Vcc is 5V.
The AD620 receives a differential voltage from the KMZ10B magnetoresistive sensor, which has a Wheatstone bridge construction.
I wanted to amplify voltage from KMZ10B (which is about several tens of millivolts). In my circuit this voltage depends on direction of the magnetic induction of the neodymium magnet. A direct voltage results gives such a characteristic:
On the above graph:
- X-axis = Neodymium magnet angle (degrees)
- Y-axis = voltage (V)
Then I decided to implement the voltage reading from AD620.
HOW I DID THAT:
I used 4.7 kOhm resistor between pin 1 and 8. Datasheet of AD620 says that this resistance (called Rg) must be calculated like this:
Rg = 49.4 kOhm/(G-1)
G - gain
I used 4.7 kOhm resistor, so gain is 11.51. Reference voltage is about 2.5 V (from second op-amp: MCP6022) and voltage divider (half of 5V from Vcc)
I created a chart of this circuit (voltage measured between output of AD620 and GND) and it looks like below:
Now I decided to compare signals before and after amplifying.
I took the results in Excel made before using AD620, I multiplied them with Gain (11.51) and added a 2.5 volt reference (below).
It was supposed to show if the amplifier really strengthens as the calculations show. And look, I compared both charts:
LEGEND FOR ABOVE GRAPH:
- Blue chart is a real, amplified signal
- Orange chart is a calculated signal.
It looks like something's going wrong and in my opinion it depends on in-amp AD620. The signal rise up quickly and falls slowly. What can it be caused by?