By Brian Black, Product Marketing Manager, and Glen Brisebois, Senior Applications Engineer, Linear Technology Corporation

Photodiodes transform light into a current or voltage which can then be used in electronic circuits in applications ranging from solar cells to optical data networks, from precision instruments to chromatography to medical imaging. What all these applications have in common is the need for circuitry to buffer and scale the photodiode output. For applications requiring high speed and high dynamic range, transimpedance amplifier (TIA) circuits like the one shown in Figure 1 are often used. In this figure, the feedback capacitance is shown as a parasitic capacitance. In many applications this will be a deliberately placed capacitor to ensure stability.

This circuit has the photodiode in “photoconductive mode” with a bias voltage applied to the cathode. The virtual connection between the two op amp inputs holds the anode at ground, thus applying a constant reverse bias voltage across the photodiode. A photodiode can be thought of as a current source (proportional to light intensity), a capacitor, a large resistor, and a so-called dark current (the small current when no photons are present) all connected in parallel. The larger the bias voltage across the diode, the smaller the photodiode capacitance tends to become. While this is good for speed, it is limited practically by the capability of a photodiode to withstand large reverse voltages.

The current generated by the photodiode (IPD) is amplified by the TIA circuit and converted to an output voltage through the transimpedance gain resistor (also referred to here as the feedback resistor, or RF). Ideally this current flows through RF (i.e., IFB = IPD), but in practice the amplifier introduces an error current in the form of op amp input bias current. This bias current results in an error voltage at the output and limits dynamic range. The larger the gain resistor, the greater this effect. It is important to select an amplifier with sufficiently low bias current (as well as input offset voltage and input offset voltage drift) to achieve the required dynamic range and overall accuracy.

One other consideration is the effect of op amp input current variation over temperature. Op amps with bipolar input stages have fairly constant input current, but this current is so high even at room temperature (nA or even μA) that unbuffered bipolar amplifiers are not suitable for many high transimpedance gain applications. For this reason, op amps with a FET input stage are often preferred over bipolar amplifiers because they have inherently lower input current – often in the single digit picoampere range or even lower at room temperature. But input ESD protection diodes leak as they get hot, causing the input current to rise exponentially with temperature. It is not unusual for an op amp with picoampere bias current at room temperature to have nanoampere input current at 125°C. One alternative is to use a discrete FET to buffer the photodiode at the amplifier input, but this requires an additional component and the associated board space and has relatively high input capacitance. Another option is to use the new LTC6268 femptoamp bias current op amp, which has input bias current of just 3fA typical at 25°C. It uses replicas of the input voltages fed into split ESD diodes to effectively bootstrap them and keep the voltage and current across the diodes extremely low during normal operation. Typical input current performance is shown in Figure 2. While this current still increases over temperature, it is orders of magnitude lower than that of other amplifiers. Guaranteed maximum input current is 0.9pA at 85°C and 4pA at 125°C. The pinout of the LTC6268 was carefully chosen to help minimize board leakage currents, which can contribute to measurement error.

At the femtoamp level, unexpected leakage sources can come from adjacent signals on the circuit board, both on the same layer and from internal layers, any form of contamination on the board from the assembly process or the environment, other components on the signal path, and even the plastic of the device package. The LTC6268 is available in SOT-23 and SOIC packages. Although the SOT-23 version has a smaller footprint for a board space advantage, the SOIC is the best choice for low input bias current applications. For this package, pin spacing is wider and V- is moved to the other side of the package, away from the inputs. Also, pins 1 and 4, next to the inputs, are left unconnected to facilitate guard ring routing. This is especially useful for applications that experience electrically noisy environments. For more information on using methods such as guard rings to protect against leakage currents, see pages 17 and 18 of the LTC6268 data sheet.

Since dynamic range is the ratio of maximum output signal to noise, it is also important to select an op amp with sufficiently low noise. Op amp current noise and voltage noise both matter, in varying degrees depending on the value of RF and CIN. The input capacitance, CIN (see Figure 3), is a combination of the photodiode capacitance, the amplifier input capacitance, and stray board capacitances. In transimpedance amplifier circuits, the current noise is multiplied by RF, causing noise to appear as an output voltage error. Also, the amplifier’s voltage noise is multiplied by the noise gain. So for higher RF values, current noise (in) becomes more dominant, and for circuits with high CIN, voltage noise (en) dominates. Finding an op amp with both low current noise and low voltage noise is challenging. The input-referred voltage and current noise of the LTC6268 is 4.3nV/√Hz at 1MHz and 5.5fA/√Hz at 100kHz, respectively, striking a good balance between the two.

Input capacitance also limits bandwidth. One way to think about this is to consider the impedance of the input capacitor as the gain resistor (RG) in a conventional inverting op amp configuration. The larger the capacitor, the smaller the impedance and the larger the effective gain the op amp “sees” (1+RF/RG), often called the noise gain. Since an amplifier’s bandwidth is inversely proportional to gain due to the constant nature of the gain-bandwidth product, this means that a large input capacitance limits the circuit bandwidth. This can also be thought of in terms of stability. Capacitance at an op amp input can create a pole in the frequency domain or a lag in the time domain. This pole can be compensated to make the circuit stable by adding a (deliberate, rather than parasitic) feedback capacitor (CF). The larger this capacitance, the more limited the circuit bandwidth. Thus it is important to choose an amplifier with low input capacitance and to carefully lay out the board to avoid stray input capacitance and feedback capacitance. See pages 14 and 15 of the LTC6268 data sheet for some practical ideas for reducing stray feedback capacitance which in practice achieves greater than 4x improvement in circuit bandwidth. With just 0.45pF input capacitance, the LTC6268 contributes only a small portion of the total circuit capacitance, preserving high bandwidth.

The LTC6268 is a good example of an amplifier optimized for the performance required by high speed, high dynamic range photodiode circuits described in this article. The LTC6268 offers 500MHz gain bandwidth, enabling the single-stage circuits shown in the LTC6268 data sheet from 20kΩ transimpedance gain with 65MHz bandwidth to 499kΩ transimpedance gain with 11.2MHz bandwidth. Also, the LTC6268 has wide bandwidth, low distortion, and high slew rate, making it suitable for high speed digitizing applications such as is shown on the last page of the LTC6268 data sheet. Its very high impedance makes it ideal for buffering high impedance or capacitive sources. A dual channel version LTC6269 is also available. Versions with individual shutdown pins reduce current consumption when amplifiers are not in use and make them suitable for multiplexed applications.

Although hundreds, if not thousands, of op amps are available on the market, finding a suitable transimpedance amp for high speed, high dynamic range photodiode circuits is remarkably challenging. Each requires its own unique set of performance characteristics, including extremely low input bias current and input current temperature drift, high speed (e.g., gain bandwidth product and slew rate), the right balance of low voltage and current noise, and low input capacitance. Special attention should also be given to board layout to minimize leakage currents and stray capacitances, which would limit the accuracy and speed of the circuit. The LTC6268 represents a new class of op amps, precisely optimized for exactly these high performance TIA applications.

Linear Technology (UK) Ltd

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