Monday, May 6, 2019

Electronics Design Lifecycle: From Discrete, to Hybrid, to IC to gone...

While looking for Op-Amp noise articles, I ran across this old Analog Dialog from 1969[1] that had an interesting new product announcement in it.

Press release from the March 1969 issue of Analog Dialog announcing a very fast 10 bit, 1 uSec ADC [1]. Clip copyright Analog Devices, 1969.

In the Analog Dialog there is an announcement that Analog Devices bought Pastoriza Electronics and one of their products was a 10 Bit A/D converter that made a 10 bit conversion in 1 microsecond. The Pastoriza ADC-F was a 4.5 x 2.5 by 1 inch “Plug In” form factor. This was all back in March of 1969.

Interestingly, one of my first projects as an engineer was to do this very thing again. In 1980, I was tasked with designing a 10 bit, PCB mount, modular ADC that would convert 10 bits in 1 microsecond all in a 2 x 4 x 0.4 inch modular package.

Another engineer working for the firm had designed a fast 8 bit R/2R based DAC, so I took that, added 2 more bits and some more precision resistors, an AMD LS-TTL 2502 Successive Approximation Register (SAR) IC, a Precision Monolithics CMP-01 Comparator as I recall plus, some other spices and herbs and a few Tantalum capacitors. It worked decently and probably sold for $150 in 1980 dollars. At that time the majority of the modular business was in 12 to 14 bit data converters that operated at slower data rates.

This design used through hole components on a double sided PCB that was hand taped by a true artist, it looked great.

Moving up the food chain:
I then went to work for a company that was making Sampling Oscilloscopes for semiconductor testers and they were rebooting their product line to be 10 times faster then before. Since I was responsible for the analog signal processing electronics, I needed to come up with a 10 bit, 1 microsecond conversion ADC to replace the 10 microsecond converter that the previous product used. I knew that they didn't really want to have a design that they would have to calibrate and they had no room for a discrete design, so we turned to a smaller, but more expensive hybrid solution. Datel [2] made the same basic ADC as a hybrid package, probably 1 x 2 x 0.25 inches.

By this time I was testing the ADC on my newly acquired Apple ][ computer using some data acquisition boards that I had hand built - you have to love open architecture hardware and that simple and slow Motorola 6502 bus (and later ISA bus of the first IBM PC).

Using this setup I could measure and plot the transfer function of the whole front end including the ADC using the ADC Crossplot Test that was in use by all the ADC manufacturers at the time [3]. The idea was to dither the input across a few LSB's and convert the ADC LSB output bits to a voltage that was displayed on a standard Analog Scope, later the scope display was replaced with the Apple ][ monitor. With this setup you could find all sorts of “funny” things with ADC's operation like missing codes and non-monotonic behavior. This is still a useful and simple test technique today. The ADC input is basically DC, but the ADC runs at it's full data rate.

The Apple ][ belonged to me and I didn't want to leave it at work. I'm sure it was quite a show to watch me pack it up and take it home every night! Apple had one of the the first ‘portable’ computers. Although you could not carry it all at once.

By 1985 the whole ADC design could be made up of a few IC's (a DAC + SAR + Comparator and Reference voltage IC), bigger but a whole lot cheaper than the Hybrid. By 1995 a single IC could implement the entire ADC and that finally did in the majority of the Module and Hybrid business.

Today you can buy the Analog Devices AD7298, a 10 bit, 1 microecond ADC that is 4x4 mm, costs the less than the price of a Latte and runs off of +3.3 volts. My original design required +/-15 and +5 Volts at probably 500 mWatts total.

Today we mostly use very high quality sine wave ‘tones’ and run automated FFT’s on the converter output to see with our eyes distortion and signal to noise ratios. This is usually much more indicative of actual system performance than the cross plot testing. Although if you are making a 24 bit ADC for a weigh scale, then the Crossplot test is still an excellent method of testing ADC’s.

I found that I was first a ADC builder, then moved on to be a ADC user then I moved on to be a Historian of the whole business cycle.  :-)

Pastoriza was gobbled up by Analog Devices as was Precision Monolithics, Datel Data Conversion died. The companies where I designed the ADC and where I used the next generation of the very same thing I designed also died. Others came and went without so much as a whimper, but the industry chugged on, built on the body of knowledge that these pioneers painstakingly figured out.

The only really, relevant thing today is that Crossplot testing is still a viable technique of testing ADC’s!

[1] Analog Devices magazine, 1969

[2] Datel was another Massachusetts based electronics company making Modular, Hybrid data converters and later modular power supplies. The data conversion side of the business died, but the Modular power supplies still live on in name at least as the Datel division of the Murata brand. Datel also made digital panel meters and this too lives on as Datel Panel Meters of Chandler Az.

[2] See ADC Crossplot Test in, Analog Devices, “Testing ADC Converters”

Article By: Steve Hageman

We design custom: Analog, RF and Embedded systems for a wide variety of industrial and commercial clients. Please feel free to contact us if we can help on your next project.

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