[Technology Report]
Field Programmability Pervades Analog Design
User-friendly CAD programs reconfigure on-the-fly analog circuit requirements for a system during operation.
Last year was a major turning point for the analog world. Like programmable logic devices (PLDs) and field-programmable gate arrays (FPGAs) in the digital domain, field programmability and in-system reconfigurability finally pervaded the analog front. Several single-chip field-programmable analog arrays (FPAAs), in-system programmable filters, and analog building blocks emerged, giving system designers a new capability.
With accompanying easy-to-use CAD programs and Windows-based integrated design environments (IDEs), engineers can now define complete analog functionality on a PC screen, view the results instantly, and download the circuitry onto the associated chip by only clicking a button. Also, these programmable analog ICs can be reconfigured on-the-fly to adapt dynamically to real-time system requirements.
Gone are cumbersome pc-board layouts, passive and discrete soldering, and tedious trials and errors. According to Hans W. Klein, Lattice Semiconductor's director of mixed-signal products, "what you see is what you get" with pinpoint accuracy. Plus, the design is laid out and implemented instantaneously. Weeks of laborious design and development work is accomplished in minutes with point-and-click software.
The easy-to-use graphical Windows-based software makes programming and reconfiguration jobs look like a piece of cake, notes Shane Messerly, a senior design engineer at Inceptio Medical. This Farmington, Utah-based company makes ultra-sound imaging products. Messerly has deployed Lattice Semiconductor's in-system programmable ispPAC80 as an anti-aliasing filter in front of a 12-bit successive-approximation-register (SAR) analog-to-digital converter (ADC) with a 500-kHz sampling frequency.
"Compared to a discrete solution, this is much better," he says. It offers a steep roll-off and takes only a few bypass capacitors for power supplies to complete the solution. Satisfied with the performance, designers at Inceptio Medical plan to take this programmable low-pass filter to production. While the present programmable cutoff frequency range for the device is 50 to 500 kHz, Messerly hopes to see it offer a much broader range in the future, with extensions at the lower and higher ends.
Unwrapped early last year, a single ispPAC80 chip contains all of the components required to implement almost any popular continuous-time fifth-order LPF topology with programmable gains of 1, 2, 5, and 10, and programmable cutoff frequencies from 50 to 500 kHz. Supporting it is an IDE, labeled PAC-Designer, which includes a search engine and a filter database of over 8000 configurations and a simulation tool.
The ispPAC 80 lets designers configure a desired response on a PC screen using parameters like cutoff or corner frequency (fC), ripple, roll-off, and stop-band attenuation. Upon satisfaction, the user simply downloads the filter configuration onto the chip's EEPROM memory with a button click.
Every popular response known can be implemented, including Butterworth, Bessel, Chebychev, Legendre, elliptical, Gaussian, and linear-phase types. From a single programmable device, one can obtain thousands of different responses (see "Programmable Analog IC Provides Popular Filter Responses," Electronic Design, Feb. 21, 2000, p. 66).
This programmable filter comprises a programmable-gain high-speed instrumentation amplifier, a fifth-order filter core, a buffered differential voltage output, two nonvolatile configuration memory registers, a precision voltage reference, and auto-calibration. Plus, it includes fixed resistors and variable capacitors arranged in seven arrays. Each array offers hundreds of variations in capacitor values. It has a differential signal path from input to output to guarantee high performance and signal integrity. So that engineers can configure the device when it's soldered to a pc board, it offers a digital I/O interface.
Because it eliminates external components and clocks, it's a robust device that is insensitive to component and temperature variations. This approach yields repeatable filters that can simplify a production job, Lattice Semiconductor designers claim.
Serious about changing the analog design methodology, the company has extended its in-system programmable (ISP) PLD technology to create more programmable analog parts for addressing various application needs. Presently, the portfolio includes two other programmable parts aside from the programmable filter. The ispPAC10 and ispPAC20 are aimed at designing analog front ends for data-acquisition and control applications.
Jeff Berry, a senior design engineer with Atlantek, a producer of electromechanical printers based in Wakefield, R.I., uses the ispPAC20. He has tapped this device to realize a servo current drive for a subfractional dc motor that drives ribbon in the printer. "It provides all standard analog components needed on a single chip, and it could be configured in the system using the SPI interface," he says. "The CAD tool is fairly convenient to use."
But it isn't yet perfect. For Berry's application, the capacitor values were inadequate. "For the kind of frequency response and loop-gain configuration we needed, we had to go off-chip to realize a long time-constant circuit," he notes. Despite these caveats, Atlantek is in the prototype stage with this approach and intends to take it into production soon.
We are today designing with microprocessors and op amps for good electronic power control cards. Your EPAD seems good if cost becomes low to use in India. I want to remain in touch with this technology until it becomes avilable at low price.
Vaidya Amol -September 09, 2003
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