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?
Can you help us to choose the schema which gain the voltage from current shunt and the gain signal go to differential ADC like AD7687. It would nice if the shema gain the differential mode. The current which flow through a shunt may be ac or dc.