TL082 opamp input range?

Usually when a manufacturer makes an IC family using a part number that's a takeoff on another popular part, such as the TLC082 compared to the TL082, they retain as many of the main features as possible. For example, these two opamp families have similar slew rate. But here TI does an unusual thing; whereas the TL08x JFET opamp's input common-mode range is to +Vcc, the TLC08x CMOS input range is to -Vee.

This is the kind of thing could really catch someone who was making a substitution. Perhaps LMC324 or TLC324 might have been a better name for the TLC084, and TLC358 for the TLC082.

Other things that might catch one up would be the 36V maximum supply-range reduced to 16V, and the rather high 23pF input capacitance of these CMOS parts. 23pF is high enough to make a low-gain amplifier using commonplace 10k feedback resistors ring like a banshee, from the unexpected extra 700kHz pole at the input. The TL08x opamp doesn't suffer from this pole, but even if it did one might not notice, due to its slower (3MHz compared to 10MHz) speed.

The high input capacitance is one of the penalties for lower noise, 8.5nV for the TLC08x, compared to 18nV for the TL08x.

It's a curious design choice: most users must be attracted to this family for its speed, or its 50 to 100mA rail-rail output capability, rather than a reduced but still fairly-high noise. If TI's designers had used say 6x smaller input transistors, the capacitance would have been a relatively-innocuous 4pF and the noise would have been similar to its TL082 namesake, which people have accepted OK. And the die would have been smaller (the TLC082 costs $1 qty 100, 40% more than the JFET TL082).

I really didn't know about these TLC08x. Interesting devices as they are even faster than TL08x as I can see, and more than enough for the low frequency involved in my circuit. I must admit they should have kept a low input capacitance or, at least, give us the choice with another series with low capacitance input at the expense of input noise comparable to the TL08x series. Marketing decisions are sometimes weird!

To go on my experimentation, I picked an oldy 3140 out of my stock and, at least, it goes to ground and is moderately fast. Noisy figure of 40nV though but it doesn't show (or matter) for my application. I notice the output can't go more than 6.7V (from the 9V supply). Anyway it was just for a try. I'll have to buy and trone one those TLC stuff.

Dan