COVER STORY FEATURE


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 from Linear Technology, 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 minimise 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. This device 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, available on the company’s website. 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


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


, 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 current noise (in) becomes more


values,


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 optimised for the performance


/ ELECTRONICS Figure 2:


LTC6268 input bias current remains low over temperature


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 digitising 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 minimise 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 optimised for exactly these high performance TIA applications.


Figure 3:


Input capacitance includes sensor, board, and amplifier capacitance


Linear Technology (UK) Ltd. Now part of Analog Devices Inc www.linear.com 01628 477 066


ELECTRONICS | OCTOBER 2017 13


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