Pour your own programmable analog

Rather than representing opposing disciplines, as they are so often miscast, the crafts of analog and digital design have acted synergistically for decades; witness the great success of mixed-signal ASSPs (application-specific standard products). But despite significant strides in mixed-signal design, the analog discipline has not, by and large, evolved to include devices whose functions, like digital devices, are either statically or sequentially determined by programming, not by wiring.

Analog-IC manufacturers have used the term "programmable" in a variety of ways, mostly in place of "adjustable." In such cases, the term is often adjacent to a qualifier, such as "pin" or "resistor." Thus, programmable-gain amplifiers allow you to set the closed-loop gain with a fixed resistor, continuously variable resistor, or a network of fixed resistors and switches. A number of analog-IC manufacturers sell op amps that are bias-current-programmable with respect to gain-bandwidth product, slew rate, and power dissipation. Others offer digitally programmable potentiometers and other R-DAC-based cells whose behaviors vary by a digital word acting most often on a single parameter. These fixed functions, though indeed useful, do not offer the sort of functional programmability you think of when you consider digital constructs any more advanced than simple combinatorial logic.

Only a few companies, most notably Zetex, Lattice Semiconductor, and Anadigm, offer topological programmability within analog ICs (references 2 and 3). These devices provide programmable connectivity as well as parametric control of analog-signal-processing resources, and their manufacturers often compare them with FPGAs; indeed, Anadigm refers to its device as an FPAA (field-programmable analog array).

Though topologically programmable analog ICs are relatively immature from a market-acceptance perspective, their manufacturers are demonstrating an increasingly sophisticated understanding of the market needs. The evaluation/development package from Anadigm that I recently testdrove is a case in point. The package, which supports Anadigm's AN220E04 FPAA, comes with an evaluation board, AnadigmDesigner2 software, a serial cable, and documentation. Among the documentation is a hard-copy version of the 24-pg AN220D04 evaluation-board user manual, a must-read that also appears with the AN220E04 user manual and the AnadigmDesigner2 user manual in electronic form on the CD-ROM. The AnadigmDesigner2 software itself contains much information about the parts it supports and presents that information through a context-sensitive help facility and help index similar to those that familiar Windows applications provide. Notably, to Anadigm's credit, I was able to find answers to a greater proportion of my questions through the company's various software-based help facilities and in less time that I've experienced with "office"-oriented software packages. Though you could argue that Anadigm's application enjoys a more narrowly focused scope and is therefore easier to support, I found the experience no less refreshing and, from a productivity perspective, no less important.

The AN220D04 evaluation board is festooned with jumpers, connectors, and pin headers—a testament to its versatility (Figure 1). The board facilitates numerous signal, configuration control, clock, and power options. It includes a daughterboard connector for applications requiring two and three FPAAs. Two chip applications route their analog signals through the daughterboard only, bypassing the FPAA position on the main board. The evaluation board also provides an extension connector for cascading multiple AN220D04 boards.

You can set up the board's analog inputs and outputs independently for either balanced or unbalanced signals. A differential input attaches either through a pair of SMA connectors or through a set of headers that provide direct access to all of the AN220E04's pins. Two single-ended signals can connect through the SMAs or through a standard 1/8-in. TRS jack, such as you commonly find on sound cards, laptops, and many consumer-audio products. The AN220E04 FPAA's inputs are differential structures, so the evaluation board provides a pair of Analog Devices AD8132 differential amplifiers to convert single-ended input signals. The input path also includes 50Ω terminations that you can connect or disconnect with jumpers J23 and J25. Be sure to check these jumpers as part of your initial setup.

Similarly, the FPAA provides differential outputs that you can use directly, again either through header pins or SMA connectors or through a pair of Analog Devices AD8130 differential receivers, which provide single-ended signals to the SMAs or another 1/8-in. TRS jack. The SMA connectors also provide the signal paths for cascaded boards.

