1. Field of the Invention
The present invention relates to a comparing circuit, a comparator, a level determining circuit and a threshold voltage setting method of the level determining circuit, which are preferable for determining levels of micro input signals.
2. Description of the Related Art
In-vehicle radio systems represented by an ETC (Electric Toll Collection System) and so on are designed to have low consumption current for extending their battery lifetimes. The in-vehicle radio system is, as a means for providing the design, operative to wait-state (sleep-state) its units which need not operate during, for example, a stationary operation period, and to operate its wakeup circuit to output wakeup signals to the Units when the wakeup circuit determines that the predetermined wakeup condition is fulfilled.
An in-vehicle ETC unit, as an example of the in-vehicle radio systems, is operative to receive radio signals sent from an on-road antenna and output an wakeup signal, when the received signal exceeds the predetermined threshold value, to a micro-computer which keeps its sleep-state so as to wakeup the micro-computer.
FIG. 7 shows an electrical configuration of the wakeup circuit used for conventional in-vehicle ETC units. The wakeup circuit 1 which operates at a single polarity power source is provided with an input terminal 2 and an output terminal 3. To the input terminal 2, a voltage Vin of a demodulation signal Sdm passing through a detector is inputted and an wakeup signal Swk is outputted from the output terminal 3. The wakeup signal Sdk turns into a H (High) level when the demodulation voltage Vin exceeds the threshold voltage Vth. The wakeup circuit 1 also has a reference voltage generating circuit 4 which is provided with a D/A (digital to analog) converter. The reference voltage generating circuit 4 is operative to output a reference voltage Vref that it can gradually set from 0 V by a predetermined voltage according to a decode signal of n bits. The wakeup circuit 1 is provided with a comparator 5 having a reverse terminal common to the input terminal 2 and a non-reverse input terminal. The comparator 5 compares the demodulation voltage Vin inputted to the reverse input terminal 2 and the reference voltage Vref inputted to the non-reverse input terminal, outputting a reference signal on the basis of the compared result. The outputted signal from the comparator 5 is inputted to an inverter 6 of the wakeup circuit 1 so as to turn into the wakeup signal Swk.
In this configuration, the comparator 5 has an offset voltage caused by characteristic differences of its elements which occur in the manufacture thereof so that the offset voltage causes the threshold voltage Vth of the wakeup circuit 1 and the reference voltage Vref to be not necessarily in agreement with each other. An adjustment of each threshold voltage Vth of each in-vehicle ETC unit is therefore required. The adjustment of threshold voltage is performed as follows.
That is, the decode signals are sequentially switched while the radio signal having threshold power which permits an object unit to be woken up is provided, and the lowest reference voltage in the range within which the wakeup signal Swk has an L (low) level is set as the threshold voltage.
The adjustment, however, causes a problem in cases where the comparator 5 has a negative offset voltage, that is, the output of the comparator 5 does not become a low level unless the voltage of the reverse input terminal exceeds that of the non-reverse input terminal by only the absolute value of offset voltage. Namely, in cases where the absolute value of offset voltage Voffset is higher than the threshold voltage Vth, even if the reference voltage Vref is set to 0 V which is the lowest voltage in the range, the wakeup signal Swk does not turn into the H level unless the moderation voltage Vin is not less than the absolute value of offset voltage Voffset. As a result, it is difficult to adjust the threshold voltage Vth so that the threshold value Vth is not more than the absolute value of offset voltage Voffset, causing the receiving sensitivity with respect to the input signal to the comparator 5 for the wakeup of the object unit to be lowered.
For solving the problem, an negative reference voltage Vref may be generated, or a comparator 5 having a lower offset voltage may be adopted. However, the former measure requires a new negative power source and the former and latter measures complicate the circuit configuration of the wakeup circuit, causing the design and manufacture costs thereof to be increased. The complicated circuit configuration of the wakeup circuit causes the consumption current to be increased so that the result is contrary to the original design concept of the in-vehicle radio systems having low consumption current.