Simplify Direct Time-of-Flight Receiver Design with an Integrated Current-to-Bits Module

Time-of-flight (ToF) systems serve a broad set of applications, including light detection and ranging (LiDAR), laser rangefinders, 3D depth sensors, drone altimeters, and optical time-domain reflectometers (OTDRs). Architectures that use separate components in the receive path support flexibility but expose designers to challenging tradeoffs among bandwidth, dynamic range, saturation recovery, and noise performance.

This article describes the role of ToF receivers and their associated design challenges. It then introduces an Analog Devices ToF receiver design that addresses these challenges through an integrated architecture.

Why matching challenges complicate discrete ToF system designs

Although simple in concept, a ToF receiver is complex to implement. It must capture return pulses lasting on the order of tens of nanoseconds without rounding their edges, span a dynamic range that runs from faint long-range returns to overwhelming reflections from close targets, recover from saturation quickly enough to avoid losing the next pulse cycle, and keep its input-referred noise below the optics-limited signal level so that range and accuracy are set by the optics rather than by the receiver electronics.

When a receiver is built from discrete parts, the design faces additional challenges due to matching issues that can arise at every block boundary. The transimpedance amplifier (TIA) noise must be matched to the gain of an avalanche photodiode (APD), the TIA bandwidth to the analog-to-digital converter (ADC) sample rate, and the single-ended TIA output to the ADC's differential input, while an anti-aliasing filter protects the sampler.

In its most basic form, a pulsed direct-ToF receiver for short-range applications typically consists of only a few stages: a photodetector such as an APD or a positive-intrinsic-negative (PIN) photodiode, a TIA, and an ADC. In applications that use detector arrays, matching problems become more acute (Figure 1).

Figure 1: Working alone or as part of an array, a discrete receive chain links a photodetector, TIA, and ADC in series, leaving the designer to match each stage to the next. (Image source: Analog Devices)

How an integrated solution meets ToF receiver challenges

Analog Devices' ADA4356 current-input µModule (Figure 2) offers an integrated solution to address these challenges. Designed to simplify ToF receiver design, the ADA4356 consolidates the receive chain into a single current-input device packaged in a 12 × 6 millimeter (mm) 84-ball ball grid array (BGA). The module's receive-chain input pin (INPUT) connects to a programmable-gain, field-effect transistor (FET) input TIA that converts the photodetector’s current output into a voltage. The TIA's output then passes through a signal-conditioning stage comprising a fully differential amplifier (FDA) and a programmable low-pass filter (LPF), before being digitized by a 14-bit pipeline ADC at sampling rates up to 125 megasamples per second (Msamples/s).

Figure 2: Bringing the TIA, FDA, LPF, and ADC behind one current-input pin moves the block-boundary matching inside the device. (Image source: Analog Devices)

By consolidating the receive chain, the ADA4356 module achieves a small footprint and alleviates the traditional burden on designers to solve the block-boundary matching problem described earlier. Rather than dealing with performance that varies with discrete-component choices, designers start with a well-characterized device with a known receive-chain noise floor. Because the module maintains nearly constant noise performance across varying photodetector source capacitances, it also provides designers with a more predictable starting point for receive-path budgeting (Figure 3).

Figure 3: The noise spectral density of the ADA4356 remains nearly constant across photodetector source capacitances, providing designers with a fixed budgeting reference. (Image source: Analog Devices)

Programmable features target specialized requirements

Diverse ToF applications rely on the same fundamental receive chain but differ in specific requirements, including physical footprint, thermal management, and performance characteristics. With the ADA4356, designers can configure their receive chains to meet a broad range of these requirements.

For example, the module meets the needs of space-constrained devices. Operating across the -40°C to +85°C industrial range, the module can run off a single 3.3 V supply. Its internal bypass capacitors on every supply rail eliminate the need for external decoupling, further reducing the footprint. When space is at a premium, designers can use the module's on-chip 1.8 V low-dropout (LDO) regulator to power the ADC core. Alternatively, designers can disable the internal LDO and use an external 1.8 V source optimized for their target efficiency, noise, or performance characteristics.

