Part Number:THS3121
Hi TI expert,
I use TI’s high speed op amps to generate voltage within -12V and +16V. The circuit is simple and is as follows, where op1, op2 are THS3061 and the three behind are THS3121.
VDA is within 0V and 1V. Then, due to op1 and op2’s gain of 4 and 7, the voltage at R2 is within -12 and +16V. THS3121s are adopted to enhance the output capacity and drive the next-stage push-pulls, which are removed and don’t exist in experiments.
Fig1
In experiments, I found Vo and VDA have serious oscillations, which made me first think it’s the DAC layout that brings the bad waveforms.
I do several tests.
1. Only remove R1 and let op1 work alone. VDA is good and has no oscillations.
2. Only remove R2, and let op1 and op2 work. VDA is still good.
3. When R1 and R2 are all put back, Vo and VDA are both very bad. And THS3121’s input nodes’ voltage are better than Vo.
Then, I think the oscillations don't result from DAC. I think that THS3121 is unstable because of small RF, and I try RF from 1.2k to 22k, but the oscillations are still there and as follows. Fig2 is @RF=22k, where blue trace is VDA, purple is Vo. I even change 3121 to 3061, but it’s not better and shown in Fig3. Indeed, VDA's large parts of severe oscillations come from Vo due to oscilloscope channel's interference, but testing VDA alone shows it has some abnormal oscllations.
Fig2
Fig3
My TINA simulation file is provided for you but it shows there’s no oscillations.
3121’s PCB layout is no different from 3061, and I’ve etched out internal copper around its inverting and output terminal.
TI experts, is my circuit design wrong(But, the simulation result is ok)? If necessary, I can give you PCB photo or PCB file.
(Please visit the site to view this file)
Best regards
Yatao