Analog Devices partnered with First Sensor to design a reference design featuring the First Sensor 4-channel APD array, Maxim quad TIA with multiplexer MAX40662, and Maxim single fast comparator MAX40026.
This document highlights how easy it is to interface these three critical components when designing the front end of a receiver in a light detection and ranging (LiDAR) module. The MAX400662 and MAX40026 are AEC-Q100 qualified. So, these are highly suitable for automotive applications. These can be used in any industrial or commercial applications needing LiDAR.
This document provides some tips and recommendations seamlessly applied to systems with a higher count of channels.
The quad-channel LiDAR receiver front-end reference design features a 4-channel avalanche photodiode (APD) array from First Sensor, interfacing the Analog Quad TIA MAX40062 and Analog single fast comparator MAX40026.
The reference design implements test points to check each part of the board independently. Table 1 describes each critical piece of the board in detail with a focus on how to interface the APD to the trans-impedance amplifier (TIA) and the TIA to the comparator.
A special paragraph is dedicated to the layout recommendations.
EVKIT PART - IC; AMP; QUAD TRANSIMPEDANCE AMPLIFIER WITH INPUT CURRENT CLAMP AND MULTIPLEXER FOR LIDAR; PACKAGE OUTLINE DRAWING: 21-100204; PACKAGE CODE: T1644Y+5C; LAND PATTERN: 90-0070; TQFN16-EP
U2
1
EVKIT PART-DIODE; FIRST SENSOR APD ARRAY; SMT; PIV=200V
The J1 jumper is implemented to adjust the gain of the TIA. Set the gain to 25kΩ or 50kΩ in a static way in the application by connecting the GAIN pin to the GND or VCC permanently or have a dynamic signal controlled by a microprocessor to change the gain on the fly as required in the application. See Table 2 for more details.
Table 2. Jumper Table (J1)
Jumper
Shunt Position
Description
J1
1 TO 2
Connects GAIN to GND (Gain = 25kΩ)
J1
2 TO 3
Connects GAIN to VCC (Gain = 50kΩ)
Table 3. Jumper Table (J2)
Jumper
Shunt Position
Description
J2
1 TO 2
Connects LP to GND (Low-Power Mode On)
J2
1 TO 3
Connects LP to LP External Connector for External Control
J2
1 TO 4
Connects LP to VCC (Normal Mode On)
Channel Selection
The SEL0 and SEL1 jumpers are used to select the channel sent to the comparator. The channel selection is static in this reference design. In a normal application, these two pins are connected to a microprocessor to control the channel to output dynamically. See Tables 4 and 5 for more details.
APD Description
A 4-channel APD array for near infrared (NIR) detection of First Sensor is used. It is placed in a ceramic carrier type non-hermetic SMD package with AR-coated glass window. The used APD arrays are engineering samples for LiDAR applications (autonomous driving, industrial, and commercial) with the following technical parameters.
Table 4. Jumper Table (SEL0, SEL1, J1, J2)
Jumper
Shunt Position
Description
SEL0
1 TO 2
Connects SEL0 to VCC (1) – Refer to Table 5 for Channel Selection Decoding Table
SEL0
2 TO 3
Connects SEL0 to GND (0) – Refer to Table 5 for Channel Selection Decoding Table
SEL0
1 TO 2
Connects SEL1 to VCC (1) – Refer to Table 5 for Channel Selection Decoding Table
SEL0
2 TO 3
Connects SEL1 to GND (0)– Refer to Table 5 for Channel Selection Decoding Table
The APD outputs are connected to the TIA in a DC-coupled fashion. There is no need for AC coupling capacitors in this case although it is left to the discretion of designers to select from the AC or DC coupling based on the application requirements. Both schemes are allowed.
Connectors were added in this design to access the TIA independently. These are AC-coupled (Figure 3).
Alternative Input to Bypass APD
The APD array can be bypassed and each TIA input channel can be fed from an external electrical source generator. Unsolder all the four 0Ω resistors R11, R12, R13, and R14, and populate the following resistors with 0Ω: R3, R7, R8, and R10.
TIA to Comparator Connection
The TIA outputs in this design are sent to the comparator inputs in an AC-coupled mode using the C10 and C11 capacitors. Then, R16, R17, R18, and R19 allow to create the necessary offset between IN+ and IN- for the proper operation of the comparator.
Comparator Output
The MAX40026 provides a low-voltage differential signaling (LVDS) output, which can be sent directly to any processing circuit.
Figure 4. 4-channel APD schematics.
Figure 5. AC-coupled external input to the TIA.
Figure 6. Interface between the TIA and comparator.
A 4-layer PCB board is used for this reference design with a high-performance substrate material like Rogers. The top and bottom layers implement all the signals, while layer 2 and 3 are used as ground planes.
The following guidelines are implemented:
A differential microstrip is used for the MAX40662 outputs with terminations close to the outputs. Avoid unwanted stubs by removing the ground below the traces not part of the termination line leading into the input pins. The parasitic capacitance created between the traces and ground slow down and even distort the signals by creating reflections on the path.
The input trace connecting each of the photo-diode array to the IN_ of the MAX40662 should be as short as possible and have ground etched/removed underneath. This reduces unwanted parasitic capacitance created in the PCB. Longer trace lengths increases the parasitic inductance in the signal trace paths.
There are four input traces in the design. It is critical to include a ground isolation between them to minimize channel to channel coupling.
Mount one or more ceramic capacitors between the GND and VCC as close to the pins as possible. Multiple bypass capacitors reduce the effect of trace impedance and capacitor ESR.
Bypass capacitors are chosen for minimum inductance and ESR.
Minimize any parasitic layout inductance by keeping the traces as short as possible.
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