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ELECTRONICS

can interrupt the operation of the amplifier, or even, in the worst-case scenario, cause the has protective diodes for its own
integrated circuit to fail. voltage rails, it would make sense to
Unlike typical op amps, which experience such problems, amplifiers with OTT can place a resistor at V OUT. This would limit
tolerate differential input voltages of up to 80 V. In this state, the output level is saturated to the possible currents flowing to the
the positive supply (+V S). The output retains its ability in this state to sink or deliver current downstream circuit.
within the data sheet limits. Once the inputs come back to the normal operating range (–V S The ADA4098-1 has a dedicated
to +V S), the output level will also return to the usual linear range without any damage to or SHDN pin to put the amplifier in a very
degradation of the DC accuracy. The case is similar for common-mode voltages of up to 70 V. low shutdown state when this pin is
asserted as high. A logic high is defined by
Application examples and tips for amplifiers with OTT technology a voltage of ≥1.5 V applied to the SHDN
A few current measurement examples can be found in figure 2. The ADA4098-1 is the low pin with respect to the –V S pin. The V OUT
power version, whereas the ADA4099-1 has a higher bandwidth and a higher voltage rise rate. pin is then in a high impedance state. As
In the low-side measurement, the gain comes from resistors R2 and R3. Diode D1 an alternative method, the amplifier can
improves single-supply accuracy at low load currents. effectively be placed in a low power state
In the high-side current measurement, 1 kΩ and 100 Ω (on the top) resistors are decisive through removal of the positive supply. In
for the gain. The resistors at the amplifier inputs provide, among other things, filtering. In both of these off modes, OTT is still active
this case, a 1% resistor would be optimal. The possible input bias currents result in a voltage and voltages of up to 70 V over –V S can be
drop through these resistors, and close tolerances such as 1% will help to minimize voltage applied to the input pins.
drop ranges here. Other uses for the OTT amplifier
The output from the ADA4098-1 can swing under no load from rail to rail within 45 mV besides current or power measurements
of both supplies. The output can source 24 mA and sink 35 mA. The amplifier is internally are sensor front ends or 4 mA to 20 mA
compensated and can drive a load capacitance of 200 pF (min). A 50 Ω series resistor can current loops. Detailed information, further
be interposed between the output and higher capacitive loads to extend the capacitive load application examples and calculations can
drive capability of the amplifier. be found in the data sheet.
If the output V OUT drives a circuit with a lower potential and this downstream circuit
Conclusion
This article has demonstrated how Over-
The-Top amplifiers can provide protection
against overvoltage. Due to the intelligent
and precise internal circuit, Over-The-Top
amplifiers simultaneously offer robustness
and accuracy.
Analog Devices’ fifth-generation OTT
amplifiers bring the latest overvoltage
protection from the lab to your circuit
design. OTT op amps, such as the
ADA4098-1 and the ADA4099-1, deliver
higher voltage tolerances beyond the rails
while achieving lower offset errors and
noise values. n

For more information on Analog Devices
contact Conrad Coetzee at
ccoetzee@arrow.altech.co.za

About the Author
Hakan Uenlue is a senior field applications
engineer at Analog Devices. He has an
M.Sc. degree in electrical and electronics
engineering from the University of
Stuttgart. After previously working as a
hardware developer and field applications
engineer, he joined Analog Devices in
2015. He can be reached at
Figure 2. Current measurement examples with the ADA4098-1. hakan.uenlue@analog.com.

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