The AN220D04's versatility extends to the evaluation board's control-logic options as well. In its default mode, the board takes configuration data by way of an RS-232 serial link from your PC through the board's DB-9 connector. The onboard 16-MHz oscillator provides the FPAA's analog clock. A freestanding mode makes use of either an SPI EEPROM, such as Atmel's AT25080 8-kbit device, or an FPGA EEPROM, such as Atmel's AT17C65 65-kbit chip. In this mode, the onboard clock drives the configuration process during the turn-on sequence and provides the timebase for the analog clock. If yours is an embedded application development, you can directly drive the digital signals through the DIO (digital-I/O) ribbon connector. Though the current software version does not support USB, the board's USB-capable microcontroller, a Microchip PIC16C745, is wired to an unpopulated connector site. Should Anadigm choose to include USB support in a future software revision, you will need only to solder to a connector and plug in.

Though most of us are in the habit of hooking up a new piece of gear while the documentation is still in the shrink wrap, this board offers so many options that I recommend perusing the user manual and familiarizing yourself with the jumper's functions, locations, and initial states before powering the product. Anadigm provides a full set of board schematics at the end of the book that helped confirm my understanding of the board's architecture. Unfortunately, those schematics are small, and you may find yourself tempted, as I was, to reach for a magnifying glass or low-power microscope. Thanks to Adobe Acrobat's zoom and image-rotation capabilities, the .PDF version of the manual on the CD-ROM rectifies the problem. Thus, with the evaluation board on the lab bench, the schematics on my laptop's display, and the hard-copy manual in my hand open to the introductory narrative, I was able to familiarize myself with the board's functions and settings in just a few minutes.

Most critical to review are the power-supply settings and turn-on behavior. You can provide power from either regulated 5V or unregulated 9V supplies. The 9V power port is a 2.1- mm jack that mates with common wall-wart supplies. You can use an unregulated supply as long as its output voltage is bounded by the board's absolute ratings of 8 and 12V, minimum and maximum, respectively. The 9V power port feeds a 5V linear regulator that drives the rest of the board through a jumper, J28, which you must remove if you are going to provide regulated 5V directly to either the banana jacks or the screw terminals. Because neither the screw terminals nor the banana jacks provide a mechanically polarized connection, these power inputs connect to the rest of the board through a Schottky diode to prevent reverse-polarity damage. If your application cannot accommodate the 200-mV diode drop, you can add a jumper that shunts the Schottky device.

The three power paths—the output of the linear regulator, the banana jacks, and the screw terminals—join at the Schottky diode's cathode. It is important, therefore, that you connect no more than one port at a time. Also note that, although the documentation states that the 5V regulated input is the default configuration, the board I received had jumper J28 fitted. Had I simply hooked up a 5V supply and flipped the switch, I would have applied power to the output of the linear regulator while its input was at ground. As an Anadigm field-application engineer confirms, this condition could damage the 5V regulator, so be sure that the jumpers are set for your power connection.

When the AN220D04 operates without a daughtercard, its maximum supply current is 350 mA, except at turn-on, when a 1.2A peak current is typical. This turn-on surge is not, as the board's data table suggests, an inrush current. The narrative portion of the manual warns that if the power-supply current limit prevents the supply output from reaching 5V while satisfying the turn-on current demand, the board may power up in a "high current state." With a regulated bench supply, the board reliably achieved this "high current state" every time I energized the supply, as indicated by a sustainable 1.1A on the power supply's current monitor and a comparator that detects the undervoltage condition and drives a board-mounted, red-LED indicator. The recommended way to deal with this ungraceful start-up behavior is to pull the analog power jumper (J5) before turning on the power supply and to fit the jumper back after the supply reaches regulation.

