Page 20 - EngineerIT December 2021

ELECTRONICS

Protect against analogue front

end overvoltages with OTT

amplifiers

Hakan Uenlue, Senior Field Applications Engineer

Introduction corresponding preventive measures using external protection circuits with, for example,
The possibility of high voltages occurring external diodes or resistors. However, these extra components need space on the board
is a constant concern in industrial and have disadvantages such as leakage currents, additional capacitance and noise. For this
applications. Finding ways of providing reason, an integrated IC solution with Over-The-Top technology is a top choice.
protection from them has always been and
will continue to be an important task for How Does Over-The-Top Work?
developers. This design tip illustrates how For a simplified explanation, the insides of the latest-generation ADA4098-1 or ADA4099-1
developers can accomplish this by utilising can be considered. These OTT op amps each have two input stages. The first is a common-
Over-The-Top® (OTT) amplifiers. emitter differential stage consisting of PNP transistors that work for input signals between
Even industrial applications sometimes the negative supply (–V S) and up to approximately 1.25 V less than the positive supply (+V S).
experience voltages that are higher than The second is a common-base input stage consisting of further PNP transistors that work
the system supply. Although the potentials for input signals whose common-mode voltage is +V S – 1.25 V or higher. An example of the
are not as high here as, say, in automotive internal circuit is shown in Figure 1. The first stage is designed with transistors Q1 and Q2,
electronics, they can often be higher than whereas the second stage has transistors Q3 to Q6.
the usual system voltages. Some of the These input stages thus lead to two different but complementary operating ranges. The
system voltages may even be too high offset voltages of the two input stages are tightly trimmed and given in the data sheets.
for many op amps. This presents a huge When the common-mode voltage of the inputs approaches +V S, the second stage is
challenge to analogue front ends (AFEs). activated, and the op amp is then in Over-The-Top mode. This can be a case of overvoltage
Higher voltages can, for instance, make the in various applications. For example, for high-side current measurement, the voltages
internal input diodes of a typical amplifier might exceed the system supply potential due to parasitic or load-related effects, even if
conduct. The longer this state exists, the temporarily. Typical amplifiers allow signal voltages up to the supply voltage range. If the
more likely it is that a malfunction or even inputs far exceed this range, internal diodes are usually turned on and a significant electrical
failure will occur. Developers can take the current flows through them. Depending on the signal voltage and currents, these spikes

Figure 1. A simplified representation of the internal structure (taken from ADA4098-1 as the latest generation).

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