I have a 4-20 mA current loop that I am running through two INA122 circuits in series. The first circuit in the loop has a 50 Ohm resistor across the input of the INA and the second circuit has a 40 Ohm resistor across the input of its INA. On the very bottom of the loop are a couple 1N4002 diodes in series to get the bottom of the loop above the common mode voltage.
There are no gain resistors installed which sets the gain for the INA122s at 5. The bottom circuit (40 Ohm) works fine but the top circuit (50 Ohm) has a 0.1 Volt offset in the output. At 4 mA, the differential across the input is 0.2 V and at 20 mA, the differential across the input pins is 1.0 V. The outputs at these two points are 1.1 V and 5.1 V respectively. Vref (pin 5) is tied to pin 4 which is tied to 0 VDC.
Does anyone have an idea of where the 0.1 V offset on the output could be coming from?? Thanks for looking!
There are no gain resistors installed which sets the gain for the INA122s at 5. The bottom circuit (40 Ohm) works fine but the top circuit (50 Ohm) has a 0.1 Volt offset in the output. At 4 mA, the differential across the input is 0.2 V and at 20 mA, the differential across the input pins is 1.0 V. The outputs at these two points are 1.1 V and 5.1 V respectively. Vref (pin 5) is tied to pin 4 which is tied to 0 VDC.
Does anyone have an idea of where the 0.1 V offset on the output could be coming from?? Thanks for looking!