I have a TI INA126 (datasheet) instrumentation amplifier that I want to use to amplify a noisy, low-level signal from a fabric-based resistive stretch sensor.
The resistance ranges for the fabric sensors vary depending on how they are cut. Sometimes the range is 50Ω-100Ω. Ideally it is 300Ω-600Ω. Let's assume the range 200Ω-300Ω at first.
With a simple voltage divider (R2=250Ω) and working with 5V, I get a voltage range of about 0.5V from 2.27V to 2.77V.
(250/(250+200))*5 - (250/(250+300))*5 = 0.5
So, I need to remove the common-mode of 2.77V and amplify the difference into the output swing of the INA126 (0.8V-4.2V).
The equation for the INA126 is:
Vo = (Vin+ - Vin-)*G
G = 5+(80kΩ/Rg)
Setting the gain is very easy (one resistor across pins 1 and 8). A table is provided in the datasheet.
I'm having trouble figuring out how to set everything up. I have it set up as follows:
- Rg (no resistor for 5x amplification)
- V-in (3.3V? 5V? 2.7V? grnd?)
- V+in (sensor in)
- V- (grnd)
- Ref (grnd)
- Vout (output)
- V+ (+5V)
- Rg (no resistor for 5x amplification)
With the above setup and V-in connected to 2.2V from a DC power supply and V+ connected to the Arduino 5V supply, the output range becomes:
2.65-2.98
This doesn't make sense according to the calculations:
(2.27-2.1)*5 = 0.85
(2.77-2.1)*5 = 3.35
Generally the amp doesn't seem to do what I want it to. What am I doing wrong?
EDIT1: I have access to a DC Power Supply (w/ negative voltage). Perhaps this will help me?
EDIT2: I'm going to try once more with a Wheatstone bridge and the dual power supply. If that doesn't work, I'm ordering amplifiers that are less fussy in single-supply mode (along with some 1% resistors) and trying again on Monday.