Page 22 - EngineerIT September 2021

MEASUREMENT

Input DC-coupling considerations dynamic range essentially reduces the receive chain’s SNR.
A DC-coupled input is straightforward. At a high level, the With enough ambient light, the TIAs dynamic range is severely
DC-coupled input shown in Figure 7 allows the TIA to recover reduced to zero dynamic range, blinding the system.
quickly from saturation and is limited only by the TIA’s saturation The most commonly used methods to reduce ambient light
recovery. The disadvantage of this approach is that it will allow often involve optical filtering, like that shown in Figure 8 or the use
DC to pass from the APD to the TIA input. Unfortunately, ambient of active circuits to remove the offset shown in Figure 9. Optical
light, dark currents and leakage caused by the detector can band-pass filters can be coated directly on the APD window or
use some or most of the TIA’s input linear range. This reduced on the lens. On a high level, optical band-pass filters will reduce
the effects of ambient light. The optical filter does not help with
reflections from the internal optics, which can cause a large,
unwanted signal. However, it is a good first line of defense. An
active DC cancellation circuit injects an opposite current into the
input of the TIA to counteract the input DC components. This
scheme requires a closed loop from the TIA output to its input
and great care is needed to preserve the noise and the switching
performance of the TIA. Since we are looking at TIA gains of 10k
to 100k, loop stability is also challenging. Circuit techniques and
architectures for this will not be covered in this article. The big
takeaway here is that once the stability and input capacitance
are mitigated in the circuit, the DC cancellation gives the best
performance for saturation recovery. However, this comes at the
price of added cost and complexity.

Mutually exclusive design choices
There are many trade-offs when considering AC vs. DC
Figure 7: DC-coupled TIA input. coupling or current cancelling a TIA input for LIDAR. The ideal
characteristics of a multi-plexed LIDAR receiving analogue front
end would be that it has a high dynamic range, recovers quickly,

Figure 8: Optical band-pass filter for removing most ambient light. Figure 9: DC cancellation scheme.

Table 1: Input coupling trade-offs

Input Coupling DC AC DC with current cancellation
Advantages Fast saturation recovery Maximum dynamic range even with Maximum dynamic range even with
Fast channel switching ambient light and ADP leakage ambient light and ADP leakage
Simple to design Fast saturation recovery
Fast channel switching

Disadvantages Dynamic range loss from ambient Several design considerations Complex circuit that needs careful
light and ADP leakage Slow saturation recovery considerations

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