dc-offsetinstrumentation-amplifier
I'm trying to add a DC offset level to a sine signal through an instrumentation amplifier (as the figure below shows.)
The problem is that, in the simulation, everything works perfectly, however, in reality I can't achieve the DC displacement of the signal.
The input signal parameters are: 0.7Vpp and 100Hz
The image shows the wanted behavior of the circuit, however, I'm not able to obtain that in real life.
What could be the problem? Maybe the AD620 is not the best option to do that?
You will never be able to get accurate amplification of a 1mV full scale DC signal with a simple circuit using discrete components (I do know a way, but it's not simple). Even a crappy LM324 will do better (because the transistors are matched and on one chip).
Tell us what you have and we can, perhaps, help. An instrumentation amplifier can be made with three op-amps, and as I said, even a crappy 10-cent LM324 will be better than trying to use discretes.
In general, an LM324 will probably drift about 5-10uV/K so you can expect (at best) errors commensurate with that- about 5g ~ 10g/°C. A better amplifier would reduce those errors.
By comparison an B-E junction (as in your voltage follower) changes -2mV/K so you'd see 2kg/°C change- worse than useless.
In an ideal in amp the equation you have is true. In a real in amp we have to deal with something called offset voltage. This will be the real difference between the inverting and non-inverting inputs, as opposed to the 0 found in an ideal in amp.
Looking at the datasheet we see that it has a worst case value of 125uV. Multiplying this by the gain of 381 gives us about 48mV. Your output is therefore completely within spec.
Best Answer
The AD620 negative supply (-Vs) is at 0 volts but if you do this, you need to respect the input common mode range and that is specified as: -
In other words, tying pin 2 (one of your inputs) to ground is an illegal condition and all bets are off. The lowest voltage allowed on your inputs is 1.9 volts.
Just in case it's important to your design, you should also consider that the output signal is limited to this following range: -
So, with a single 5 volt supply, the output can typically be between 1.1 volts and 3.8 volts but, over the full temperature range this might reduce to 2.1 volts to 3.6 volts.