Page 20 - EngineerIT September 2021

MEASUREMENT

give a worse signal-to-noise ratio (SNR) to the detector.
Luckily, APDs are a happy medium, offering enough optical
gain for this space, but not too much ENF to negatively impact
the SNR.
When a photon strikes the APD, an electron hole pair is
created at the junction. The APD’s high electric field acts like
a slingshot and accelerates the electron to knock off more
electrons. This increases the number of released electrons
per received photon. This effect is called the avalanche effect
and it adds a multiplication factor (M factor). This gain, which
is bias dependent, can allow us to see weaker signals since
the TIAs are generally the limiting factor for SNR due to its
noise floor. The goal is to match the noise floor of the next
stage in the signal chain, in this case, matching the TIA’s
noise floor by providing enough gain in the APD to slightly
dominate the signal chain noise and give the best SNR to the
system. This noise matching concept is widely used in many Figure 3: RB is needed to AC couple the TIA.
signal chains where the sensor noise floor is not the limiting
factor. In practice, this increase in receiver performance This parallel path will negatively impact the gain of the APD
translates to an extended detection range. Another important since its signal will be shared. The magnitude of this APD
advantage of APDs is a fast saturation recovery. Again, the gain degradation is determined by the ratio between the TIA
TIA is the limiting factor in this and LIDAR specific TIAs are input impedance and the value chosen for R B. Additionally,
designed to reduce the saturation times to avoid blinding the C IN has an impact on the circuit - when the input is subjected
LIDAR system. The only downside of APDs is their relatively to currents from the APD, a voltage is produced on the
high bias point (hundreds of volts) and the temperature capacitor. This effect is due to the integrating current nature
coefficient associated with it. of capacitors, where the voltage is a function of current over
LIDAR has unique requirements for TIAs. Low current noise time and capacitance value. The goal is to make C IN small
and high bandwidths are typical for all optics applications. enough to minimise charging effects but large enough to
However, low power is a necessity. A system’s power budget allow it (to have low enough impedance) to pass signals at
may come under strain very quickly, since current systems have the frequency of interest. In other words, if C IN is too large,
64 or more TIA APD channels. As such, lower power modes then it takes longer to discharge, but if C IN is too small, you
are necessary when the TIA is not in use. Also, these devices will lose some of your signal as a voltage drop across it (or
need to wake up quickly to optimise their power budget. distortion of the pulses have a long relative timescale to the
Another requirement for modern LIDAR TIAs is clamping capacitance). Any of these effects will severely compromise
circuits for saturation events and to balance and trade-off input the signal chain.
referred noise and bandwidths.
One major difference between the normal optical signal Sizing RB and CIN
chain and LIDAR is the environment. In fibre applications, the Let’s illustrate how sizing C IN too small hurts your full-scale
system is enclosed and is very stable. However, in LIDAR measurement: at 200 MHz, a 33 pF capacitor looks like 24 Ω,
we have the sun to contend with, as well as other LIDAR which creates a voltage divider with R IN (usually on the order
systems. The sun could cause a DC input that saturates the of a few hundreds of ohms for LIDAR TIAs), taking 10% off the
receive chain linear range. This is one of the first challenges signal’s actual value. A 10% hit to your signal can easily wipe
engineers will have to overcome for designing these systems. out the hard engineering work done to optimise other areas
Unfortunately, the solution is not easy and will be addressed in such as the optics design.
this article. The pitfall with this AC-coupling approach to cancel
DC becomes clear when sizing R B. R B should be large in
Input AC-coupling considerations comparison to the TIA input impedance to prevent gain
Let’s explore a simple approach to block the DC signal and one degradation, but small enough not to compromise the
that many engineers try to implement without much success: saturation recovery. An impossible balance of choosing RC
connecting an AC-coupling capacitor between the APD to the time constants is compounded by the fact that the input signal
TIA. By placing a capacitor, we can mitigate DC effects, but this of the detector is unipolar. The square wave nature of the
introduces a new set of challenges. input pulse is averaged on this RC and will remove the TIA’s
dynamic range. Additionally, the TIA can potentially charge C IN
RC trade-offs when channel switching or by using output multiplexing. For
Firstly, adding an AC-coupling capacitor to the input of the example, with the LTC6561, the input of the TIA of an active
TIA input also requires connecting a DC path to the detector. channel is nominally 1.5 V. When the channel is inactive, the
By placing a resistor, R B, the APD’s bias point can be set, voltage of the input drops to 0.9 V. When an AC-coupling
allowing you to AC couple the TIA input with C IN, as shown in capacitor is inserted in between the detector and the TIA’s
Figure 3. One sacrifice you make with this bias path is that input, the capacitor must recharge back to 1.5 V for the
it creates a parallel path for the APD current to flow through. channel to become active again.

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