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ADALM1000 Design Document

Overview

The ADALM1000 is an USB analog multitool offered by Analog Devices Inc. for early explorations of physical and electrical systems. Offering power output equivalent to a few AA cells or a 9v battery, it is designed for interactive use in conjunction with an external system connected to the analog outputs of the device.

While standard electrochemical battery cells offer a convenient portable source of electrical power for exciting systems, they suffer from a fixed set of outputs, no current consumption measurement or control, and poorly defined operation over a range of attached loads and discharge levels. The USB powered ADALM1000 is a convenient source of electrical power, without the limitations enumerated above - offering high speed analog waveform output and measurement, with well behaved operation across several orders of magnitude of Amplitude, Frequency, Voltage, and Current, it allows a wide array of explorations to be carried out using just the device, spare parts, and a computer of some sort.

Analog Front End

Living up to the “Analog Multitool” moniker requires careful design to the “business end” of the ADALM1000, or the “Analog Front End” or AFE. To be useful in the widest array of contexts without degrading performance or increasing precise remains a challenge, but the final solution achieves higher precision, higher power, and lower noise than any comparable device.

Analog electrical systems interact in at least one of two domains - voltage, and current. The traditional tools wielded by electrical engineers operate exclusively in the voltage domain, and trade current and power for performance to an extent which makes it challenging to interact with even the most basic of real-world systems.

Function generators, while invaluable tools for generating test signals to stimulate systems, typically offer an output supply in series with a 50Ω resistor, and as such require external components to buffer their output signal to a point where it is of use for stimulating driving physical systems like DC motors or incandescent bulbs. Oscilloscopes, while powerful tools for working with high speed signals spanning into the gigahertz, require potentially thousands of dollars of add-ons in order to measure the current, and in turn the power into or out of an electrical system.

Introductory exercises typically require six or more electrical connections to be made perfectly before measurements can be made, and literally dozens of (useful!) knobs and buttons to be dialed in specifically for the system at hand. These same introductory engineering exercises often require significant amounts of time spent building up external supporting circuitry to afford functionality similar to that of the M1K. Exposing the voltage across *and* the current through a single connection at a reasonable speed allows for basic explorations to be made with two connections and zero wires. Even sophisticated explorations require at most a half-dozen connections.

Dynamic Range

The problem becomes one of dynamic range, the number of orders of magnitude between your biggest and smallest signals. Oscilloscopes, the bread and butter of measurement devices, typically offer between eight and twelve bits (maximum of 4096 different values) in their data conversion, relying on sophisticated systems of variable gain amplifiers to “zoom in” to the signal of interest. This equates to a dynamic range of approximately three orders of magnitude, requiring human intervention to view signals ranging more than this. On the other hand, human senses offer extraordinary dynamic range, capable of reading a book in both the light of a single candle and the light of full noonday sun. A candle offers microwatts of incident optical power at typical reading distance, whereas peak sunlight can reach almost a kilowatt per square meter. These nine(!) orders of magnitude easily saturate the dynamic range of most measurement tools. To attempt to accomdate the wide array of explorations one might want to carry out, the M1K was designed with sixteen bit data converters, capable of representing up to 65536 different values between the maximum (5v or 200mA) and minimum (0v or -200mA) range of the device. While not able to match the dynamic range of human senses, the depth of the device allows for users to carry out explorations ranging in millivolts or milliamps across the full input range of the device.

Output Power

Analog systems encountered in early engineering typically range from milliwatts to tens of watts of electrical power. One is unable to supply power for a large motor with the paltry supply afforded by USB, but it seemed ideal to allow explorations spanning up to at least one full watt of electrical power, in comparison to the relatively low power handling capabilities (~100mW max for Analog Discovery) of many instruments. One watt channels also allow a two channel device to fall within the 2.5w power budget of USB without substantial difficulty. This one watt of power is more than adequate for experiments with analog circuitry, optical transducers such as incandescent bulbs or light emitting / laser diodes, thermal transducers such as Peltier arrays or temperature-dependent resistors, and a variety of small electromagnetic mechanical transducers such as voice coils, DC / AC motors, and solenoid coils. These transducers and power levels map voltage and current to arbitrary characteristics well within the range of human senses, offering users additional tools for working with systems connected to a M1k.

Sample Rate

Sample rate was one of the final parameters to consider. The inherent proportional relationship between sample rate and signal resolution make it favorable to offer as fast of conversion as possible, as frequently as possible, while the proportional relationship between power consumption and sample rate provide motivation to choose conservatively, allowing for maximum power to be available for the end user. The blazing fast AD9625 12-bit analog-digital converter offers up to 2.5 gigasamples per second, but at a power consumption which is almost twice the entirety of the budget of the M1K, fully loaded!

Human senses are typically substantially slower than the devices used for electrical test and measurement, but early exploration often occurs with signal speeds within the realm of human hearing - slower signals are impacted less by the small parasitic (inadvertent “bonus”) resistors, capacitors, and inductors which are present in all electrical circuits. Working with slower signals also offer pedagogical benefits - it can be tremendously enriching to gain an understanding of filter circuits by listening to the waveform at subsequent nodes. Before ubiquitous and affordable instrumentation, it was normal for introductory courses to teach the use of crystal earpieces for probing and exploring the analog world - those familiar with the Radio Shack / Tandy “150-in-1” introductory circuits kits are already familiar with the technique. These aspects of learning set a minimum sample rate of forty four kilohertz, a sampling rate often used by electrical devices designed for audio work. An upper limit of approximately million samples per second was determined by the desire to offer continuous data streaming at the maximum rate over the ubiquitous high-speed USB2.0 interface. Budget, interface, and availability made the AD5663R two-channel sixteen-bit digital to analog converter and the AD7682 four-channel sixteen-bit analog to digital converter, good fits for the data conversion aspect of the design, each offering a peak sample rate of a quarter of a million samples per second. Such a rate offers the end user the ability to work with sound signals with substantially higher fidelity than even a traditional PC sound card, and allows exercises to stretch up into ultrasound, a domain often used for distance sensing (sonar) and actuation (with piezoelectric transducers.)

Tradeoffs

The design of the M1K required making tradeoffs across voltage, current, power, and speed. With the intent of building an affordable tool for introductory exploration of complicated electrical and mixed electrical/physical systems, the tradeoffs were balanced to offer high dynamic range, the ability to source waveforms higher frequency than the human ear can hear, and enough electrical power to allow the direct interface with just about anything that runs on batteries. Offering this functionality via USB makes a wide array of exploration immediately achievable without requiring local 120VAC wall power or much in the way of site infrastructure for data collection and processing.

university/tools/m1k/design.txt · Last modified: 11 Jan 2017 15:00 by rgetz