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LM9044 Datasheet(PDF) 8 Page - Texas Instruments |
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LM9044 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 14 page LM9044 SNOSBP4D – FEBRUARY 1995 – REVISED MARCH 2013 www.ti.com APPLICATION INFORMATION CIRCUIT DESCRIPTION The LM9044 is a single channel device intended to act as a linear interface between a zirconium dioxide oxygen sensor and an A-to-D convertor. The LM9044 is fabricated in Bipolar technology and requires two supplies: a nominal 12V automotive supply (i.e. VBATTERY), and a well regulated 5V supply. The IC consists of a single channel differential input amplifier with a nominal DC gain of 4.5 V/V. The differential inputs have a specified common mode voltage operating range of 1V above and below ground. The circuitry also contains provisions for default output voltage in the cases of cold sensor and open sensor wiring. Additional support circuitry includes one pin for an optional user programmed low pass filter. COLD SENSOR Typically, a Lambda sensor will have an impedance of less than 10 k Ω when operating at temperatures between 300°C, and 500°C. When a Lambda sensor is not at operating temperature, its impedance can be more than 10 Meg Ω. Any voltage signal that may be developed is seriously attenuated. During this high impedance condition the LM9044 will provide a default output voltage. While the Lambda sensor is high impedance the internal non-inverting input bias current (380 nA typical) will flow through the differential input resistance (1.2 M Ω typical) and out the inverting input pin to ground. This will cause a voltage to be developed across the differential inputs: VIN(DIFF) = 380 nA x 1.2 MΩ VIN(DIFF) = 456 mV The 456 mV across differential input resistance will be the dominant input signal, and the typical VOUT will be: VOUT = VIN(DIFF) x 4.50 VOUT = 456 mV x 4.50 VOUT = 2.0V As the Lambda sensor is heated, and the sensor impedance begins to drop, the voltage signal from the sensor will become the dominate signal. The non-inverting input bias current is scaled to the VREF voltage. As the VREF voltage increases, or decreases, this bias current will change proportionally. OPEN INPUT PINS DEFAULTS In any remote sensor application it is desirable to be able to deal with the possibility of open connections between the sensor and the control module. The LM9044 is capable of providing a default output voltage should either, or both, of the wires to the Lambda sensor open. The two inputs handle the open circuit condition differently. For the case of an open connection at the non-inverting input, the device would react exactly the same as for the Cold Sensor condition. The internal non-inverting input bias current (380 nA typical) flowing through the differential input resistance (1.2 M Ω typical) would cause the typical output voltage to be at a value defined by: VOUT = ((380 nA x 1.2MΩ) x 4.50 ) VOUT = 2.0V The inverting input would still be connected to the Lambda sensor ground, so common mode signals would still need to be considered in this condition. For the case of an open connection of the inverting input, the device output stage switches from the amplifier output to a resistive voltage divider. The LM9044 has a comparator to monitor the voltage on the inverting input pin, and a 65 μA (typical) current source that will force the pin high if the pin is open. When the voltage on the inverting pin goes above typically 1.5V, the comparator will switch the output pin from the amplifier output to the resistive voltage divider stage. In this case, the default VOUT is not dependent on the gain stage, and any signal on the non-inverting input will be ignored. 8 Submit Documentation Feedback Copyright © 1995–2013, Texas Instruments Incorporated Product Folder Links: LM9044 |
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