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There is no universal FPAA function list. The CAB architecture determines which operations are native, which require several blocks and feedback, and which are impractical at the required accuracy or frequency.
What an FPAA contains
An FPAA is a reconfigurable analog integrated-circuit fabric. Instead of fixing one analog circuit at manufacture, it combines reusable analog cells, programmable interconnects, input/output circuits and configuration memory so the same silicon can be rewired for different signal-processing jobs.
The configurable analog block (CAB)
A CAB is the reusable unit that creates analog behavior. Depending on the device, it can include:
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- An operational amplifier or operational transconductance amplifier (OTA)
- Programmable capacitors and other passive elements
- MOSFET or CMOS analog switches
- Bias-current or bias-voltage controls
- Local signal routing and feedback connections
A University of Hertfordshire record describing a 5×8 prototype used programmable OTAs, programmable capacitors and MOSFET switches to build OTA-C filters. Its reported filter range was from several kilohertz to a few megahertz; that is evidence for that prototype, not a general FPAA bandwidth specification.
Interconnect and configuration
Programmable switches connect CABs to one another and to I/O. A global switch fabric can reach many destinations but adds switch resistance and parasitic capacitance to signal paths. Local or OTA-based interconnect can reduce switching in the signal path, although it may restrict placement, signal range or routing flexibility. Configuration memory and its software tools determine how circuits are loaded, changed and retained.
Generic analog functions an FPAA can implement
The following functions are common targets, but their availability and quality depend on the specific CAB design.
| Function | Typical CAB arrangement | Common uses | Availability |
|---|---|---|---|
| Pass-through or buffer | Direct routing, a voltage follower or an OTA stage | Isolation, level transfer and signal conditioning | Broadly representative, but loading and signal range vary |
| Gain | Amplifier feedback, programmable transconductance or capacitor ratios | Programmable-gain amplifiers, sensor conditioning | Representative FPAA function; gain range and linearity are device-specific |
| Filtering | RC, switched-capacitor or OTA-C poles and feedback paths | Low-pass, anti-aliasing, band-pass and notch filtering | One of the most established FPAA application classes |
| Integration | OTA charging a capacitor, or a sampled-capacitor integrator | Filter states, control loops and analog computation | Available where the CAB exposes the required integrator elements |
| Addition and subtraction | Current summing, differential amplifier connections or routed amplifier stages | Mixers, error signals, multi-input filters and control laws | Demonstrated in current-mode CABs; implementation differs by architecture |
| Multiplication and division | Translinear, current-mode or other specialized nonlinear circuitry | Analog arithmetic and adaptive or control functions | Architecture-specific, not a capability to assume for every commercial FPAA |
| Oscillation and waveform generation | Integrator, feedback loop and nonlinear or limiting element | Clocks, test waveforms and signal sources | Possible when the array provides suitable feedback and nonlinear behavior |
| Nonlinear transfer functions | Configured nonlinear cells, piecewise paths or local switch networks | Approximation, compression and specialized analog computation | Active research area; usually less general than filtering or gain |
Pass-through, buffering and gain
A CAB can route a signal directly, isolate it with a buffer or configure amplification. Programmable-gain amplifiers are a representative FPAA use because the gain can be changed by selecting feedback elements, transconductance settings or capacitor ratios rather than redesigning the chip.
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In practice, check the allowable input and output swing, common-mode range, gain error, bandwidth and load-drive capability. “Gain” on an FPAA does not imply the precision or drive strength of a dedicated instrumentation amplifier.
Filtering
Filters are among the most broadly useful FPAA circuits. By connecting integrators, capacitors, OTAs and feedback paths, a design can realize low-pass, anti-aliasing, band-pass, notch and related responses. OTA-C implementations can tune gain, bandwidth and notch frequency independently in suitable architectures.
Filtering may be continuous-time or sampled. The choice affects clock requirements, usable frequency range, noise, tuning method and susceptibility to switching artifacts.
Integration
An integrator accumulates a signal, typically by using an OTA to charge a capacitor or by transferring charge between capacitors on clock phases. Integrators are fundamental state elements for higher-order filters, control loops, oscillators and analog computers. Their accuracy is limited by capacitor variation, finite OTA gain, leakage, bias errors and, in sampled designs, clock feedthrough and settling.
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Addition and subtraction
Summing can be performed by combining currents at a node, while subtraction commonly uses differential connections or opposing current paths. Voltage-mode arrays can also construct sums with configured amplifiers and routing. The number of inputs, allowable coefficients and signal headroom are architecture-dependent.
Multiplication and division
Multiplication and division are possible in some CAB designs, but they should not be treated as standard features of every FPAA. A current-mode CAB described in a 2022 paper, with a 2022/2023 publication record in Computers, Materials & Continua, selected six operations: addition, subtraction, integration, multiplication, division and pass-through.
