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Sunday, May 15th, 2011

The Infernal Op Amp 2

Categories: [ DIY ]

I don't have a proper explanation as for the why, but the op amp problem I had recently is solved by using a virtual ground set to 1/2 Vsupply.

My understanding there is that the differential amplifier acts as both an inverting and non inverting amplifier, and that it needs a negative supply (or a virtual ground, which amounts to the same thing) for the non-inverting part. Using a virtual ground as mentioned above has two drawbacks:

  • it brings the reference point of Vout to 1/2 Vsupply, which makes it much less nice to use with the Arduino's analog input (only the upper-half of the input range is used because of this offset) and
  • the maximum allowable range for the signals is between 1/2 Vsupply and Vsupply-1.5V (for the LM324), which is not much even when Vsupply is 12 V.

Moreover, the differential amplifier needs well matched resistors: R1||R2 = R3||R4 (R1||R2 is the equivalent resistance of R1 in parallel with R2), othwerwise it has an additional DC offset which gets amplified by the gain of the differential amplifier, making the whole thing useless for my purpose of amplifying the very small voltage across a shunt.

On the bright side though, I found another op amp, the LT1495, that can accept inputs beyond Vsupply and with a very low input offset. Its only drawback is that it costs 25 times more than the LM324 (which was very, very cheap, but still).

[ Posted on May 15th, 2011 at 20:52 | 1 comment | ]