I inherited a design when someone left the company and I am trying to figure out why it is performing the way it does. We are using an Instrument amp to provide some ground isolation on an analog input (4-20 mA). From the input terminal +, the signal goes through a zener diode (1N5221, 2.1 V), through a 46.4 Ohm resistor and another 1N5221. We are using a INA118 Instrument Amp with a 12.1K Ohm gain resistor, giving a gain of 5.13. Since all signals are positive voltage, we are using a single 24VDC power supply to power the Instrument Amp.
The problem is that the gain decreases with an increase in signal. At 4 mA, the gain is 5.14 and at 20 mZ, the gain is only 4.79. The output of the INA118 goes through a 100 Ohm resistor to a MicroChip 16F819 analog input.
I had a INA128 available and installed it. The gain at 20 mA decreased to 3.81.
Any help or suggestions will be appreciated. Thanks for looking!
The problem is that the gain decreases with an increase in signal. At 4 mA, the gain is 5.14 and at 20 mZ, the gain is only 4.79. The output of the INA118 goes through a 100 Ohm resistor to a MicroChip 16F819 analog input.
I had a INA128 available and installed it. The gain at 20 mA decreased to 3.81.
Any help or suggestions will be appreciated. Thanks for looking!