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.
Best Answer
I don't really see a reason for this not to work the way you expect with the connections you have listed here. Might I suggest some debugging steps?
1) See what happens to the output range when you put a large-ish Rg ( say 27k ohms) in the circuit. Does the output swing change at all? 2) Have you checked the inputs to make sure they are as you expect with the amplifier connected? If so, how about using a larger resistance POT to simulate your resistor divider just to remove the sensor from the picture until you get the circuit working. 3) Perhaps the simplest explaination might be that the INA126 itself is bad. Do you have a spare you can drop in to test?