Node Voltage Transferring for Squib Resistance Measuring on Cross-Coupled Systems
Original Publication Date: 2003-May-25
Included in the Prior Art Database: 2003-May-25
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The squib measurement in cross-coupled systems is difficult. The general problem consists of measuring appropriately the voltage across the squib with a system that is immune to the introduction of noise such as EMI (Electro-Magnetic Interference), ground shifting, etc. Accuracy of the measurement is very often important to verify the proper functionality of the squib which is used e.g. in an airbag of a car. A special problem to solve is how to transfer a node voltage from one IC (Integrated Circuit) to another without interference and noise introduction. It is suggested to introduce a buffer and a filter to transfer the signal externally, which permits the use of accurate voltage differential measurement instead of the sequential one. The buffer must have a greater bandwidth than the cornering frequency of the filter so that any time delay introduced by the buffer becomes negligible. Also the output node of the buffer must be low ohmic so that it can source or sink the leakage currents of two pads. The filter is needed to filter out EMI and to get the average of the transferred signal. In Fig. 1 an example is shown. There are two additional pins per IC, these are SEN_O (SENse_Output) and SEN_I (SENse_Input). There is a buffer connected to the SEN_O pin, so that this node becomes a low ohmic node and can withstand the leakage current in the pads. This node is buffering one of the signals which need to be transferred to the other IC. Since it is low ohmic it absorbs most of the noise due to EMI, however it is not sufficient to guarantee a perfect replica of the signal in the other IC, because the buffer has an offset and has a time delay depending of its' bandwidth. So it is necessary to introduce a low pass filter after the pin SEN_I. If this low pass filter has a cornering frequency much lower than the cornering frequency of the buffer, then the time delay, introduced from the buffer, will be negligible. By filtering the replica of the signal, its' average will be the outcome, and this is what is desired. This low pass filter, at the input of the gain amplifier (connected to the Analog Out pin), will also filter out noise introduced from EMI.