MAX1455 Diagnostic Clip Boost

Abstract: The MAX1455 high-performance low-cost signal condiTIoner includes output clipping capability, which is very important diagnosTIc tool in the automoTIve applicaTIons. This feature can detect various failure mode of the transducer module, including the failed sensors. When a resistive load is applied to the OUT pin during clip, the clip voltage is pulled away from the nominal set point. A solution is presented that prevents output loading from changing the clip voltage value. This is accomplished by using the on-chip uncommitted op amp as a buffer. The PGA and clip circuit output the full voltage range from clip-low to clip-high and the op amp drives the resistive load. This document also describes a way of customizing the pre-programmed clipping limits.

The MAX1455 has a unique feature: programmable analog clip-levels on the output signal. This feature can detect open circuits and short circuits in the wiring harness and connectors. It can also detect failed sensors.

The output clip functions, Clip-Top and Clip-Bot, set the upper and lower output voltage limits of the programmable gain amplifier (PGA). The clip functions are highlighted in the MAX1455 block diagram in Figure 1.


Figure 1. Block diagram shows the diagnostic functions clip-top and clip-bot at the PGA output.

A typical application diagram is shown in Figure 2. The sensor assembly contains a pressure or force transducer and the MAX1455 signal conditioning IC. A wiring harness joins the sensor assembly to the primary Electronic Control Unit (ECU). The ECU provides the power-supply voltage (5V typical) to the sensor and receives the analog voltage signal from the sensor. The three harness wires are often bundled with other wires and may pass through several connector systems between the sensor and the ECU. It is desirable to detect an open circuit in the wires or connectors so that the ECU can take appropriate action. This action may consist of shutting down a subsystem, disregarding all measurements on the suspect line, or activating warning lights to indicate the fault condition.


Figure 2. Typical sensor wiring configuration.

It has become common practice to use a pull-up or pull-down resistor on the Vout line in the ECU that will pull the line to a defined state when the Vout line is an open circuit, as shown in Figure 2. Values for RPU or RPD of 2.2KΩ to 10KΩ have been used. When the Vout line is an open circuit the resistor pulls the line to a supply-rail and keeps it stable at that value. Normally this would be confused with a no-load, or full-scale load condition. By limiting the normal operating output of the sensor to values that are slightly away from the supply-rail voltages, a supply-rail voltage then is defined as an anomaly or fault condition (i.e., an open circuit in the Vout line).

When low values of RPU or RPD are used, and precise control of the clip voltage value is desired, it is sometimes necessary to buffer the clipped voltage to prevent the load from pulling the Vout signal away from the programmed clip setting.

The possible clip voltage settings for the MAX1455 are shown in Table 1.

Table 1. Output Clip Voltage Settings with VDD = 5.0VDC
CLIP[1:0]