I am using OP177G for amplifying a strain sensor o/p - 1mV / Kg. I am configuring it as a difference amp as per the picture.
- The difference between the non-inv and inv pin is 2.49 mV.
- The voltage gain is 470.
- The expected output is 1.1703 V (2.49m x 470).
- The actual output is 1.27606 V (as per sim).
- A difference of 0.10576 V.
- Even if we account for the Vos max 60uV the expected output should be 1.1985.
- If we account for the thevenin resistance (814R or 407 on both inputs) of the wheat-stone bridge the gain changes to 334 (470k / 1.407k). The expected output falls.
I have practically built this circuit with 1% resistors. The wheat-stone bridge has been constructed from discrete 1% resistors.
- The diff between S+ and S- is 3.8mV ( 2.391V - 2.387V )
- However when I connect this to my circuit the S+ drops to 1.934 and S- drops to 2.292
My questions are:
- In the simulation why can't I achieve the expected output within reasonable limits?
- I have measured the Vos of my current IC to be 1mV using the Fig 3 in the OP177 DS. Rf = (390k + 10K) and Rs = 100R. The Vos is coming to be 1mV!! Much higher than what the DS claims. Am I measuring it incorrectly or is this expected?
- The Vos nulling via Fig. 4 doest seem to work. With nothing on the i/p wouldn't the op/amp pick up noise and fluctuate randomly? I have connected a 100K pot with 27K in parallel to get closer to the 20K in the DS. However the o/p doesn't change stays at 0.485 volts regardless of the pot position.
- What is happening on my circuit that is causing the drop in S+?
- INA AD620 (can be procured quickly) has Vos much greater than OP177. I believe because of that it is a poorer substitute to the difference amp that I am making. Am i correct?