ADA4522 opamp hangup

The cool thing about brainstorming, kicking problems around, is that ideas evolve and there is usually no one person to credit (or blame.)

Most people are too insecure to play the game, especially online.

Brainstorming as originally defined has a bunch of rules that I can't even remember. Inventing stuff with a few smart people and a white board is the most fun you can have standing up. Part of what I do is to teach people the rhythm of doing that.

Depends a lot on the environment. You need a reasonably thick skin unless you've been round long enough to know who you're dealing with. ;)

Cheers

Phil Hobbs

I am more than happy to have helped in any small way*. I've never had any problem with making mistakes/ falling down/ saying something thing stupid. It's my #1 method of learning. Basically my electronics life is a list of all the mistakes I have made. (I've made a few mistakes twice, which is embarrassing, you look in your data book.. I had this working before.. dang, head slap.) Reading about someone else's mistakes, just allows me to recognize it more quickly when I do the same thing for the first time.

The great thing about electronics, is the result is right there in front of you. (on the 'scope or whatever.)

George H.

You also need smart advice/criticism.. That's what I mostly count on SED for, so far, so good.

George H.

Me too.

Cheers

Phil Hobbs

There are some pretty nasty people here, who don't seem to design electronics, or are too paranoid to discuss it. Ignore them.

Ditto. Stupid ideas often evolve into great ideas, unless they are murdered at birth.

It's surprising how many mistakes and failures turn out to be valuable later, sometimes years later. Which is why we document and archive our experiments, even the dumb ones. We assign a project number to every experiment or breadboard, and have a folder on a server, J:\PROTOS\Zxxx where we keep them.

We also archive parts tests and notes in our inventory database, in H:\MAX\PDATA.

Yes. I pity mechanical and civil and aeronautical engineers who don't have the visibility that we do. I can probe and thermal image every point on a circuit board. The only tricky part is figuring out where the current is going.

You owe me for that! Medium-Dark chocolate is always acceptable compensation.

There is no such hang-up mode according to fig 73 of the datasheet, and that's a pretty low frequency signal at 400 seconds/div:

You have something else going on.

Look at page 30 of the datasheet. It's a documented bug (a.k.a. "a feature").

It really does this trick, I can confirm. In other configurations it works just fine. I've probed around with an FFT analyser - the spikes both in the voltage and current noise are not huge, and they start in the tens of kHz. Below that it's well-behaved.

Among the chopping amps with low voltage rails, OPA388 is worth mentioning.

Cheers, Nikolai

Right.

"May rail to the positive supply" is wussydatasheetspeak for "hangs up." All opamps rail around their positive supplies.

It will come out of the latched rail condition if the input signal yanks the input down pretty hard... about a mA of sink maybe. The cited fig 73 does that.

Our product is a resistor simulator, and we can't predict what the customer might do that could latch it up. A resistor is not expected to sit there and generate 11 volts. We can latch it up by connecting it to some auto-ranging DVMs that can output a bunch of voltage on some of their ohms ranges.

Except for that little glitch, ADA4522 is fabulous.

Choppers are getting way better lately, but too many are low voltage things, we need a high voltage follower. We have bootstrapped the power supplies of low voltage amps to do this, but that is sort of a nightmare. Or really a nightmare.

te:

lt

d that's a pretty low frequency signal at 400 seconds/div:

re").

Ummm, no. The caveat on pg 30 refers to a power supply ramp up issue, and t he fixes in Table 11 have Vin within the input CMR for each case. Your obse rvation is about the amp input being overvoltaged, and fig 73 clearly shows the *output* clipped so as to keep IN(-) within the ICMR. Your working wit h a broken chip. Someone zapped it with ESD or something. If you can't repr oduce fig. 73, your chip is broken. And this sound like the wrong part to t rust to the outside world anyway. You either protect the input better or ge t another part.

ks just fine. I've probed around with an FFT analyser - the spikes both in the voltage and current noise are not huge, and they start in the tens of k Hz. Below that it's well-behaved.

ng.

What can I say, after I say, "You don't understand this" ?

s a

keep

=1

ll

volt

n.

and that's a pretty low frequency signal at 400 seconds/div:

ature").

d the fixes in Table 11 have Vin within the input CMR for each case. Your o bservation is about the amp input being overvoltaged, and fig 73 clearly sh ows the *output* clipped so as to keep IN(-) within the ICMR. Your working with a broken chip. Someone zapped it with ESD or something. If you can't r eproduce fig. 73, your chip is broken. And this sound like the wrong part t o trust to the outside world anyway. You either protect the input better or get another part.

What could "power supply slow ramp time" possibly mean other than a turn on latch-up type of fault?? There's something wrong with your chip if it does n't self-clip as shown in fig 73.

Indeed, such bootstraps are tough.

I've been drooling over the datasheet of OPAx189:

• CMRR: 168 dB (!)
• Open-loop gain: 170 dB (!!)
• en = 5.2 nV/sqrtHz, in = 165 fA/sqrtHz
• drift: 5 nV/deg C typ.
• huge input Z
• Vsupply up to 36V

Has anyone used these yet?

Cheers, Nikolai

Well for a few bucks you should buy a few and stop drooling. :^)

No, but I like the opa192. Say the opa189 has the same type noise bump above ~100kHz as the ADA4522. (though not as much)

George H.

That's a great amp, but its input common-mode range, near V+, is even worse than the ADA4522. All the super chopamps seem to have that problem. This one doesn't have input back-to-back diodes, so it won't have my hangup problem.

DC psrr is great, but AC psrr is terrible. That could be a real-life noise limit.

One big problem we had with bootstrapping a low-voltage chopamp was instability caused by psrr.

Yup. CMRR and PSRR are input-referred, i.e. the error gets multiplied by the gain of the stage, which can be a bit of a rude shock.

Cheers

Phil Hobbs

What's the problem with AC PSRR? I mean - once you provide clean rails and adequate decoupling?

Bootstrapping the supplies means e.g. running a 5V chopamp from +-18V rails using a couple of BJTs and a zener to keep the amp from seeing more than 5V. In that topology the output feeds back to the supply rails, so there's the opportunity for feedback oscillation via the AC PS and CM sensitivity.

Cheers

Phil Hobbs