Part Details for AD8608 by Analog Devices Inc

This circuit uses the AD7689 16-bit, 8-channel, 250 kSPS PulSAR ADC along with two AD8608 low cost precision quad op amps to provide the entire signal conditioning and digitizing function in the data acquisition system. The only other op amp required is the AD8605 that is used to buffer the reference voltage from the AD7689.

The circuit, shown in Figure 1, incorporates a dual axis ADXL203 accelerometer and the AD7887 12-bit successive approximation (SAR) ADC to create a dual axis tilt measurement system.<p>The ADXL203 is a polysilicon surface micromachined sensor and signal conditioning circuit. Acceleration in the X or Y axis will produce a corresponding output voltage on the XOUT or YOUT output pins of the device. The X axis and Y axis are perpendicular to one another. The AD8608 quad op amp buffers, attenuates, and level shifts the ADXL203 outputs so they are at the proper levels to drive the inputs of the AD7887. The rail-to-rail input/output AD8608 is chosen for its low offset voltage (65 μV maximum), low bias current (1 pA maximum), low noise (8 nV/√Hz), and small footprint (14-lead SOIC or TSSOP).<p>The AD7887 is configurable for either dual or single channel operation via the on-chip control register. In this application it is configured for dual channel mode, allowing the user to monitor both outputs of the ADXL203, thereby providing a more accurate and complete solution.<p>The system maintains an accuracy of 1° over 90° and over temperature. The circuit provides this precision, performance, and range in a low cost, low power, small footprint, calibration dependent solution. The ADXL203 is specified over a minimum temperature range of −40°C to +105°C and is available in an 8-terminal ceramic leadless chip carrier package (LCC).

The input stage of the circuit is an RTD signal conditioning circuit using a compensated 3-wire connection to the RTD. The circuit translates the RTD input resistance range (100 Ω to 212.05 Ω for a 0°C to 300°C temperature range) into voltage levels compatible with the input range of the ADC (0 V to 2.5 V).

The circuit consists of an input signal conditioning stage and an ADC stage. The current input signal is converted to voltage by charge-to-voltage converter (charge amplifier of the U1A op amp and capacitor C2) and amplified by a noninverting amplifier (the U1D op amp and the R7 and R8 resistors). The buffered and attenuated (the U1B and U1C op amps and the resistors R1 and R2) voltage reference (VREF =2.5 V) from the ADC is used to generate an offset HREF of 1.25 V for conditioning the ac signal from sensor to input range of the ADC. Op amps U1A, U1B, U1C, and U1D are one quad AD8608. The output of the U1D op amp is 0.1 V to 2.4 V which matches the input range of the ADC (0 V to 2.5 V) with 100 mV headroom to maintain linearity. Resistor and capacitor values can be modified to accommodate other sensor ranges as described in this circuit note.