Designed to capture return pulses on the order of tens of nanoseconds, the module can nevertheless be configured by designers to meet specialized operating and capture requirements. For example, the module's internal analog LPF can be set to a 100 megahertz (MHz) cutoff to preserve the edges of narrow pulses or serve as an anti-aliasing filter for the ADC. Alternatively, the LPF can be set to 1 MHz to lower the noise floor for slower or wider-pulse applications.

The module’s gain-programmable TIA enables developers to adjust the receiver's sensitivity with three feedback resistor settings of 4.54 kΩ, 11 kΩ, or 133 kΩ, accommodating recommended linear input currents of 3 µA to 300 µA, 1 µA to 100 µA, and 100 nanoamperes (nA) to 10 µA, respectively. To handle peak currents beyond the native range, an external current divider can be used to extend current detection up to 60 milliamperes (mA) (Figure 4).

Figure 4: A compact external current divider placed ahead of the input extends the usable range to higher peak currents without changing the receiver architecture. (Image source: Analog Devices)

Even without an external current divider, the module maintains performance during transients, such as when close, highly reflective targets drive large transient currents into the receiver. The ADA4356 withstands an analog input current of 40 mA without damaging the TIA. Internal overload-current protection, combined with fast input overload recovery, quickly returns the device to normal operation. At the ADC stage, out-of-range recovery occurs within a single clock cycle, so a saturating return does not blind the receiver to the subsequent pulse.

Configuring one device across different applications

Because the receive chain is configured rather than selected by component, the same ADA4356-based design can serve applications such as OTDR that require dramatically different sensitivities and bandwidths. OTDR used in data centers resolves closely spaced events over short fiber runs, where bandwidth is the priority, pairing the lowest 4.54 kΩ gain with the 100 MHz filter. Long-haul OTDR instead measures returns across long fiber spans using wider pulses, with the priority on the lowest possible noise floor, pairing the highest 133 kΩ transimpedance gain with the 1 MHz filter. When an application can tolerate the added acquisition time, real-time averaging further lowers the effective noise floor (Figure 5).

Figure 5: At the 133 kΩ gain and 1 MHz filter used in long-haul OTDR, averaging significantly lowers the noise floor. (Image source: Analog Devices)

Designers can use a similar approach to rapidly deliver receiver solutions for other direct-ToF applications. Sensitivity-limited cases with longer pulses favor higher transimpedance gain and a narrower filter, while bandwidth-limited cases with narrow or closely spaced pulses favor lower gain and a wider filter. A designer applies these operating points via the gain-select and filter-select pins or over the Serial Peripheral Interface (SPI). Thus, moving the same device from one application to another is a configuration change rather than a board redesign or a different part.

Moving quickly from evaluation and prototyping to custom designs

Analog Devices provides a comprehensive set of resources to help developers evaluate, prototype, and create custom ADA4356-based designs. The EVAL-ADA4356EBZ evaluation board and accompanying software provide a complete platform for understanding the ADA4356’s configurability and performance. It comes configured for immediate use with Digilent's ZedBoard development board, which connects through the evaluation board's field-programmable gate array (FPGA) mezzanine card (FMC) connector (Figure 6).

Figure 6: The EVAL-ADA4356EBZ evaluation board, connected to a controller board via its FMC connector, provides access to a complete ADA4356 subsystem for evaluation and prototyping. (Image source: Analog Devices)

Following the step-by-step instructions in Analog Devices' EVAL-ADA4356EBZ user guide, developers connect the EVAL-ADA4356EBZ to the ZedBoard via the FMC connector, load software from the secure digital (SD) memory card included with the evaluation board, and power up the board set using a 12 V wall adapter. An Ethernet connection between the ZedBoard and a host PC completes the setup and enables communication with the board via evaluation tools or a terminal program, such as Windows PowerShell, for basic functions like system shutdown and debugging.

To work with the on-board ADA4356, the evaluation board offers two methods for providing input to the module. The default input source is a buffered Howland current source (BHCS) driven by a voltage source, such as a function generator, connected to the evaluation board's SMA input. The board's alternative input pathway provides a 3-pin APD slot with an onboard bias circuit intended to support prototyping ADA4356 configurations with specific APDs. An onboard current-divider circuit scales down the input current from the BHCS or APD to the ADA4356's linear input current range.

