OPA197 c-load stability

Not quite the low, low input bias current, but cap drive on the output is excellent.

It doesn't. A big enough capacitor means that the oscillation isn't driving enough current into the capacitor to produce enough voltage swing to be detectable - or sometimes not even even enough to be bigger than the Johnson noise at the oscillation frequency in the series resistance of the capacitor. At that level the oscillation doesn't take the input stage out of its linear region (+/25mV, for bipolar transistors, a volts or so for FET and MOSFET inputs), so it doesn't mess up performance enough to notice, but it is still oscillating.

Pity about that.

Just because you don't see a voltage across a 1uF cap, doesn't mean that the thing driving current into the node isn't going unpredictably nuts. Case temperature?

RL

I'd be wary relying on that "Williams effect". Even a very small resistor - DC feedback after it, some AC prior to it - should be a lot more reliable. Or a transistor in the loop, well you know what I mean. Just not that "Williams effect", feels awful to me.

Sort of the reverse of Schawlow's law: "Anything will lase if you hit it hard enough." ;)

It's worth putting a sense resistor in the supply leads to check for oscillations of very low amplitude. THat's been known to happen even when the output looks steady on a scope.

Cheers

Phil Hobbs

The 1u case seems to have a limit-cycle oscillation that dies out pretty fast.

Oscillation would increase supply current, and I don't see that.

The data sheet has a chart of recommended damping resistor vs cap load, table 3. The last entry is 2 ohms and 1 uF. Why did they stop there? The next step could have been 10 uF and zero ohms.

And why the 47r and 100 pF point?

Makes no sense.

Supply current looks OK.

I need to drive a net to Vcc/2, and it has a dozen 10 uF ceramics to ground, and I want a low impedance drive from DC up.

Looks like the added 47 uF tantalum is prudent. That adds some ESR damping.

My boss assigned me to rev this board

The bottom of the board is paved with parts; there's not much room to add things.

A previous fix hung a 1500 uF aluminum cap on the rail, an ugly hack on top.

I recall--the lead sneaking round the edge of the board was thrillingly gnarly. ;) (Not that I'm above doing the same, when pressed sufficiently.)

The C load moves the output pole to lower frequency, and when it's too close to the zero-cross of the main+tail poles, you wind up with instability.

A large, higher-ESR cap is often a good way to stabilize switchers and LDOs, too--it's a shunt version of the usual lead/lag network used in feedback amps. There's no reason that should be a problem in an op amp loop, in principle. Doing stuff outside the datasheet's guaranteed limits puts the responsibility on us, but oh, well--that's where it winds up anyway.

Cheers

Phil Hobbs

thanks for that link Bill. Although I've read the same info elsewhere this one also mentioned *external* compensation is useful. And I thought external comp was just for "old" op-amps, a previous-century idea! ;0)

Note in the opamp table above some of the amps have "unlim" capacitive load drive capability.

In most opamps, there is a buried internal compensation pole, and adding a cap load creates a second pole in the loop, causing instability.

In some opamps, adding a c-load just slows the open-loop response but doesn't add another pole. So it gets more stable, not less.

OPA197 has a number of patented features, but the data sheet doesn't name the patents, so it's not obvious what the internal circuits are.

One other problem is the 12 heavy transformers. A sufficiently aerobatic flight path to the floor, with the box landing on its top, will break the PEMs that hold it to the bottom of the box.

There are big cutouts along the pcb edges, to let air flow up into the fan, which don't help.

Bifurcation?

RL

In short, you don't want a power supply, you want ground, but you're making it with an amplifier from power supply as input. Are there current surges on this pseudo-ground? If not, current sources (by the dozen, if necessary) into parallel RC loads are a way to get well-filtered voltage levels without power supply ripple sensitivity. Takes one op amp and a pass transistor per branch.

Then again, why not use the ECL trick of +3.2V and -2.0V power supplies? Signal circuitry ought to be well-characterized against power supply noise, but the signal-splitter application has a gain of 0.5 on power noise, and adds another round of filter capacitance, multi-branched, to boot. It's arguably better to use split supplies (like, +1.8 and -1.8V), instead, and a single-point ground topology like the old guys did 50 years ago.

That 'been known' and 'looks like' means off-the-spec-sheet design. The thought makes me... itch. Maybe it's reminding me of bugs?

Yeah, sounds like the itchy feelling isn't just me.

The big reason to avoid split supplies, is ... the mindset of the student with a small project to complete for class. He will always design a negative ground system, usually with a microprocessor/ADC that accepts no negative signal input. Next year, he'll be writing up applications literature, based on his 'experience', which will guide the next generation of students.

Spec sheets are often incomplete or downright wrong. Experimenting and thinking are both worthwhile. Dremeling and soldering and measuring are a break from screens and mice too.

Spice models of opamps are typically not realistic. And this is TI, who have their own version(s) of Spice.

Our synchro box workes nicely with a single +24 supply from a big wart, without a big + to - converter. So it's handy to reference signals to a clean +12 rail.

The original version had some channel-to-channel crosstalk via that rail, and it was fixed with a gigantic aluminum cap to ground. I thought I'd do something more elegant for the next rev.

No students were involved.

I found this TI patent, but it may not apply to this opamp.

So why do you not want a 20-30 Ohm resistor (plus one say 1k and a few pF of a cap) if you have 12V headroom? The opamp is fast enough, what it cannot do in 1-2 uS will be done by the bypass caps you have. If this is your original setup and it took the huge aluminium cap to filter the crosstalk I very much doubt shorting the opamp's output to all the bypass caps will buy you anything. Did it?

At low frequencies, the closed-loop output impedance of the opamp follower will be less that the impedance of any reasonable cap. Adding

And why not do what's simplest? And learn something along the way?

Well learning something is always worth it of course. But the 20 Ohms closed in the loop does not mean you add 20 ohms to the output impedance, especially with all the 12V headroom that you have. To make sure we are talking about the same thing: 20 ohms between output and load, 1k between load and - input, a couple of pf between output and

- input to ensure stability.

I've done that, but it will still present a higher bus impedance at some frequencies... assuming that the opamp doesn't peak, which it seems not to do. The real test is to snoop the transient response to a small load step.

There will always be some transient of course, this is where the bypass caps come in. If it takes a "huge aluminium cap" to filter out to levels you need then I understand your experiment but I doubt it will bring much of an improvement, the opamp will still have to respond with current etc. (And I'd be nervous about having a batch work out of spec and not knowing if the next one will behave the same but well, it may be of no concern in many cases). The way to reduce the size of the aluminium cap I would go to would be the 2 resistor and a cap circuit and a faster opamp and keeping the compensation as close as practical. But whatever you do you will need enough capacitance to filter out the transients, it is just a matter of how much is enough. With this opamp I'd say 100uF would be plenty. opamp,