OPA2244: Aol sim

Thank you Jerry for your answer and suggestion. I tested +/-15V to see if it has an impact on the simulation results and don't change back to +/-5V before importing the file to this thread. My design requires +/-5V.

TLC2252 seems to be a good option. One important point for my design is to have a low impedance circuit to drive INA ref pin. TLC datasheet provides output impedance vs frequency. I tried TLC2252 spice model but don't get the same curve for +/-5V at G=1 (datasheet p.38). (closed loop impedance simulation inspired from https://training.ti.com/modern-op-amps-output-impedance). Do you know if this model can simulate closed loop impedance? Thank you. 

Benoît,  

TLC2252_zout.TSC

Hello,

Sorry I didn’t pay attention to the version of the model. Thank you. In the attached simulation I used the TLC2252.TSM file (REVA) by using the menu “Insert\Macro… (Ctrl + M)”. I checked and the macro model is the same as TLC2252.LIB (REV B).

I tried to simulate closed loop impedance for unity gain and gain x10 but not sure I’m using the correct schematic/configuration (figure1 below) since:

• curves are different from figure 31 found in the datasheet. Simulation seems closer to figure 31 for G=10 and G=100 instead of G=1 and G=10.(figure 2, figure 3 below)  
• In the table (datasheet p.8) closed loop impedance is specified at 190Ohms for Av=10 and f=25kHz. Simulation is about 670Ohms. (figure 4 below)

Do you know what I’m doing wrong? Thank you for your time and your help. 

Best regards, 

Benoît, 

Figure1:

Figure 2:

 Figure 3

Figure 4:

 TLC2252_zo2.TSC

Below please see graphical representation of the difference between GBW vs unity BW of OPA2244.  

As far as precision alternative parts go, you could also consider OPA2191 with max offset of +/-25uV and max drift 0.8uV/C or OPA2187 with max Vos of +/-10uV and max drift of +/-0.015uV/C

Thank you for your suggestions. OPA2191 / 2187 are good alternative but with higher power consumption. TLC2252 seems to be suitable for my application but I want to understand the differences between simulation and specifications given in the datasheet. Any idea? Thanks. 

Best regards, 

Benoît,   . 

The TLC2252 open-loop output impedance, Zo, is show below.  You may notice Zo=~800ohm at 25kHz.

The TLC2252 close-loop output impedance, Zout=Zo/(1+LoopGain), simulates 654ohm at 25kHz for G=10 - see below.

I believe the error comes from the fact that in G=10 TLC2252 effective bandwidth is only 20kHz (see graph below) and thus at 25kHz the op amp runs out of loop-gain, LG.  Since LG=AOL*beta, Zout=Zo/(1+LG), where LG<<1, which results in Zout=~Zo - see attached.

2211.Open-loop Zo vs Close-loop Zout.ppt

Sorry for my late reply. Thank you for your answer and the ppt. 

You are probably right. It is surprising that datasheet makes ref. to impedance at a Gain / frequency where the op amp runs out of loop-gain.

I struggle to correlate datasheet information and simulation data. Simulated TLC2252 close-loop output impedance graph gives about 46Ohms @ 1khz whereas the datasheet specifies between 4 and 5 Ohms. I think I misunderstand something here since it seems to have a x10 factor between datasheet and simulation. Please let me know any suggestion you could have.   

Best regards, 

I took a closer look at the TLC2252 Zo vs Zout for G=10 and have confirmed discrepancy between datasheet Zout graphs and simulation model.

The simulation of the TLC2252 open-loop output impedance, Zo, to be 822ohms between 10Hz and 100kHz - see below.

Simulating AOL and 1/beta for G=10 shows LoopGain (AOL*Beta) to be ~25dB at 1kHz and -3dB at 25kHz.  Using equation for the close-loop output impedance, Zout=Zo/(1+AOL*beta), result in Zo of 44ohm and 482ohm while the Fig 31 show 5ohm and 190ohm, respectively, at 1kHz and 25kHz - see below.

Direct simulation of TLC2252 shows Zo to be at 46ohms and 654ohms at 1kHz and 25kHz - see below.

In the G=10, the effective bandwidth of TLC2252 is around 20kHz (BW=GBA/Gain).  Thus, from the stability point of view, only error in the effective Zo at 20kHz would matter.  But Zo simulation error is relatively small at 20kHz and thus model reliably assures stable operation.

However, I believe at lower frequencies the graph in Fig 31 does not properly show Zo as it should go down at 20dB/decade to dc Zout=Zo/(1+AOL*beta) = 4kohm/[10^(96dB/20)] =~60mohm and NOT 3.2ohm shown below.  All in all, the low frequency Zo graphs shown below do not make sense and contradict results of above simulations and calculations.  But this fact would only affect reliability of your simulation in high gains where effective bandwidth is very low.