# Datasheet-a-Day W30 2020

I tried one new thing this week with the datasheets. I took a number of datasheets for “standard” diodes, and just looked through them to get a sense of the kinds of parameters that are usually quoted for these things. It was a useful exercise, and I’ll do the same for some transistors next week. Otherwise, I continued with ploughing through the Analog Devices “mini-tutorials” series. They’re of varying difficulty and relevance, but they all feel like things it would be good to know about.

## Various diode datasheets

I looked at:

A bit of a random mix. I think they were just datasheets that I’d downloaded from some course website once and had lying around.

Anyway, the important characteristics for these things seem to be:

• Forward voltage

• Reverse voltage limits

• Current limits

• Junction capacitance

• V/I curve

• Switching times (for Schottky diodes)

• Zener characteristics (for Zener diodes!)

All mostly obvious, but it’s interesting to see the variation in how these things are quoted by different manufacturers. I would have thought that datasheets for these kinds of standard components would be more standardised, but they’re not. Also, there are some things like diodes that claim to be “fast switching” without quoting any measured switching times, which seems a bit cheeky.

One other thing that was useful to see was that tolerances for Zener voltages are 5% for the 1N7xxx parts, but you can get other Zeners down to 1% tolerance.

## Aperture Time, Aperture Jitter, Aperture Delay Time—Removing the Confusion (Analog MT-007)

A continuation in the series of “things that can go wrong with ADCs”. This one’s about how to think about jitter in the timing of the sample pulse in the sample-and-hold amplifier.

I didn’t understand it all, and it seems to be targeted at high-end ADC applications, so I’ll come back to it in the future.

## Converting Oscillator Phase Noise to Time Jitter (Analog MT-008)

This one is sort of a continuation of MT-007, but it’s thinking about jitter in the ADC sampling clock. I found that much easier to understand than the discussion of aperture jitter in the previous app note.

Oscillators usually have their “jittery” characteristics quoted in terms of phase noise, which gives oscillator output in 1-Hz bandwidth buckets relative to the output at the fundamental frequency of the oscillator. A perfect oscillator would have all its output in the bucket centred on the fundamental frequency. Real oscillators have some power in other buckets, some of which is a simple spread of the fundamental peak, and some of which is a broadband “background”.

Phase noise plots as dBc/Hz (dB relative to “centre”, i.e. fundamental) vs. log frequency offset are usually composed of a number of line segments, each of the form $1/f^n$, with n = 0 (broadband white noise), 1 (flicker noise), 2, 3, and so on. You can think of this as a superposition of noise from different sources with different mechanics. It’s not a perfect way of thinking about real phase noise plots, but it’s pretty close, and it makes the calculations much easier.

There’s a relatively simple method to calculate jitter from the phase noise plots, and it allows for comparison of different oscillators for ADC applications.

And then we get to the bit where you realise that it’s one of those app notes. They’re talking about oscillators with jitter in the 50fs range...

## AD620 Low Cost Low Power Instrumentation Amplifier (Analog)

“Low cost” is relative: €11.87 in unit quantities from Mouser...

The claim is that integrated instrumentation amplifiers give better performance (error, power consumption) than 3-opamp designs, presumably because of better resistor matching. (I read some lecture notes the other day talking about this, and about how it’s possible to match temperature characteristics of resistors in ICs by placing them on isotherms determined by looking at where the main heat dissipators on the die are. I’d not thought about that aspect of matching before, and it’s pretty clever!)

The datasheet here shows something I’ve not seen too often yet: it has distributions of parameters for some things, along with the component sample sizes used to generate them. There’s nothing in the datasheet about how the devices are sampled from production, but I guess that if I went looking, Analog might have a document to describe it somewhere. (Didn’t find one with a quick search, but it must exist.) I guess this extra care is part of what you pay for when you buy Analog parts.

This is quite a monster of a device: it has 10 GΩ input resistance!

It’s also quite a monster of a datasheet. Lots of graphs, lots of applications. Good datasheet!

## LT1017/LT1018 Micropower Dual Comparator (Analog)

This is a random pick from the component list in LTSpice that I’ve been using for some simulations.

It comes in two flavours, one optimised for low power, one for speed.

Unlike a lot of comparators that have open collector outputs, this one has a built-in pull-up current source, so no external pull-up resistor is needed.

Looking at datasheets for these “standard” components (opamps, comparators, etc.) raises the question of what’s the best way to compare all these different parts? You can do parametric search within a single manufacturer’s products, and you can do some parametric search on Mouser or Digi-Key, but it generally seems quite hard to do detailed comparisons across manufacturers without sitting down with all the datasheets and making yourself a big table. Has anyone made a big spreadsheet of opamp/comparator parameters across manufacturers?

They comment that the internals of these things are trimmed. Is that normal for comparators? Does it make a big difference to the price?

Some application examples show using it as an opamp. Why would you do that? What really is the difference between a normal opamp and a comparator?

This datasheet has lots of applications. Don’t understand the more complicated ones... Something to revisit after reading more about opamps, I think.

## Using DRV to Drive Solenoids - DRV8876/DRV8702-Q1/DRV8343-Q1 (TI SLVAE59)

This one is about driving solenoids and relays with H-bridge or half-bridge circuits. It’s very much like driving a motor, unsurprisingly.

A low-side or high-side FET plus a freewheeling diode is the simplest way to do it. A push-pull driver with two FETs: better for high frequencies. Or use a H-bridge...

Motor drivers can do this. Some have special “independent drive” modes to drive multiple solenoids.