The story of analog integration in test instruments is long, storied, and rife with names renowned in the annals of analog design who innovated upon each others' design to realize the foundation of today's circuits.
So what is the state of integration of the analog circuitry in test instruments? Measurement on a chip started long ago and continues with Intersil, Maxim, and other companies offering integrating ADCs with auto-zeroing capability in 24 or 40 pin ICs.
They have been around for decades as the core of digital panel meters (DPMs) and digital multimeters (DMMs). DMMs as instruments lead the integration effort; cheap ones sell new for as low as $10 US. To achieve the voltage range required of a typical instrument, discrete resistive dividers are used, along with off-chip capacitors, fuses, trim-pots, and switching. Cheap DMMs do not use the ADC in a DIP package but apply board-level chip-on-board integration, as shown below. The circular black epoxy dot encapsulates the DMM IC on-board.
The most non-integrated aspect of these cheap, handheld DMMs is the electromechanical switching. If you have taken apart a handheld DMM, you will have noticed that the big, many-position rotary switch also has many tracks (shown above as seen through the board) and several rotated metal contacts to connect adjacent tracks, sometimes on both sides of the board (or on multiple boards). This switch performs two functions: it is the main element of the human interface (or front-panel ) and it also embodies the encoding logic for the circuit interconnections that configure the instrument settings .
Another instrument to receive some integrative attention is the function generator (FG). Exar and Intersil came out with FG ICs as long ago as the mid-1970s. Exar's XR2206 and Intersil's ICL8038 integrated most of the transistor circuitry required except for the output amplifier. That portion of the circuitry is better implemented with an op-amp or discrete transistors.
These two FG versions have quite different circuit designs, each with its relative merits. An early, semi-custom IC foundry, Interdesign, offered Monochip Application Note APN-8 (by James Knapton) titled “Waveform Generator”. It included, as did the Exar and Intersil FGs, a triangle-wave generator (TWG) — the core of a FG – and sine shaper. The TWGs all produce a square-wave in digital form. None of these three FG TWGs had the same design, though all of them were capable of sloppy 1-MHz waveforms. The sine-wave at the high-frequency end of the instrument is cleaned up by bandwidth-limited circuitry, but the triangle-wave loses its sharp peaks and the square-wave starts looking more like a sine-wave.
Maxim introduced a next-generation FG IC in the 90s as the MAX038. (Apparently, someone at Maxim was more impressed with the 8038 than the 2206.) It was a major improvement and had instrument-quality capability. It had a maximum frequency of 20 MHz and also included a phase detector for phase-locking the TWG to an external frequency source. TWGs are a kind of voltage-controlled oscillator (VCO) — or rather, a timing-current controlled oscillator — and can be swept in frequency, making them easy to accommodate in a phase-locked loop. For reasons inexplicable to me, Maxim dropped this part from its repertoire of available products.
Back in the 70s, two talented engineers at Tektronix — Barrie Gilbert (now a Fellow at Analog Devices (ADI)) and Art Metz — each devised different schemes for converting triangle-waves to low-distortion sine-waves. These schemes are readily integrable. They are not (to my knowledge) being used in commercial FGs though they are superb schemes.
For a while, ADI had an FG-like device in their portfolio, the AD639 trig function generator. It was designed by Gilbert after he moved there. The device has since been dropped from their portfolio — also inexplicable.
The 8038 used diode breakpoint circuits to approximate the sine function with multiple linear segments. The XR2206 used the tanh function of BJT diffferential-pairs to approximate a sine. Gilbert took this idea much farther with his multi-tanh circuit concept. Metz did something very simple and clever using translinear circuitry invented by Gilbert, a consequence of creative minds in close proximity.
This brief historical survey of measurement instrument integration is but the first part of a much longer saga. In the coming parts, we'll transition from history to look at possibilities for future integration of analog circuitry for test instruments.