WEBENCH Tools and Photodetector Stability

The first priority for photosensor applications is to achieve good stability in the transimpedance amplifier circuit. WEBENCH ® Designer Tool TI developers strive to provide customers with photosensor designs that have 60° phase margin, which is about 8.7% overshoot of the step input signal.

The WEBENCH Designer tool has powerful software algorithms and a visual interface that can generate complete power , lighting, and sensor detection applications in seconds. This capability allows users to compare system and supply chain values ​​before designing. One of the many tools embedded in the WEBENCH environment is the Photodiode section of the Sensor Designer . This article will focus on the embedded photodiode circuit stability of the WEBENCH Sensor Designer.

Consequences of Ignoring Stability

Many light-sensing applications use photodiode preamplifier (preamp) circuits. These circuits convert light information from an LED or light source into an effective voltage. When using precision photoconductance circuits with zero bias voltage (photoZB) and high-speed photoconductance circuits with negative or reverse bias voltage (photoRB), the embedded circuit phase margin is critical. Some precision photoZB applications using photodiode preamps include CT scanners, blood analyzers, smoke detectors, and position sensors. These precision circuits require voltage feedback amplifiers with low input bias current, low offset voltage, and low noise. Low-precision photoRB applications that utilize detection of digital light signals include barcode scanners and fiber optic receivers. These high-speed application circuits require voltage feedback amplifiers with greater bandwidth.

The simplest way to design a photodiode preamplifier circuit is to place the photodiode between the amplifier inputs, connect the non-inverting input to ground, and place a resistor in the feedback loop. This allows you to configure the light-sensitive photodiode with or without a bias voltage. In the precision photoZB configuration (see Figure 1a), the input amplifier needs to have a FET or CMOS input structure with low input bias current and low offset voltage. In this circuit, the photodiode cathode is connected to the amplifier inverting input, while the photodiode anode is grounded. The photodiode sensor in this circuit is zero biased. Note the direction of the current I _PH with respect to the anode and cathode of the photodiode .

Figure 1 Photodiode pre-amplification structure

If digital speed and fast response time are important, the photoRB structure (see Figure 1b) uses a reverse biased voltage on the photodiode. This reverse bias voltage creates a leakage current in the photodiode. However, the parasitic capacitance of the photodiode is significantly lower than that of the photoZB structure. The reduction in the photodiode capacitance increases the bandwidth of the circuit. The amplifier used in the reverse biased photodiode preamplifier configuration can use FET, CMOS, or bipolar inputs; however, the higher the bandwidth of the amplifier, the better.

In either configuration, incident light on the photodiode causes a current (I _PH ) to flow through the diode from cathode to anode. This current also flows through the feedback resistor, R _F , causing a voltage drop across the resistor. The amplifier input stage holds the amplifier inverting input at approximately ground level.

The easy solutions shown in Figures 1a and 1b will not usually work. Figure 2 shows how a step input light signal can produce horrible ringing at the amplifier output, V _OUT . If we are lucky, this photosensitive circuit may not exhibit ringing, but it is best to understand and compensate for this stability problem.

Figure 2 Uncompensated photoZB photodiode circuit

_{In Figure 3, the addition of capacitor CF} in the feedback loop changes the overall phase margin of the circuit and eliminates the ringing of the output signal. However, this simple solution overcompensates because the value of _CF is set too high, causing the amplifier output to propagate too slowly.

Figure 3 Transition compensation photodiode circuit

In photoZB applications, the overcompensation shown in Figure 3 may be acceptable, but it consumes more power and has higher noise than a properly compensated circuit. For photoRB applications, this circuit response is unacceptable because it does not produce a good square wave response. Since the photoRB circuit relies on a noise-free digital square wave signal, we need to pay more attention to the structures shown in Figures 2 and 3 to obtain correct compensation.

Photodiode compensation factors

The target phase margin for this transimpedance amplifier is 60°. This phase margin allows for an 8.7% overshoot in terms of step response (see Figure 4). Some designers would say that the correct phase margin for this bipolar system is 45°. As shown in Figure 4, the step response of the 45° phase margin circuit is 22.5%.

