Datasheet AD8214 (Analog Devices) - 11

Data Sheet AD8214 COMPARATOR OFFSET AND HYSTERESIS
values for these resistors can be chosen based on the desired threshold voltage using the equation: 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  2.4    × R1 = TH V (+IN) (1) the device.  R1+R2  For proper operation it is recommended that the internal 2.4 V regulator not be loaded down by using small R1 and R2 values.
VOH
Figure 11 shows the proper range for the total series resistance.
INPUT-REFERRED DYNAMIC ERROR VH
Frequently, the dynamics of comparators are specified in terms of propagation delay of the response at the output to an input
V V OL OS
pulse crossing the threshold between two overload states. For 033 this measurement, the rise time of the input pulse is negligible
VOS = INPUT OFFSET VOLTAGE
06193-
VH = HYSTERESIS VOLTAGE V
compared to the comparator propagation delay. In the case of
TH VTH = THRESHOLD VOLTAGE V
the AD8214, this propagation delay is typical y 100 ns, when the
OH = OUTPUT HIGH VOL = OUTPUT LOW
input signal is a fast step. Figure 29. Hysteresis and Input Offset Voltage Definition The primary purpose of the AD8214 is to monitor for over- Figure 29 shows the relationship between the input voltage and the current conditions in a system. It is much more common that in output current. The horizontal axis represents the voltage between such systems, the current in the path increases slowly; therefore, the positive (+IN) and negative (–IN) inputs of the AD8214. The the transition between two input overload conditions around vertical axis shows the output current for a given input voltage. the threshold is slow relative to the propagation delay. In some cases, this transition can be so slow that the time from the VTH represents the point where the inputs are at the same voltage actual threshold crossing to the output signal switching states is level (+IN = –IN). The output of the AD8214 remains low (VOL) longer than the specified propagation delay, due to the provided (–IN) is at a higher voltage potential than (+IN). As the comparator dynamics. input voltage transitions to +IN > –IN, the output switches states. Under ideal conditions, the output is expected to change states at If the voltage at the input of the AD8214 is crossing the set exactly V threshold at a rate ≤100 mV/µs, the output switches states TH. In practice, the output switches when the inputs are equal ± a smal offset voltage (V before the threshold voltage has been exceeded by 15 mV. OS). Once the output switches from low to high, it remains in this state Therefore, if the input signal is changing so slowly that the until the input voltage fal s below the hysteresis voltage. Typical y, propagation delay is affected, the error that accumulates at the this occurs when +IN is 10 mV below –IN. input while waiting for the output response is proportionately smaller and, typically, less than 15 mV for ramp rates
SETTING THE INPUT THRESHOLD VOLTAGE
≤100 mV/µs. 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 Rev. A | Page 11 of 16 Document Outline Features Applications Functional Block Diagram General Description Revision History Specifications Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Comparator Offset and Hysteresis Setting the Input Threshold Voltage Input-Referred Dynamic Error Applications Typical Setup and Calculations High Side Overcurrent Detection Outline Dimensions Ordering Guide Automotive Products