Machine Design

Analog on the fly

Field programmable analog arrays, like the AN10E40 from Anadigm Inc., are easy to program. The accompanying AnadigmDesigner software has an extensive library of analog functions that lets designers build and test circuits without the need for breadboarding and prototyping.

The interface circuitry for an LVDT consists of a 2.5 kHz sine-wave generator, low-pass filters, rectifiers, and summing amplifiers, all of which are implemented in an FPAA.

The AN10E40 FPAA offers 20 configurable analog blocks (CABs). Each CAB contains an op amp, comparator, five switchedcapacitor arrays, and other support circuitry.


The AN10E40 FPAA offers 20 configurable analog blocks (CABs). Each CAB contains an op amp, comparator, five switchedcapacitor arrays, and other support circuitry.

Analog design has finally caught up to digital. For years, digital designers have had the luxury of field programming complete digital systems in single-chip FPGAs and PLDs. Now designers of analog systems have that same ability.

One example comes from Anadigm Inc., Crewe, U.K. The AN10E40 field programmable analog array (FPAA) contains a matrix of configurable analog blocks consisting of switched-capacitor op-amp cells surrounded by a programmable interconnect and I/O structure. One cell can handle common analog signal-conditioning functions, with more complex functions implemented in two or more cells. The chip also contains internal voltage references and programmable clocks with maximum frequencies to 1 MHz.

Engineers can design circuits using a free CAD tool called AnadigmDesigner. The software library has over 50 analog functions including gain stages, amplifiers, filters, oscillators, Schmitt triggers, differentiators, integrators, and more. For example, in the case of a biquad filter, the user specifies cut-off frequency, pass-band gain, and Q factor. Circuit configuration is held in on-chip SRAM, which is initialized on power-up from EPROM, or through the chip's microprocessor peripheral interface.

One place where FPAAs are welcome is in designing interface circuitry for linear variable-displacement transducers (LVDTs). Ready-to-go units are available, but at a cost. FPAAs offer a welcome alternative.

A concrete example involves a Schaevitz E200 LVDT and a typical signal-conditioning circuit. The Anadigm FPAA provides a 2.5-kHz sine-wave excitation voltage from its large range of frequencies and can drive directly into the E200 primary impedance. This still leaves enough capacity for signal conditioning circuits in the same device.

The overall output is a dc signal proportional to the mean amplitude of the rectified input signal. Outputs feed to a microcontroller's digital-to-analog converter that computes ratiometric measurement and finally adjusts it against a linearization curve. This compensates for using the E200 beyond its nominal linear range and for any nonlinearities in the circuit.

Building this circuit using conventional analog components would take hours in design, construction, debug, trimming, and tuning. Production would require stocking many components, consume valuable board space, and increase manufacturing costs. Not to mention that changing the design would mean starting over from scratch.

To increase sensitivity, a discrete circuit would require a complete redesign. This means restocking inventory and possibly scrapping work in progress. With the FPAA, only the receiver's gain settings need be adjusted to get more sensitivity.

Implementing the new circuit with the FPAA does not require a new PCB layout or new component inventory, just a quick download. In some instances, multiple coarse and fine resolution transducer systems could be replaced by a single LVDT and FPAA. In a few milliseconds, the LVDT system could retune itself to whatever the current application environment dictates, with no need to relayout the PCB.

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