10.2.1.2.5 Input Attenuation and Level Shifting

For this design, the bipolar ±5-V input must be attenuated and level shifted so the differential voltage is within the input range of ±V_REF / 2, or ±0.625 V. The accuracy of the op amp output and ADC input may degrade near the supply rails and V_REF voltage, so the output is designed to produce a 0.125 V to 1.125 V output, or ±0.5 V for a ±5 V input. Scaling the output this way also increases the allowable input range to ±6 V, and allows for some underscale and overscale voltage measurement and protection.

Use Equation 12 to scale the ±5-V input to a ±0.5-V differential voltage, as shown in Equation 15.

Equation 15.

where

• R₁ = R₄ = 100 kΩ

R₁ and R₄ dominate the input impedance for this design and are therefore selected to be 100 kΩ. Higher values can be selected to increase the input impedance at the expense of input noise.

With the value for R₂ and R₃ selected as 20 kΩ, the value for R₅ is calculated, as shown in Equation 16:

Equation 16.

where

• R₂ = R₃ = 20 kΩ

In order for A1– to equal to V_REF / 2, R₆ must equal R₇. Two 47-kΩ resistors are used in order to conserve power without creating an impedance too weak to drive the ADC input.