Because microcontrollers with integral Analog-to-Digital Converters (A

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Because microcontrollers with integral Analog-to-Digital Converters (A

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Single-Chip Bridge Sensor Microcontroller Interface Using ispPAC30
Because microcontrollers with integral Analog-to-Digital Converters (ADC's) have become very inexpensive recent years, there many applications provide embedded `intelligence'. example turn `dumb' transducers into `smart sensors'. Many transducers common use, however, require significant amount interface electronics both provide excitation transducer, acceptable signal microcontroller. Some requirements bridge-to-microcontroller interface are: Provide stable constant-voltage excitation transducer bridge. Provide precise stable gain. Provide differential single-ended conversion. able trim offset errors. ispPAC®30 provide these functions when configured shown Figure Figure ispPAC30 Bridge Interface Internal Configuration
ispPAC30
Bias/Gain Adjust
VREF1=256mV MDAC1
OUT1 Bridge Bias
G=10 Bridge Transducer
G=10
Offset Adjust
VREF2=2.5V MDAC2
OUT2 Microcontroller ENSPI Control Signals
this circuit, combination VREF1, multiplying MDAC1, instrumentation amplifier provide bias voltage transducer. this example, setting reference 256mV, setting IA1's gain results nominal bridge excitation 2.56V. MDAC1 allows adjustment range ±256 (±10%). Because resistive bridge ratiometric, meaning that sensitivity proportional excitation voltage, varying excitation voltage also convenient perform gain (span) adjustment. This technique provides simple adjust system gain while using fixed gain amplifier. Because ispPAC30 analog outputs capable driving 10mA, this circuit directly drive transducers with bridge resistances down approximately 300. differential voltage output transducer sensed input pair ispPAC30. This signal IA3, which provides fixed gain Because input signal sensed differentially, value signal this point will zero balanced-bridge condition. able represent bipolar signal OUT2, which single-ended, signal will need offset. this purpose, VREF2 2.5V summed into OA2. This effect offsetting signal appearing OUT2 that balanced-bridge condition represented 2.5V. Additionally, MDAC2 also connected VREF2 summed into OA2. This makes possible adjust
www.latticesemi.com
cs1017_01
Lattice Semiconductor
Single-Chip Bridge Sensor Microcontroller Interface Using ispPAC30
offset output signal compensate imbalances transducer. arrangement shown Figure provides ±250mV offset adjust range with resolution (input referred). settings gain offset stored ispPAC30's internal memory, from which they will automatically loaded when power applied device. configuration data programmed into chip serially through integral JTAG port. This feature makes straightforward perform end-of-line test calibration procedures adjusting sensor assemblies individual basis. Figure Block Diagram
ispPAC30 Transducer Bridge OUT1 IN1+ IN1TCK ENSPI MOSI MISO DOUT OUT2 Microcontroller (with optional SPI)
Optional Interface
also possible microcontroller change calibration factors ispPAC30 on-the-fly through device's interface. Figure shows necessary connections. When ENSPI ispPAC30 pulled high, pins normally used JTAG programming become port. ability program ispPAC30 under control valuable several situations. first where multiple ispPAC30's slaved single microcontroller, designer wants store calibration data centrally microcontroller. Another situation where control might desirable when packaging makes difficult impossible access ispPAC30's JTAG pins. example, packaged sensor assembly have communication interface microcontroller (e.g. RS-232, RS-485). addition being able read sensor data through this interface, also possible supervisory microcontroller perform calibration operations writing ispPAC30's on-chip memory.
Technical Support Assistance
Hotline: 1-800-LATTICE (Domestic) 1-408-826-6002 (International) e-mail: ispPACs@latticesemi.com

Because microcontrollers with integral Analog-to-Digital Converters (A

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