AoE x-Chapters, High-Speed op-amps section, DRAFT

It would be good if the x-chapter had tips on how to simulate these things, where a model is unavailable.

You might add the LMH6733 to your table. 3 wideband CFB amps with shutdown pins, somewhat similar to OPA3695.

Also could be worth mentioning that the shutdown pins on many of these parts are referenced to the positive rail, not ground. That just hosed me the other day. The shutdown pin on the LMH6733 and similar parts is best thought of as an analog input, not a digital one. Some chips like the newer THS3491 have a separate reference pin just for that purpose, but most don't. For the LMH6733 the disable/enable thresholds are specified at 3.2V and 3.6V for 5V rails, and you need to add a volt to them if you max out the rails at +/- 6 volts.

That could be (and was) a problem when relying on an open-drain output to pull the shutdown pin up to 5 volts, since the current drawn by the shutdown pin is not negligible. (Worse, the open-drain pin in question was on an I2C extender whose data sheet specifies 5.5V compatibility at the GPIO pins without regard to its own supply voltage, but didn't mention the ESD diode that limits the "open drain" voltage to 1 volt above the Vdd rail, which was 3.3V in my case.)

-- john, KE5FX

Arrgggh! Yes, it would be good. Do we have a guest author?

Thanks, I'm adding that one in.

Excellent note John, we'll try to do the story justice.

One of my guys got an old Pspice model and hacked it into LT Spice. It was ugly.

LTC6228 has a multifunction shutdown pin. Different voltages (wrt the posive rail) will shutdown the whole opamp, or enable / disable the input bias current cancellation circuit (allowing one to trade input current noise for input bias current).

Allan

Wow, Allan, that's one fascinating op-amp. It's new and I had missed it, thanks!

Awesome.

One interesting part you don't list (except dissing its offset voltage in an aside) is the LT1228, which is a 75-MHz OTA with a 100-MHz CFA attached. It's a real OTA, not like that three-terminal OPA861 thing.

The LT1228 is kind of noisy--about 20 dB above shot noise, and worse at higher current--but you can do a lot with a fast OTA.

Cheers

Phil Hobbs

Interesting. Usually they're OK unless you use too small a feedback impedance.

Cheers

Phil Hobbs

Yes, thanks Phil for point that out. I'm a longtime fan of the LT1228, and have found it super useful in several projects. We'll carve out a spot for it.

LM324. ;)

National used to make a combo LM358/LM393 amp/comparator. (LM611 and

IIRC JT said that the two were the same silicon with different metallization.

But for slow stuff that has to be very cheap, using a section of a 324 as a comparator works fine.

Cheers

Phil Hobbs

The horror!

One thing about the classic National LM324 is that if you rail any section, it wrecks the shared bias supplies for the other sections. So one comparator switching *really* messes up whatever the other three amps are doing.

I think some peoples' later versions didn't do that.

Or too large, insufficient speed.

LM392, alive and well, multiple manuf.

I remember JT claiming that was the designer's fault, we should never let an opamp saturate.

The 324 is my favourite opamp ever. You get so much for so little.

NT

Single SOT23 opamps don't have a shared bias supply problem!

Many new rrio opamps behave beautifully as comparators. They come off the rails fast and clean. Some older amps would wind up some internal node and take forever to unstick. One Intersil part took *seconds*.

I've used RRIO opamps in things like ideal rectifier circuits, just let'em rail.

Yes, but it's trivial to add one transistor, and circumvent the problem. I felt we should blame Jim, rather than have him blaming us for our standard use.