Regards, Thomas

Precision Amplifiers Applications Engineering

Thank you Thomas.

I couldn't notice VCM range because of the different results. I need a ultra low noise buffer op-amp for the same circuit. I'll be glad, if you help me.
I will use it between 40Khz-80Khz as unity gain/buffer. VDD=5V, VCM=x

I tested some op-amps that are low input voltage noise and low input current noise density and found Opa365 that is best low noise for 70khz in tina.

Can you suggest me another op-amps?

Regards,

Hi Aykut,

to find the latest model of OPAmp, go to the product site of OPAmp and open the "TINA-TI Reference Design". Draw your circuit in this reference design or copy and paste the model into your circuit.

aykut_opa365.TSC

But the total noise is a bit high because the OPA365 has a wide bandwidth:

A simple low pass filter at the output of circuit can decrease the HF noise:

aykut_opa365_1.TSC

Kai

Dear Kai,

thanks a lot for your help

My circuit structure is buffer, preamplifier(THS4551) and narrow band BPF(THS4551). I added the tina file and picture without BPF.

Buffer and preamplifier has wide band and I want to add low pass filter between buffer and pre-amplifier like you.

But differential pre-amplifier includes low value resistors for low noise and low pass filter too so I don't want to reduce the signal level too much

how should I put low pass filter in circuit?  

AU_Pre.TSC

Hi Aykut,

can you tell a bit more about your application? Where is the input signal coming from? And what do you want to connect to the output of THS4551?

Kai

I want to read an hydrophone signal

hydrophone -> Buffer -> preamp(ths4551) -> 4.th BPF(THS4551) -> VGA -> ADC

by the way, I added parallel 22-47pf to gain resistors in preamplifier, but not effective.

additionally , VGA or BPF, whichever comes first, I'm undecided.

Hello Aykut,

Since this is a low voltage, low noise application I suggest the LMP7731 bipolar op amp. Its Input Referred Voltage Noise Density at f = 1 kHz, and VCM = 4.5 V is typically 2.9 nV/√Hz for 5 V supply operation. It is usable with supply voltage down to 1.8 V, and has a 22 MHz GBW which should nicely meet the requirements of your 40 to 80 kHz application.

The LMP7731 provides the best noise performance of any of our low voltage Precision Amplifier op amps. You can find more information about the LMP7731 on its web page:

https://www.ti.com/lit/ds/symlink/lmp7731.pdf

The OPA365 is supported by TI's High Speed Amplifier product group. If you decide to move forward with the OPA365, please use their e2e forum for best assistance.

Regards, Thomas

Precision Amplifiers

Hi Thomas,

thank you for your suggest.

i am asking to learn,  LMP7731, why I couldn't understand better than opa365?  Please tell me where I made a mistake

yes, LMP7731 has low voltage noise density than OPA365 but its current noise density is a little bigger. (1.1pA > 0.8fA )

input impedance of my circuit (100K) is big value so current noise density is more dominant.

in simulation

total noise with opa365 : 117uV ( with 40dB gain)

total noise with LMP7731: 153uV ( with 40dB gain)

plus, I will amplify the signal 60db with VGA. (total 100dB) so

under these conditions, VGA is saturated earlier by noise of the circuit that includes lmp7731 buffer. I'm wrong?

thank you in advance

Hi Thomas,

I got it thank you,

Hi Aykut,

can you increase your C1?

Kai