Data Sheet AD8214
Rev. A | Page 11 of 16
COMPARATOR OFFSET AND HYSTERESIS
The AD8214 features built-in hysteresis to minimize the effects
of noise in the system. There is also a small offset at the input of
the device.
06193-033
V
OL
V
H
V
TH
V
OH
V
OS
V
OS
= INPUT OFFSET VOLTAGE
V
H
= HYSTERESIS VOLTAGE
V
TH
= THRESHOLD VOLTAGE
V
OH
= OUTPUT HIGH
V
OL
= OUTPUT LOW
Figure 29. Hysteresis and Input Offset Voltage Definition
Figure 29 shows the relationship between the input voltage and the
output current. The horizontal axis represents the voltage between
the positive (+IN) and negative (–IN) inputs of the AD8214. The
vertical axis shows the output current for a given input voltage.
V
TH
represents the point where the inputs are at the same voltage
level (+IN = –IN). The output of the AD8214 remains low (V
OL
)
provided (–IN) is at a higher voltage potential than (+IN). As the
input voltage transitions to +IN > –IN, the output switches states.
Under ideal conditions, the output is expected to change states at
exactly V
TH
. In practice, the output switches when the inputs are
equal ± a small offset voltage (V
OS
).
Once the output switches from low to high, it remains in this state
until the input voltage falls below the hysteresis voltage. Typically,
this occurs when +IN is 10 mV below –IN.
SETTING THE INPUT THRESHOLD VOLTAGE
The AD8214 features a 2.4 V series regulator, which can be used
to set a reference threshold voltage with two external resistors.
The resistors constitute a voltage divider, the middle point of
which connects to +IN. The total voltage across the resistors is
always 2.4 V. (See Figure 28 for proper resistor placement.) The
values for these resistors can be chosen based on the desired
threshold voltage using the equation:
)(
4.2
INTHVR1
R2R1
+=×
+
(1)
For proper operation it is recommended that the internal 2.4 V
regulator not be loaded down by using small R1 and R2 values.
Figure 11 shows the proper range for the total series resistance.
INPUT-REFERRED DYNAMIC ERROR
Frequently, the dynamics of comparators are specified in terms
of propagation delay of the response at the output to an input
pulse crossing the threshold between two overload states. For
this measurement, the rise time of the input pulse is negligible
compared to the comparator propagation delay. In the case of
the AD8214, this propagation delay is typically 100 ns, when the
input signal is a fast step.
The primary purpose of the AD8214 is to monitor for over-
current conditions in a system. It is much more common that in
such systems, the current in the path increases slowly; therefore,
the transition between two input overload conditions around
the threshold is slow relative to the propagation delay. In some
cases, this transition can be so slow that the time from the
actual threshold crossing to the output signal switching states is
longer than the specified propagation delay, due to the
comparator dynamics.
If the voltage at the input of the AD8214 is crossing the set
threshold at a rate ≤100 mV/µs, the output switches states
before the threshold voltage has been exceeded by 15 mV.
Therefore, if the input signal is changing so slowly that the
propagation delay is affected, the error that accumulates at the
input while waiting for the output response is proportionately
smaller and, typically, less than 15 mV for ramp rates
≤100 mV/µs.