Figure 4 Relationship between overshoot response and phase margin

In theory, both phase margins allow for a stable circuit design; however, we have not yet considered variations in amplifier bandwidth, resistance, capacitance, and stray capacitance. These variations can have a significant adverse effect on a circuit with a 45° phase margin.

Proper compensation of the simple circuit shown in Figure 3 requires a clear understanding of the effects of capacitance and resistance. Figure 5 shows a system model that includes a feedback network (R _F and CF ₎ and an op amp . The following discussion will show you how all the capacitive components combined have a direct impact on the frequency response of the circuit. Before installing hardware or performing manual calculations, we can first use the WEBENCH Sensor Design tool to generate a design with good system stability.

Figure 5 System model of photodetector circuit

The transfer function of the bipolar system circuit shown in Figure 5 is:

Where β is the inverse of the noise gain, that is:

Z _IN is the input network impedance, that is:

Z _F is the feedback network impedance, that is:

Using algebraic calculation methods, the equations for the system pole frequency f _p and the system zero frequency f _z are:

Figure 6 shows the frequency response of the system in a graphical form. In this figure, fi _is the cutoff frequency between the feedback system (1/β) and the amplifier open-loop gain (AOL). The frequency fBW _{is the} gain-bandwidth product of the amplifier. In this system, the DC gain G1 is determined by the resistors RF _and RSH _. Note that the feedback resistor (RF ₎ is in the numerator of the second term, while the input resistor (RSH ₎ is in the denominator. The high-frequency gain G2 of the system depends on the capacitance of the system. Note that the numerator of the second term contains the sum of the input capacitances, while the denominator contains the feedback capacitance of the circuit (CF ₎ .

Figure 6 Frequency response of the photodiode circuit

Stability design principles

The distribution of the pole frequency (f _p ) and the cutoff frequency between 1/β and A _OL determines the stability of the circuit. The point where the feedback curve intersects the amplifier open-gain curve determines the stability of the circuit. In particular, the phase margin of _{fi determines the type and amount of ringing or overshoot produced by the circuit. For example, if f p} equals _fi , the circuit has a phase margin of 45°. A 45° phase margin produces an overshoot of ~22.5% on a square wave input signal. If the circuit has a phase margin of ~60°, the corner frequency of the pole occurs before the amplifier A _OL curve crosses (see Figure 6). If the corner frequency of f _{p is lower than the A OL} cutoff frequency, a design with a 60° phase margin is possible. A 60° phase margin results in an overshoot of ~8.7% on a square wave input signal.

WEBENCH Implementation

_{The WEBENCH implementation of the PhotoRB sensing network involves selecting the correct feedback capacitor (C F} ) for the ideal 60° phase margin , choosing the correct amplifier, and following the circuit's ADC recommendations. The WEBENCH Sensor Design Tool provides a working circuit and a printed circuit board that can be purchased without components installed. Figure 7 shows a block diagram of the WEBENCH photoRB system.

Figure 7 WEBBENCH implementation of photoRB application circuit

in conclusion

To design a light sensing circuit with good stability, you need to follow some methods. WEBENCH Sensor Design Tool is powerful enough to provide you with a circuit with a stable 60° phase margin.

References

1. Bonnie C. Baker, “Transimpedance Amplifier Noise Problems,” (October 2, 2008), EDN (Online), http://www.edn.com .

2. Bonnie C. Baker, “Transimpedance amplifier stability is key to photosensitive applications” (September 4, 2008), EDN (online), http://www.edn.com .

3. Bonnie C. Baker, “Improving Photosensitivity Using Integrated Photodiode/Operational Amplifiers,” Canadian Electronics, June 1, 1996.

4. Jerald G. Graeme, “Photodiode Amplifiers”, Boston: McGraw-Hill, 1996.

5. “Understanding and Using Transimpedance Amplifiers (Part 1 of 2)” by David Westerman (August 8, 2007), published in EE Times (online version), address: http://eetimes.com .

6. “Understanding and Using Transimpedance Amplifiers (Part 2 of 2)” by David Westerman (August 10, 2007), published in EE Times (online version), address: http://eetimes.com .

Related Website

Data converters: www.ti.com.cn/lsds/ti_zh/analog/dataconverters/data_converter.page

WEBENCH Design Center: www.ti.com/webench

WEBENCH Designer: www.ti.com/ww/en/analog/webench/sensors/index.shtml