Programmable Gain TIAs Deliver Precision Amplification for Signal Processing

Converting a low current signal to a voltage output is an essential requirement of a broad range of applications, especially those relying on sensors to convert physical phenomena for measurement, monitoring, and detection purposes. When those signals are predictable and steady, transimpedance amplifiers (TIAs) are a relatively simple and reliable solution, but increasingly, engineers need a more sophisticated option with precision amplification that can adapt to variable input currents or high dynamic ranges.

TIAs are used to convert input current to output voltage through a feedback resistor. They provide a relatively easy and cost-effective way to convert small currents into voltage signals.

These devices are widely used to convert currents produced by phenomena such as light, electrical charge, or radiation into measurable voltage signals that can be amplified and conditioned for signal processing and long-distance transmission. As such, they are widely utilized in fiber optics communications, light and radiation sensing, particle detection, light detection and ranging (LiDAR), medical devices, and compact systems utilizing low-power sensors.

However, most TIAs operate on a fixed gain and are not adaptable to fluctuations or wide current ranges, limiting their performance in dynamic conditions. When the current level is not within design parameters, it can result in signal distortion, reduced accuracy, or limited performance. Adapting these for more variable or dynamic conditions requires hardware modification and additional components, adding to complexity and increasing power consumption.

Programmable gain TIAs (PGTIAs) can utilize a single amplifier to handle the wide dynamic ranges encountered in applications such as high-sensitivity optical systems, precision analytical instrumentation, and electrochemical and bioelectrical signal detection.

Unlike standard TIAs, PGTIAs allow for optimization of gain for a particular signal range, maximizing output signal strength and thus overall system signal-to-noise ratio (SNR). These components can dynamically change gain to amplify weak signals and prevent strong signals from saturating the output.

With the ability to adapt to changing signal conditions and dynamically change gain, PGTIAs are suitable for applications with wide input dynamic ranges and high-precision measurement devices. For example, PGTIAs can adapt dynamically to the signal levels of LiDAR systems that measure variable reflected light.

Single versus dual-channel PGTIAs

Single-channel PGTIAs excel for applications that rely on measuring or detecting signals from a single point, such as in a simple motion detector or a bar code scanner. But many applications require an even more adaptable solution to deliver greater precision, further reduce electronic noise, analyze multiple parameters, and provide superior processing and adaptability in rapidly evolving markets.

Dual-channel PGTIAs can process signals from two independent input sources simultaneously, enabling designers to consolidate functions like differential detection, noise cancellation, and multi-parameter analysis. Integrating dual amplifier channels into one compact package is more cost-efficient than employing separate single-channel devices and can reduce the need for additional components. Each channel can be optimized for different input ranges, providing designers greater versatility for their applications.

Other advantages of dual-channel PGTIAs include more efficient power consumption, minimization of parasitic effects that could result from combining discrete components, and reduction of needed board space. The dual channels can be used for diverse application design purposes, such as:

• Acquiring data simultaneously from independent data sources to increase efficiency
• Providing redundancy of measurements to improve reliability
• Achieving comparative measurements from two signals

While dual-channel PGTIAs may be slightly more costly per unit than single-channel alternatives, that will likely be more than offset by reduced component counts, simpler assembly, and improved quality control.

ADI's highly integrated, compact PGTIA

Analog Devices, Inc. (ADI) offers a compact and flexible solution for applications requiring precision PGTIAs, such as optical networking equipment, photodetector interfaces, and precision instrumentation.

The ADA4351-2 (Figure 1) is a monolithic, dual-channel PGTIA in a 3 mm x 3 mm lead frame chip scale package (LFCSP) with no exposed pad. Each channel has two selectable feedback paths, where each feedback path’s gain is set by an external resistor.

Figure 1: ADI's ADA4351-2 PGTIA provides a monolithic, dual-channel option for precisely measuring small currents over a wide dynamic range. (Image source: Analog Devices, Inc.)

The ADA4351-2 can meet the needs of a range of applications that depend on high precision, sensitivity, and adaptability. Its versatility makes it a good fit for applications requiring precise signal amplification, high dynamic range, and integrated functionality, such as optical communication, medical imaging, spectroscopy, and scientific instrumentation. Its operating temperature range is -40°C to +125°C.

The ADA4351-2's compact design and ability to directly drive an analog-to-digital converter can simplify system architectures, reduce component count, and enhance reliability. It can directly drive two 16-bit precision ADCs (Figure 2, one shown), such as ADI's AD4695 and AD4696, providing developers with a complete analog front-end for precision current-measuring applications.

Figure 2: Schematic of ½ of the ADA4351-2 driving an ADC such as ADI's AD4695/AD4696. (Image source: Analog Devices, Inc.)

The ADA4351-2 provides distinct analog and digital inputs and can operate on bipolar supplies to accomplish high-performance analog tasks while maintaining seamless and low noise communication with ground-referenced digital systems. The digital supplies provide flexibility to control the switch logic separately from the analog supply range.

The solution simplifies design for mixed-signal environments as the ADA4351-2 can be integrated into systems requiring high-performance analog processing while maintaining compatibility with low-voltage digital control logic.

Its analog circuitry can use either a single supply (2.7 V to 5.5 V) or a dual supply (±1.35 V to ±2.75 V), enabling both unidirectional and bidirectional input signals. It can directly drive ADCs with reference voltages up to 5.5 V.

The digital input works with power supplies between 1.62 V and 5.5 V, making it compatible with common logic levels of 1.8 V, 3.3 V, or 5 V, depending on the voltage applied to the digital supply pins (DVSS and DVDD).

The two integrated, low off-leakage proprietary switches per gain setting are arranged in a Kelvin configuration to reduce inaccuracy due to non-idealities of CMOS switches. The advanced switching technology makes it an efficient solution for many applications, with a significantly reduced PCB footprint compared to using discrete components.

The ADA4351-2 has a gain bandwidth product of 8.5 MHz to handle high-frequency signals. User-programmable gain allows for optimizing dynamic range across a wide range of input currents.

Prototyping and testing the ADA4351-2

ADI's EVAL-ADA4351-2EBZ evaluation board (Figure 3) allows designers to quickly prototype, test, and optimize applications using the ADA4351-2 before moving to a custom PCB design.

Figure 3: The EVAL-ADA4351-2EBZ comes populated with the key components that allow users to run and evaluate applications using the ADA4351-2 PGTIA. (Image source: Analog Devices, Inc.)

The board supports rapid configuration for photodiode interfacing, gain selection, and other applications, making it a practical tool for developing precision analog front-end systems for optical, instrumentation, and data acquisition scenarios.

It is pre-configured with the components needed to demonstrate the key features of the ADA4351-2, including its programmable transimpedance gain, low-noise operation, and wide dynamic range. An unpopulated photodiode slot on each channel supports quick prototyping.

Open resistor and capacitor footprints at the input and output allow the installation of components with user-defined values for modifications, such as a low-pass filter (LPF) or voltage divider. Edge-mounted SMA connectors and test points allow for directly connecting test equipment to the inputs and outputs of both channels, as well as to the gain switch control pins.

Developers can explore different configurations and test the amplifier with their own signal chain components, such as ADCs or optical sensors.

Conclusion

With ADI's ADA4351-2 dual-channel PGTIA, developers can achieve more precise and reliable performance for various photodiode interfacing, optical, instrumentation, and data acquisition applications. Utilizing its integrated switching, programmable gain, and superior noise performance, it provides a highly adaptable and efficient solution for simultaneously processing signals from independent input sources.