Analog Devices' analysis-control-evaluation (ACE) software’s graphical user interface (GUI) provides an intuitive, menu-driven way to control the ADA4356 and monitor its output. Developers specify the number of samples along with the ADA4356’s frequency and gain settings, then observe the sampled signals in the time domain or in histogram or fast Fourier transform (FFT) displays (Figure 7).

Figure 7: The ACE software tool enables rapid exploration of the ADA4356 settings and their impact on its output. (Image source: Analog Devices)

Analog Devices also provides extensive support for Linux-based industrial I/O (IIO) through its open-source libIIO library and device support, enabling the use of generic IIO tools such as IIO oscilloscope, scopy, and IIO command line tools when working with the evaluation board. This IIO support extends to programmatic access through its pylibiio module, which provides Python bindings for libIIO, and Analog Devices' pyadi-iio Python application programming interface (API). Among the code samples in the pyadi-iio GitHub repository, the Python ada4356_lidar_example.py application demonstrates key design patterns for configuring and sampling from the ADA4356, analyzing its output, and displaying the results (Listing 1).

Copy
...
# Connect to ADA4356
try:
    lidar = adi.ada4356_lidar(uri=uri)
except Exception as e:
    print(f"ERROR: {e}")
    libm2k.contextClose(m2k)
    sys.exit(1)
 
# Configure TDD
tdd = lidar.tdd
tdd.enable = False
tdd.burst_count = 0
tdd.frame_length_raw = frame_length
tdd.channel[0].on_raw = CH0_ON_RAW
tdd.channel[0].off_raw = CH0_OFF_RAW
tdd.channel[0].enable = True
tdd.channel[1].on_raw = CH1_ON_RAW
tdd.channel[1].off_raw = CH1_OFF_RAW
tdd.channel[1].enable = True
tdd.enable = True
...
# Capture
adc_sample_rate = float(lidar.sampling_frequency)
lidar.rx_buffer_size = buffer_samples
lidar.rx_enabled_channels = [0]
print(f"Buffer: {buffer_samples:,} samples ({buffer_samples * 2 / 1e6:.1f} MB)")
 
time.sleep(0.3)
 
start_time = time.perf_counter()
data = lidar.rx()
capture_duration = (time.perf_counter() - start_time) * 1000
...
if mode == "fft":
...
    # --- FFT analysis ---
    N = len(data)
    data_ac = data.astype(np.float64) - np.mean(data)
    window = np.blackman(N)
    coherent_gain = np.sum(window) / N
    data_windowed = data_ac * window / coherent_gain
 
    fft_result = np.fft.rfft(data_windowed)
...
    print(f"\n--- FFT Analysis ---")
    print(
        f"Fundamental: {fund_freq / 1e6:.6f} MHz (expected {actual_sine_freq / 1e6:.6f} MHz)"
    )
    print(f"  Amplitude: {fund_amplitude:.1f} codes ({fund_dbfs:.1f} dBFS)")
    print(f"  SFDR: {sfdr_db:.1f} dB, SNR: {snr_db:.1f} dB")
...

Listing 1: Sample code for an ADA4356-based lidar application demonstrates key design patterns for initializing the ADA4356, capturing its output, analyzing the data, and presenting the results (ellipses indicate code removed for brevity). (Code source: Analog Devices)

For designers moving toward a custom board, ADI publishes the supporting reference design with open hardware description language (HDL) sources. The design can be rebuilt and modified for a target system using the published HDL sources and reference design. These resources include the hardware-synchronized timing required for a pulsed direct-ToF receiver to align data capture with laser firing, providing a documented path from evaluation to deployment.

Conclusion

ToF receivers used in many application domains must capture short return pulses over a wide dynamic range, recover quickly from saturation, and maintain low-noise performance. However, balancing these requirements is difficult when the receive chain is built from discrete components. The ADA4356 consolidates that chain into a single configurable device that adapts to different application demands while maintaining consistent performance. Using an evaluation platform and associated software, designers can quickly move from evaluation and prototyping to custom design of ToF receiver solutions.