Hi,
I am trying to develop load cell application. I am using CEL-601 45Kg load cell. As the input from the cell is low I was using INA128 instrumentation amplifier. The problem is, the output saturates to 2.5V when I apply a load greater than 5KG. Kindly help….!!!
Mandar,
This behavior is to be expected. The standard 3 op amp instrumentation amplifier topology is shown below:
The output voltage of each of the input amplifiers is the common-mode voltage + the differential voltage * (gain / 2). Sweeping the input differential voltage in the above schematic, we can see the output voltages of the input amplifiers (OP1, OP2).
Initially, the output voltages change linearly with the increase input differential voltage. However, this trend cannot continue indefinitely, because an op amp can only output a range of voltages within its power supply voltages. Near 5mV of input differential voltage, the input amplifiers can get no closer to their power supply voltages (+5V and 0V) and their outputs saturate a little more than a volt from either rail. The output voltage of the amplifier will then saturate at the difference between the two input amplifiers, here is a plot of the output:
The solution to this problem is to either use higher supply voltages, preferably bipolar supplies as well. Or alternatively you could switch to an instrumentation amplifier which uses different internal amplifiers or a different topology. The INA326 is one amplifier which might meet your requirements. Here is a schematic and the output voltage when sweeping the input:
