invalid behaviour AD8132

We developed the schema which measures the current from shunt resistor (125uO).

The first schema has ADC AD7687 and differential amplifier AD8132.

But the voltage-code characteristic wasn't linear.

Figure 1. The voltage-code characteristic

So we replaced the differential amplifier AD8132 to THS4521 which manufacture TI and the voltage-code characteristic was linear.

The simplified schema with AD8132 and the simplified schema with THS4521 are shown the figure 2.

Figure 2. The simplified schemas

Why do the schema with AD8132 work invalid?

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