# OP177G / Difference Amp

Hi,

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.

1. The difference between the non-inv and inv pin is 2.49 mV.
2. The voltage gain is 470.
3. The expected output is 1.1703 V (2.49m x 470).
4. The actual output is 1.27606 V (as per sim).
5. A difference of 0.10576 V.
6. Even if we account for the Vos max 60uV the expected output should be 1.1985.
7. 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.

1. The diff between S+ and S- is 3.8mV ( 2.391V - 2.387V )
2. However when I connect this to my circuit the S+ drops to 1.934 and S- drops to 2.292

My questions are:

1. In the simulation why can't I achieve the expected output within reasonable limits?
2. 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?
3. 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.
4. What is happening on my circuit that is causing the drop in S+?
5. 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?
Parents
• Dear Harry,

Thanks for the informative reply. I have hit the books since your reply and made an excel sheet that incorporates all the details (input offset volts, current, input bias current, Rth of the wheatstone bridge). Waiting to get back to office to test it out.

1. The budget for the opamp is very limited, max USD 1.2 . Which opamp / differential amp / INamp would you recommend?
2. Vos measurement: OP177 1mV against DS claim of 60uV. Circuit as per Fig 3 in the OP177 DS. Rf = (390k + 10K) and Rs = 100R. Am I measuring it incorrectly or is this expected?
3. Vos nulling: The correct way is to ground the i/ps and no fb. Adjust pot to make o/p zero. Please confirm.
4. Instrumentation Amp Selection: INA AD620 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?
5. Rail to Rail Operation: What is the min o/p volts for OP177? The DS mentions o/p swing but it is for +-15V.