Those six operations demonstrate what a suitably designed CAB can do; they do not establish that an arbitrary FPAA contains a multiplier or divider. When evaluating a part, look for the actual transfer-function range, linearity, temperature behavior, bandwidth and calibration method.
Oscillators and waveform generation
Connecting an integrator in a feedback loop can create an oscillator. Limiting or nonlinear elements shape the waveform and control amplitude. A usable oscillator requires adequate loop gain, startup behavior, frequency control and amplitude stability; routing delay, switch resistance and finite OTA performance can all affect those properties.
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Nonlinear arithmetic
Nonlinear functions extend beyond the traditional filter-and-amplifier emphasis. A hexagonal FPAA study proposed systematic realization of nonlinear arithmetic functions and used local switch blocks for interconnection. This is a research demonstration of a design approach, not a claim that all FPAAs provide general-purpose nonlinear arithmetic.
Switched-capacitor versus continuous-time FPAAs
The signal-time model is one of the most important architectural distinctions.
| Characteristic | Switched-capacitor FPAA | Continuous-time OTA/Gm FPAA |
|---|---|---|
| Signal model | Samples and transfers charge on clock phases | Processes signals without intentional sampling in the analog path |
| How parameters are set | Primarily by capacitor ratios, clock timing and switched connections | By transconductance, capacitance, bias and feedback settings |
| Clock requirement | Requires a clock and non-overlapping or otherwise controlled phases | Does not require a sampling clock for the core analog operation |
| Typical advantages | Accurate ratio-based coefficients and convenient programmable sampled filters | No aliasing from the core sampling operation and potentially direct continuous-time bandwidth |
| Typical concerns | Clock feedthrough, switching noise, charge injection, aliasing and settling | OTA noise, finite gain, distortion, bias drift, capacitor mismatch and tuning variation |
| Best comparison question | Is the sample rate high enough, and are clock-related artifacts acceptable? | Are tuning range, linearity, noise and continuous-time bandwidth adequate? |
Neither approach is automatically superior. A sampled implementation can make coefficient ratios predictable, while a continuous-time implementation can avoid placing a sampling operation in the signal path. The application’s bandwidth, latency, noise and clock constraints decide which is appropriate.
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Global switch fabrics
Global routing offers flexibility: a CAB can often connect to many neighbors or distant resources. The trade-off is that every switch and long metal route contributes resistance, capacitance, leakage and possible feedthrough. Those parasitics can reduce bandwidth and alter filter coefficients.
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Local and OTA-based routing
Local interconnect keeps connections short and can reduce the number of switches directly carrying the signal. The wave-active-filter literature emphasizes reducing signal-path switches, while the hexagonal FPAA work uses local interconnection. Such approaches can improve signal integrity but may constrain where a circuit can be placed or how many blocks it can reach.
What determines precision and usable frequency
- Noise: OTA devices, switches, resistors and capacitors contribute input-referred and output noise.
- Linearity and distortion: Limited device headroom and nonlinear transconductance affect large signals.
- Finite OTA gain and bandwidth: These shift pole locations and reduce integrator accuracy.
- Capacitor mismatch: Mismatch changes sampled ratios and continuous-time pole or zero locations.
- Switch resistance and parasitics: They add loss and frequency-dependent errors.
- Clock feedthrough and charge injection: These matter particularly in switched-capacitor arrays.
- Calibration: Some designs need trimming or digital calibration to meet production tolerances.
Do not infer a universal frequency limit from one prototype. The named 5×8 OTA-C array reported filters from several kilohertz to a few megahertz in 2001; that measured range belongs to that device and implementation.
How to evaluate a particular FPAA
- Read the CAB diagram. Confirm whether it contains an OTA, op amp, programmable capacitor, nonlinear cell, current mirror or other element required by your circuit.
- List native operations. Separate direct pass, gain, sum, difference and integration from functions that require multiple CABs or feedback.
- Identify the signal model. Determine whether the path is switched-capacitor, continuous-time, or a hybrid, and record clock, bandwidth and aliasing constraints.
- Map the routing. Check global versus local switch paths, available feedback connections, fan-in and fan-out, and the effect of parasitics.
- Check performance data for the named device. Use its noise, linearity, distortion, tuning range, signal swing and frequency specifications rather than a generic FPAA assumption.
- Verify configuration support. Look for the configuration-memory type, routing and schematic software, programming interface, retention behavior and whether the hardware is a research prototype or a supported product.
Examples of architecture-specific evidence
- A University of Hertfordshire/IEEE ISCAS 2001 5×8 CAB prototype realized OTA-C filters from several kilohertz to a few megahertz.
- A current-mode CAB paper recorded in 2022/2023 demonstrated selectable addition, subtraction, integration, multiplication, division and pass-through.
- A 2026 Drexel University dissertation described a 3×4 CAB array with one configurable logic block per column, illustrating that newer FPAA research may combine analog fabrics with configurable logic rather than using a single universal CAB pattern.
These examples show the range of possible architectures. They should be read as properties of the named designs, not as a specification shared by every FPAA.
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