Hi,
We designed a custom board with OPA549 amplifier.The load is an inductive 20mH with 4.5 Ohm active resistance. We saw a ringing: amplitude about 100 mV and frequency about 14,3 MHz. We added a capacitor 10pF from amplifiers output to IN-, but nothing helped us. We changed R1 from 10k to 1 Ohm then amplitude and frequency of ringing changed too: from 100 mV to 160mV and from 14,3 MHz to 13,2 MHz. How we can delete ringing at all? Power supply of opamp is +-12V.
Regards, Maxim
Thanks for the schematic Maxim,
Looking at your output waveform, couple of things stand out -
1. If you are asking for a 10V output using 12V supplies, you might be saturating as the specs say 2.7V hheadroom.
2. You are clearly slew limited on your transitions, sometimes that will cause the ringing you see as the loop closes again - you might try slowing your input transition with a cap at the V+ input.
Hello Maxim,
I ran a stability simulation on your OPA549 circuit. The load is a complex RCL combination and I was concerned that the ripple might be due to degraded phase margin. The OPA549 has an older style simulation model so it isn't absolutely ideal for a stability analysis, but it is close enough to get us in the ballpark with regard to general stability.
The Bode plot results from the OPA549 simulation circuit are shown below. It looks like the circuit has plenty of phase margin (>45 degrees) and the circuit should be stable, with little overshoot. At this point it appears that the output ripple is not due to insufficient phase margin.
I think that Michael has some good points regarding the conditions under which you are operating the OPA549. Its output is certainly slew rate limiting with the 10 Vpk, 50 kHz output pulse condition. Additionally, The OPA549 has a typical gain-bandwidth product frequency of 900 kHz and open-loop gain (Aol) at 50 kHz of about 20 dB (10 V/V), and that is low. Normally, we like to apply op amps in application circuits where the Aol is in the tens, or hundreds of thousands of V/V, to minimize errors.
Since you have the circuit in a closed-loop gain of 2 V/V (6 dB) and the Aol is only about 20 dB, the loop gain is only 14 dB (20 dB - 6 dB), which isn't high enough to achieve precise loop control. I suspect driving the output to the large signal swing levels of 10 V and then 0 V is resulting in the overshoot/undershoot ripple that you are observing. Micheal's suggestion of slowing down the input transition is a good one to try. If you find that lowering the frequency and edge rate reduces the ringing then the issue is not related to common phase margin issue, but rather very low loop gain at 50 kHz.
Regards, Thomas
Precision Amplifiers Applications Engineering
