Other Parts Discussed in Thread: INA122, INA333, INA326, TINA-TI
Good morning,
I am working on a pressure thermometer prototype, basically I need to amplify a signal at the output of a bridge configuration. I attached the design of a part of my system.
My first need is to measure very low signals some hundred uV and amplify the signal 100 times and give it to an A/D converter.
My system is supplied by +5V so the INA and the bridge has the same supply, I used RG=40K (as suggested on pag.7 of INA datasheet) to have a gain of 10. I wanted to try how the INA was responsive to the different inputs, I started by 500uV and I stopped at 6mV; I have noticed is that the gain changes depending on the inputs. How is it possible?
Attached you can find the table with the measurement results.
|
Vin (pin3-2) |
Vout (pin6) |
Calculated gain |
|
300 uV |
4.626 mV |
15.42 |
|
400 uV |
5.510 mV |
13.775 |
|
500 uV |
6.342 mV |
12.684 |
|
600 uV |
7.262 mV |
12.10 |
|
700 uV |
8.254 mV |
11.79 |
|
800 uV |
9.175 mV |
11.45 |
|
900 uV |
10.148 mV |
11.27 |
|
1 mV |
11.165 mV |
11.16 |
|
1.5 mV |
16.068 mV |
10.7 |
|
2 mV |
21.083mV |
10.54 |
|
2.5 mV |
26.097 mV |
10.40 |
|
3 mV |
31.173mV |
10.30 |
|
3.5 mV |
36.15 mV |
10.30 |
|
4mV |
21.083mV |
10.30 |
|
4.5 mV |
41.22mV |
10.30 |
|
5 mV |
51.35 mV |
10.30 |
|
5.5 mV |
56.41 mV |
10.30 |
|
6mV |
61.46 mV |
10.30 |
I would really appreciate if someone could explain to me what happens. What I need probably to add to the basic circuit I am using.
Then I will have another question concerning the lowest value I could give to the component to make it working properly...I mean, if I want to signal like 200uV can I use the INA122?
I am looking forward for an answer.
Best regards,
Mia
