Designing APD Detector with 700-1600 nm Range, Low Dark Count, High Efficiency

dheerajknambiar wrote:

(...) us Mode
Dark Count - 800 counts per sec
Calibrated Efficiency - 10% to 30%
External trigger - from CW to 100 MHz
Timing Jitter at max QE - 150 ps
Dead time range - from 100 ns to 1 ms

Designing a photon detector system based on an Avalanche Photodiode (APD) with the specifications you've provided involves several key components and stages. I'll guide you through a step-by-step approach to each part of the system, starting with APD selection and proceeding through the amplifier circuit, timing circuit, gating circuit, external trigger interface, dead time control, and finally, the cooling system.

### 1. Avalanche Photodiode (APD) Selection:
Given the spectral range of 700 nm to 1600 nm and a calibrated efficiency of 10% to 30%, along with the desired dark count rate close to 800 counts per sec, you'll need to look for an APD that specifically matches these criteria.

- Action: Research suppliers that specialize in APDs for photon detection in this spectral range. Companies like Hamamatsu, First Sensor, and Excelitas might have products that fit these specifications. Ensure the APD's quantum efficiency (QE) falls within the desired range at your target wavelengths.

### 2. Amplifier Circuit:
For the amplifier circuit, minimizing noise while maximizing gain is crucial. A trans-impedance amplifier (TIA) followed by a voltage amplifier would be a typical setup.

- Trans-impedance Amplifier (TIA): Design the TIA with a low-noise operational amplifier (Op-Amp). Consider using a feedback resistor and capacitor to set the gain and bandwidth. Op-Amps like the AD8099 or LTC6268 could be good starting points due to their low noise characteristics.

- Voltage Amplifier: Following the TIA, a voltage amplifier can further increase the signal level for processing. Ensure that this stage introduces minimal additional noise.

### 3. Timing Circuit:
Achieving a timing jitter within 150 ps is challenging and requires precision components.

- Action: Consider using a high-speed comparator and a high-resolution time-to-digital converter (TDC) for this purpose. The TDC might need to have a resolution in the order of picoseconds, which can be quite specialized.

### 4. Gating Circuit:
For switching between gated and continuous modes, you will need a gating circuit capable of handling the high speeds and synchronization required.

- Action: Use fast electronic switches controlled by a programmable logic device, such as an FPGA, which can also handle the logic for mode switching and synchronization.

### 5. External Trigger Interface:
To accommodate an external trigger from CW to 100 MHz, your system should be capable of handling these frequencies.

- Action: Implement a high-speed trigger detection circuit that can synchronize with the external trigger signal. This might involve a high-speed comparator and synchronization logic within an FPGA.

### 6. Dead Time Control:
Managing dead time between 100 ns to 1 ms requires a precise control mechanism.

- Action: This can be implemented in the FPGA logic, where after a detection event, the system can be programmed not to register another event for the specified dead time.

### 7. Cooling System:
Considering APDs perform better with cooling, especially to reduce dark counts.

- Action: Depending on the APD's requirements and sensitivity to temperature, options might include thermoelectric coolers (TECs) or even liquid cooling solutions for more aggressive cooling needs.

### Next Steps:
Before moving forward, it would be helpful to know:
- Budget Constraints: What is the budget for this project?
- Application Context: What specific application or field will this detector system be used in? This might influence certain design choices.
- Component Availability: Do you have any preferences or limitations regarding component suppliers?

Starting from these considerations will help refine the design and ensure it meets both the technical and practical requirements of your project.

dheerajknambiar wrote:

21020285

Can you give a Circuit diagram associated with the amplifier you mentioned in previous reply

Creating a simplified circuit diagram for an amplifier setup designed to work with an avalanche photodiode (APD) involves a trans-impedance amplifier (TIA) followed by a voltage amplifier stage. This setup aims to amplify the weak current signal generated by the APD into a usable voltage signal while maintaining low noise levels.

Please note: The following is a conceptual diagram and might need adjustments based on the specific APD characteristics, operating conditions, and additional requirements such as power supply voltages, impedance matching, and bandwidth. Component values (resistors, capacitors) are not specified and need to be calculated based on the desired gain, bandwidth, and noise performance. Simulation and prototyping are recommended to fine-tune the design.



### Key Points:
- APD Connection: The APD is reverse-biased and generates a current signal in response to photon detection. This current flows through the feedback resistor (Rf), generating a voltage signal proportional to the input current.
- Feedback Resistor (Rf): Determines the gain of the TIA. Its value is chosen based on the desired signal amplification and the bandwidth. A higher value increases gain but reduces bandwidth.
- Capacitors (C1, C2): C1 in parallel with Rf sets the -3 dB bandwidth of the amplifier. C2 is for DC blocking or additional filtering, if needed.
- Operational Amplifier (OPA): A low-noise, high-speed op-amp is critical for maintaining signal integrity and minimizing additional noise.

### Design Considerations:
- Noise Performance: The choice of op-amp and the values of Rf, C1, and C2 are crucial for low noise performance. It's essential to minimize the noise contribution of the amplifier to not drown out the signal from the APD.
- Bandwidth: The amplifier's bandwidth must match the application's requirements, considering the APD's speed and the signal's characteristics you aim to detect.
- Power Supply: The op-amp and APD power supply voltages (+Vcc, GND) need to be compatible with the devices used and provide stable operation.

This schematic provides a starting point. You'll need to customize component values and potentially the configuration based on your specific requirements, including the exact APD model, expected signal levels, and application constraints. Simulation tools like LTspice can be invaluable for refining this design before moving to prototyping.