AN4071 datasheet(9/27 Pages) STMICROELECTRONICS

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AN4071

Datasheet parameters

Doc ID 022939 Rev 1

9/27

In other words, input offset voltage can be represented by a voltage source applied in series

with one input of an ideal comparator. Consequently, the output doesn't toggle when

VIN+ = VIN-, as in the case of an ideal comparator, but the threshold level is shifted by the

input offset value VIO. Input offset voltage rises in the input stage as a consequence of

transistor imbalance. For comparators with built-in hysteresis, VIO is defined as the average

value of VTRIP+ and VTRIP-, and the hysteresis of VHYST = VTRIP+ - VTRIP- where VTRIP+

(respectively VTRIP-) is the input differential voltage for which the output switches from low to

high state (respectively high to low state).

Measurement

Input offset voltage and trip points can be measured using the circuit shown in Figure 10.

The first DC source sets the power supply VCC and the second sets VICM, the common

mode voltage. A 100 m VPP triangle signal is applied on the voltage divider (1/101). The

voltage divider is necessary to obtain a good accuracy on the VIO reading by the scope, and

to use the function generator with an amplitude that it can handle. The triangle signal should

be low frequency (20 Hz); higher frequency can lead to error in the VIO measurement

caused by propagation delay of the device. When the output changes its state, the actual

input voltage value VIN is read from the scope, from VIN we can simply deduce VIO = VIN /

101.

Pay attention to the fact that the oscilloscope probes and waveform generator ground are on

the inverting input pin of the comparator. Therefore, VICM and VCC power supplies must be

floating from earth-ground, or an isolation transformer must be used. The advantage is that

the VICM voltage does not need to be subtracted from the VIO reading. VICM can be easily

changed without having to adjust the offset of the input signal, making the measurement

more comfortable.

Figure 10.

VIO (VTRIP) measurement circuit

AN4071 数据表(PDF) 9 Page - STMicroelectronics