| ??? 03/12/07 19:23 Read: times |
#134809 - gain calculation?? Responding to: ???'s previous message |
Kai Klaas said:
1. Instrumentation amplifiers reference signals to signal ground (GND) normally. But you also couple Vref into the inputs. This is unusual and as long as I don't know the actual reason for that, I cannot say whether it's good or bad. I used that Vref since i thought that input at V1 is very small. So i doubted whether it would be detected. So, as you have mentioned the correct solution for that is using a bipolar supply. I got that. Kai Klaas said:
3. But even if you take a bipolar supply, the gain isn't properly adjusted. When R5 is 1k, then the first stage has a gain of 19. But R11...R14 form an additional voltage divider, so V1 is amplified by a factor of 9.5 in the first stage. Could you tell me how you calculated the gain as 19, and 9 (due to additional voltage divider R11...R14.) How i calculated the gain for the first stage is, For U2A: Assuming the point connecting R11 & R12 as 'Va' [(V1/R11)+(Vref/R12)] ----------------------- = Va [(1/R11)+(1/R12)] So, substituting the values for above would give., Va = (1/2) * (V1 + Vref) Therefore, if my input(V1 + Vref) is (1+.01) = 1.01 then Va = .505. So, this would be amplified by a factor of 10 i.e., [1 + (9000 / 1000)] which gives 5.05. Thus in the description, i said the input voltage at first stage is amplified 5 times. Have i left out any thing in the calculation or is it not the correct way to calculate? regard's Suresh. |
