8.2.1 Design Requirements

• Supply Voltage: 3.3V

• Maximum Input Voltage: ±6V

• Specified Input Voltage: ±5V

• ADC Reference Voltage: 1.25V

The goal for this design is to accurately condition a ±5V bipolar input voltage into a voltage that works for

conversion by a low-voltage ADC with a 1.25V reference voltage, VREF, and an input voltage range of VREF/2.

The circuit can function with reduced performance over a wider input range of at least ±6V to allow for easier

protection of overvoltage conditions.

8.2.2 Detailed Design Procedure

Figure 8-2 depicts a simplified schematic for this design showing the MSP430 ADC inputs and full input

conditioning circuitry. The ADC is configured for a bipolar measurement where final conversion result is the

differential voltage between the voltage at the positive and negative ADC inputs. The bipolar, GND-referenced

input signal must be level-shifted and attenuated by the op amp so that the output is biased to VREF/2 and has a

differential voltage that is within the ±VREF/2 input range of the ADC.

8.2.3 Application Curves

0

0.25

0.5

0.75

1

1.25

±6

±5

±4

±3

±2

±1

0

1

2

3

4

5

6

Input Voltage (V)

C001

Figure 8-3. OPA317 Output Voltage vs Input

Voltage

-0.00035

-0.0003

-0.00025

-0.0002

-0.00015

-0.0001

±6

±5

±4

±3

±2

±1

0

1

2

3

4

5

6

Input Voltage (V)

C00

Figure 8-4. OPA317 Output Voltage Error vs Input

Voltage

±200

±150

±100

±50

0

50

100

150

±6

±5

±4

±3

±2

±1

0

1

2

3

4

5

6

Input Voltage (V)

C003

Figure 8-5. Output Code Error vs Input Voltage

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