Part Number:INA333
Hi,
I am using INA333 to amplify a differential signal from a FSS1500NSR load cell, which is based on the Wheatstone Bridge topology (I measured ~5kOhms between sensor's differential inputs). The maximum voltage from the load cell is 210mV, therefore I am using gain of ~20, i.e. RG = 5.6kOhms. The output is connected to an 8bit ADC. INA333 is connected through a single supply of 5V.
However, I have trouble achieving stability at the inputs of INA333. I have measured resistance between pins 1 and 6 and 5 and 8, which is supposed to result in ~350kOhms (looking at the schematic in the datasheet), however I measured ~182kOhms and ~307kOhms respectfully - though the values were not constant.
Consequently, even though the load cell provides Vcm of 2.5V (V+ = 5V), connecting them to the inputs of the amplifier results in varying voltages at the inputs (Vin- = ~1.2 - 2.0V and Vin+ = ~0.7 - 1.2V). When I disconnect the load cell, I measure around 0.2V at both inputs which decreases and eventually goes down to almost 0. Thinking it might have been caused by the input bias current having no return path, I have connected 47kOhms between both of the inputs and ground but it didn't help.
Since I have my prototype on the breadboard, I used an IC socket adapter to connect the chip to the breadboard, see picture below.
The setup is shown below:
The circuit diagram:
Note: I have tried 4 separate INA333 chips and all of them suffered from a similar problem. One of them even managed to start smoking, which was quite a surprise since the 5V power supply is stable and was applied to the correct pin.
Also note that I have measured the internal resistances of one of them just having taken it out of the box - it resulted in 347kOhms between both pins 1 and 6, and 5 and 8, as expected. Having soldered it on the adapter and measured it again, resistance 5-8 changed to ~250kOhms and became unstable, therefore I suspect that the chip is sensitive to high soldering temperatures? I used 300C.
I searched the entire forum for a similar problem but couldn't find anything related, therefore I'd be very happy if someone could help me out with this. Thank you!
Regards,
Sarunas