Tips for Achieving Low-Frequency Precision and Improved Bandwidth in Photodiode Circuits - Technical Articles

In the previous article, we started a conversation about design techniques in the following areas:

.

Here, we will introduce two concepts that may need to be adjusted for transimpedance amplifiers in photodiode circuits: leakage current and bandwidth.

If you want to learn more about photodiodes, please don't forget to check out my "Introduction to Photodiodes" series. The first article in the series introduced

The current generated by the photodiode is in the nanoamp and low microamp range. Under such small trends, non-ideal behaviors that we often overlook can become obvious and even problematic.

First, carefully study the input bias current specifications of the op amp. Ideally, zero current flows in or out of the input terminal, and all photocurrent flows through the TIA's feedback resistor and contributes to the output voltage.

Unfortunately, real-life op amps require some input bias current, and bias currents that seem negligible in other applications can create unacceptable errors in the photodiode system. In the case of non-zero bias current, some of the photocurrent is transferred to the input stage of the op amp, and if the photocurrent is in the low nanoampere range, it does not need to spend a lot of current shunt to seriously change the measurement reported by the amplifier value.

Typically, you will need an operational amplifier with a FET input stage. BJT has absorbed too much bias current. However, even the FET input stage has protective diodes commonly found in IC input circuits. These diodes have leakage current, and as temperature increases, this leakage current becomes more important. If you are designing a photodiode amplifier for high temperature applications, make sure to check the high temperature specifications!     

Operational amplifiers suitable for TIA applications can achieve extremely low input bias currents. For example, I did a quick search and found the LTC6268 from Analog Devices. At room temperature, its leakage current is only a few femtoamps. However, at 125°C, the specification is 4 picoamps (maximum), an increase of three orders of magnitude!

Second, we need to remember that PCB traces are not surrounded by materials that provide infinite resistance. If the connection to the photodiode is close to a trace or toppled copper wire, a significant potential difference will be created, and the DC leakage current flowing through the PCB may be large enough to cause errors.

The input signal of the photodiode goes through a trace leading to the inverting input terminal of the operational amplifier. The inverting input terminal is usually located at or near ground because the non-inverting input terminal is maintained at ground or a small bias voltage. Therefore, the traces that are more likely to cause leakage current problems are those whose voltage is not close to zero, such as positive or negative supply voltage. In order to maximize accuracy, please leave as much space as possible between these traces and the photodiode input traces (within a reasonable range).

Many photodiode applications do not require high-frequency response, which makes life easier, because it is difficult to design an optimized photodiode circuit even when speed is not a major issue. When you make demands on mixed bandwidth, the situation can become very challenging.

The circuit diagram introduced in the previous article shows an ordinary capacitor (C

) Is included in the feedback path to ensure sufficient stability:

However, in high-speed photodiode applications, the optimal feedback capacitance may be very small-in some cases, much less than 1 pF. This is especially true in high-gain applications, because as the feedback resistance increases, the need for feedback capacitance decreases.

Therefore, the broadband photodiode TIA may not need CF, because the feedback is not at the frequency that produces instability, or because the feedback path has a large parasitic capacitance, so there is no need for deliberately installed capacitors.

Furthermore, we see that the parasitic capacitance may actually be greater than the required compensation capacitance. In this case, parasitic capacitance unnecessarily limits the bandwidth of the TIA, and the designer's task is to reduce the feedback capacitance to increase the bandwidth.

In a compact layout with short traces, we cannot do much to reduce the capacitance of the copper connection in the feedback path. However, we can reduce the parasitic capacitance associated with the feedback resistor.

First, we can try to modify the PCB size of the resistor. In theory, the capacitance can be reduced by reducing the parallel plate area of ​​the resistor end caps and increasing the distance between the end caps. Next, we can reduce the end-to-end capacitance by placing ground traces between the resistor PCB footprint pads. You can read more about these technologies on pages 14 and 15.

We have covered various interesting details related to TIA design, and hope that you will find useful information when designing or analyzing circuits containing photodiode amplifiers. If you have other tips or tricks, please feel free to share in the comments section.

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