Although this work-around might solve the problem on the bench, it raises power-supply-management questions for your application. Unfortunately, the current version of the FPAA's data sheet is a preliminary draft lacking the spec tables, so any insight you might hope to gain into the IC's start-up behavior outside the context of the evaluation board will have to wait until Anadigm releases a complete data sheet. Meanwhile, you need to manage the digital and analog supply sequencing within your application. Also, software revisions notwithstanding, until Anadigm more elegantly addresses this issue than having users pull and fit jumpers, I have doubts about seeing a USB version of the evaluation board, given USB's 500-mA load limit.

Despite the number of jumpers and connectors you need to consider while setting up the evaluation board for the first time, the process takes remarkably little time or fuss. It took less than an hour to load the software; read through the evaluation board's manual; locate and set the jumpers for my bench setup; connect the control, power, and signal leads; load a basic configuration; and confirm the signal path. Although conceptually none of these tasks is a brain buster, it's always gratifying when a sophisticated product works the first time without a hitch.

The development environment gives a block-diagram view of your circuit. By moving the cursor over any functional block, you can confirm its identity and parametric settings (Figure 2). Double-clicking on the block opens a dialogue box wherein you can interactively modify the block's parametric behavior. In addition to providing a graphical programming environment for the FPAA, the development software allows you to connect virtual signal sources and probes to drive the built-in simulation tool (Figure 3). The drawing and simulation tools are simple to operate and arranged to make intuitive sense to anyone who has used schematic-capture and Spice software. Keep in mind, however, that, for the entry price of a couple of hundred dollars, you are getting a basic tool without many of the bells, whistles, and functions that come with the five-figures-per-seat EDA suites. So, for example, Anadigm's simulator gives you some insight into the part's behavior in the time domain, but for almost everything else, plan on learning about the part's behavior by examining real signals running through the evaluation board. No apparent hooks allow you to export simulations or behavioral models to your own Spice environment, and no means exist for the native tool to incorporate any external components. This limitation is similar to that which you find with power ICs and ASSPs whose manufacturers provide limited support software but no means to connect to the rest of your EDA environment.

FPAAs will take analog designers some getting used to. Rather than combining diodes, transistors, and passives with functional blocks, such as op amps and comparators, the FPAA environment is entirely populated by more highly abstracted functional blocks that Anadigm refers to as CAMs (configurable analog modules). (See Table 1.) Consider a comparator: The off-the-shelf part you're familiar with is an amplifier whose topology and parametric behavior are appropriate for a nonlinear comparison response rather than linear amplification. However, your application circuit must provide the additional elements to set the reference potential and loop behavior, including hysteresis.

The FPAA's comparator CAM includes these elements. The CAM provides three programmable options for the inverting input: You can refer the comparator's inverting input to any of your analog signals within the FPAA. Alternatively, you can program the CAM to refer the inverting input to ground or to a dc potential whose amplitude you specify as a CAM parameter. For cases in which you would apply the threshold voltage to the comparator's noninverting input, you can invert the output in lieu of reversing the input connections. Programmable hysteresis levels include 0, 10, 20, and 40 mV. The hysteresis function is available when you drive or ground the inverting input, but not when you program the threshold voltage.

Like most FPAA CAMs, the comparator is a synchronous discrete-time block. You can program the comparator to sample its inputs on either the first or the second clock phase—a concept that simply doesn't apply to continuous-time comparators. The comparator makes its decision on the following clock phase, but the decision may not cause an immediate change in the comparator's output state. You can program the output to either follow the decision as soon as it is available or force it to synchronize the state transition with either the first or the second clock phase.

Comparators and amplifiers are the simplest CAMs, with close analogies in the world of discrete ICs. Also common in the CAM list are functions that offer a higher level of abstraction, moving you somewhat further away from the implementation details. One example, the biquadratic filter is tunable from 500 Hz to 400 kHz in three ranges corresponding to your selection from three available clock frequencies. As you would expect of a biquad, you can select lowpass, highpass, bandpass, and bandstop transfer functions. But unlike the biquads you make yourself from several op amps and a fistful of resistors, the biquad CAM provides only one of the four transfer functions at a time. The tuning and Q controls are noninteractive, but, not surprisingly, your choice of Q from 0.15 to 70 (!) does limit the gain range (Figure 4).

