Datasheet-a-Day W42 2020

26 Oct 2020electronicsreading

Bit behind on this again, but I have some interesting ones for next week, so maybe that will make me stick with it better.

LM111, LM211, LM311 Differential Comparators (TI)

These are flexible comparators that will work off a ±15V supply or a single 5V supply, with logic-compatible (open drain) outputs, high drive capability (up to 50 mA @ 50V), and very low input currents.

They also have pins for input balance and output strobing, which I’ve not seen before.

There are lots of application examples in datasheet, but these look less jellybeanish than the LMV331 that I’ve been using recently.

LF353-N Wide Bandwidth Dual JFET Input Operational Amplifier (TI)

I got hold of a pile of random opamp datasheets because I wanted to get some sort of picture of the variation there is in these things, but I think I ended up with a few oldish parts in there. This one, in particular, seems quite old and inconvenient to use. There’s a lot of stuff in the datasheet about needing to be careful about how you set it up which more modern devices don’t seem to need. For example: it needs high supply voltages, greater than ±6V. And no rail-to-rail operation for you!

It’s interesting to see some of the things I’ve been (very slowly) picking up from The Art of Electronics in these datasheets, like the temperature dependence of input bias current for JFETs (which is basically gate leakage current).

I’m starting to get a little more comfortable looking at the application circuit examples in these datasheets, but the schematics of the ICs themselves mean more or less nothing to me. I can spot a differential amplifier front end, and can guess at what are probably current sources, but most of the rest of what’s in there is still a mystery. Also, this one has an integrated Zener diode, which I don’t think I’ve seen before.

LF155/LF156/LF256/LF257/LF355/LF356/LF357 JFET Input Operational Amplifiers (TI)

These also look like old parts. The datasheet says they’re the first JFET opamps to integrate JFET and bipolar transistors, but I’m sure I’ve seen quite a few parts that do that. The datasheet describes JFETs as “rugged” compared to MOSFETs, which I suppose is true!

These things have high supply voltage requirements again. One thing I have a hard time with is working out which are the more “modern” opamps, and whether trying to find those is even a good idea. A lot of the earlier devices seem to have quite a few constraints on using them, and aren’t set up for low voltage applications. I’ve yet to settle on a jellybean opamp for general-purpose use.

LM6132/LM6134 Dual and Quad Low Power 10 MHz Rail-to-Rail I/O Operational Amplifiers (TI)

This is a more modern (well, convenient) device, supporting single supply operation from 1.8V–24V, with rail-to-rail operation and low supply current.

Would be useful to compare the operating characteristics of this (input offset voltage ~ mV, input bias current ~ 300 nA, input offset current ~ 50 nA) with other similar devices. Are these values typical?

Quite a nice datasheet: DC and AC characteristics quoted for three typical supply voltages (2.7V, 5V, 24V).

These really do look easier to use than those earlier ones!

LT1632/LT1633 45MHz, 45V/ms, Dual/Quad Rail-to-Rail Input and Output Precision Op Amps (Analog)

Another more modern device: single-supply operation from 2.7V–36V with rail-to-rail performance.

Same thing with this datasheet as with the TI LM6132: detailed specifications for 3V, 5V and ±15V supplies.

There’s an interesting design in the IC schematic to get rail-to-rail behaviour on the inputs: there are two input differential amplifiers (one NPN, one PNP), each of which is active in different input ranges. This allows them to bias each of the input amplifiers to cover one supply rail, overall giving full rail-to-rail coverage. (I didn’t work this out myself! There’s some explanation to go with the schematic in the datasheet.)

Evaluating High Speed DAC Performance (Analog MT-013)

I’ve been reading through these in order, and so far most of them have been variations on “things that can go wrong with your ADC”. For a change, this one is about DACs.

One obvious thing to want to measure is settling time when the DAC code changes, but an “extra” characteristic to go with that that I hadn’t thought about is glitch impulse area, which measures something like the energy in the glitch when the code changes.

The other two big areas to think about are distortion and noise.

When driving a DAC with a DDS sine wave, for example, glitches during code switching generate harmonics of the input frequency (e.g. the mid-point glitch gives you the second harmonic). Higher-order harmonics alias back into the Nyquist bandwidth and can’t be filtered out. In a lot of applications, you also want to measure distortion when using signals more complex than a simple sine wave. The usual things like SNR, THD, SINAD, SFDR can be determined for DACs.