1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device and, more particularly, to a semiconductor integrated circuit device containing both a digital signal processing circuit and an analog signal processing circuit.
2. Description of the Prior Art
A semiconductor integrated circuit device such as a device used as a digital controller should be capable of processing both analog and digital signals. Since an analog signal usually contains a relatively large quantity of information, the use of a semiconductor integrated circuit device that is capable of processing analog signals is beneficial for a highly precise process control or the like. In a semiconductor integrated circuit device, multi-layer wiring such as a double layer wiring or a triple layer wiring can be effected by the well-known multi-layer wiring technique for improving the integration density. In this case, the semiconductor integrated circuit device has its internal circuits or circuit elements connected with each other or one another through the multi-layer wiring.
Where a digital signal processing circuit and an analog signal processing circuit are formed into a single semiconductor integrated circuit device, a first wiring layer made of a conductive polysilicon layer formed on a semiconductor substrate through a field insulating film may be used as an analog signal line, and a second wiring layer made of an aluminum layer formed on the first wiring layer through an interlayer insulating film may be used as a digital signal line.
However, the interlayer insulating film, which is formed in the semiconductor integrated circuit, has a relatively small thickness. Therefore, it should be noted that a relatively high capacitance is established where the two wiring layers cross over one another. The undesirable capacitance formed between the wiring layers establishes a crosstalk that adversely affects the circuit operation.
Where the analog signal line is connected with an electronic circuit having a high input impedance such as an analog/digital (A/D) converter, the crosstalk imparted from the digital signal line to the analog signal line through the electrostatic coupling capacitance is increased. Also a malfunction may occur in an analog signal processing operation by undesired fluctuations of the analog signal level.
More specifically, when the level of the digital signal is changed, this level change is transmitted through the aforementioned capacitance to the analog signal line to thereby change the level of the analog signal. As a result, a malfunction takes place in the A/D converter.
For a circuit such as disclosed in copending Japanese Patent Application (No. 55-18986) which has been proposed prior to the present invention by the same Applicant, i.e., for a common external terminal selectively used as an analog input terminal or a digital input/output terminal, the aforementioned crosstalk is particularly increased.