Some of the less common functions might hold the true power of an FPAA, such as the multiplier or arbitrary periodic waveform-generator CAMs. However, to get a sense of how well the device works as a signal-chain element, I examined less exotic CAMs, such as inverting amplifiers, biquadratic filter blocks, and basic I/O cells. Most of this investigation employed an Audio Precision System Two Cascade dual domain analyzer for frequency response, noise, and THD plots.

The evaluation board is clearly designed to maximize your access to the various facilities it offers. It is, therefore, not optimized for minimum clock noise (see sidebar "Finding the baseline"). Though some care may be necessary when developing applications that make full use of the FPAA's bandwidth, which extends to about 8 MHz, the board should serve as a good development environment with little practical interference from clock artifacts for lower-bandwidth circuits.

The FPAA's data-sheet limit on differential input signal swings is 3.8V. Be careful, however, if you are going to run your signals at or near the swing limit. For example, the input cell provides an antialiasing filter with a programmable corner frequency of 34 to 470 kHz. THD+N measurements show congruent 1- and 20-kHz plot lines when the antialiasing filter is disengaged, (the blue and green plots in Figure 5). At 1 kHz, the THD+N penalty is small for a 34-kHz input filter (magenta), but as signal components approach the corner frequency, the distortion products rise with signal amplitudes as small as –20 dBV (red). Fixed amplitude spectral sweeps fill in the picture (Figure 6). Compared with the baseline measurement at 0 dBV with no antialiasing (blue), the THD+N penalty is about 10 dB for adding a 400-kHz filter on the input (red). Dropping the input to –6 dB reduces the THD+N with the 400-kHz filter to the same relative level that the 0-dBV, nonfiltered case gives (magenta). Note that the vertical scale is decibels relative to input level. With the input held at –6 dBV and the filter corner tuned down to 34 kHz, the THD+N level increases another 5 dB or so and exhibits a rising characteristic as the signal approaches the corner frequency. Considering these traits, you should operate the CAM cells at the fastest clock rates you can to push out the Nyquist frequency and reduce the need for close-in filter corners. Also, use the input filter only after paying due consideration to your expected signal levels and bandwidths, and consider a simple external filter if you need to accommodate high amplitudes with a low frequency corner.

The output cells also come with lowpass filters associated with their voltage-output mode. The cell cascades a pair of similarly tuned single-pole sections and is intended as a reconstruction filter to remove switching artifacts. Tuned to a relatively open 400 kHz, they performed better following a simple inverting amplifier than did the raw output cell. An inverting amplifier with a gain of –4 processing a –18-dBV swept sine wave produces essentially flat THD+N plots over the 20-Hz to 40-kHz band. Operating the output cell as a voltage output with a 400-kHz lowpass corner resulted in a –62-dBr THD+N. With the output cell in its raw configuration, the performance dropped to –52 dBr. Further investigation of the output structure is certainly warranted should you choose to design with FPAAs to help identify the best operating conditions for your application.

The FPAA's configuration memory includes a shadow RAM that helps minimize configuration time and thus the disturbance to a signal chain that includes FPAAs. Configuration changes may constitute a complete change of the analog array's internal resource allocation or simply cause a change in a single parameter. Operating the board from a laptop computer's serial port, a null configuration change—one in which the existing circuit and parameter set is simply reloaded to the part—imposed an interruption of about 110 msec (Figure 7). Applications that load configuration data from either an embedded processor or a co-resident PROM can optimize the configuration process to further reduce the load time.

Author Information

You can reach Technical Editor Joshua Israelsohn at 1-617-558-4427, fax 1-617-558-4470, e-mail jisraelsohn@edn.com.