Offset calibration of Op-amp - is that really ok?

Once you have a uP in the signal path, all sorts of stuff becomes free (once you write the code!): background autozero/autocal, adaptive lowpass filtering, smart dithering, synchronous detection, linearization, AC coupling, temperature compensation, crosstalk nulling, slew limiting, you name it. The problem is knowing when to stop, how deep into 2nd and 3rd-order effects you want to go before you get into real trouble.

This gadget is a 16-channel, 16-bit DAC with programmable output ranges from +-25 mV to +-12.5 volts. It uses a 16-bit bipolar dac followed by a 10-bit mdac that acts as a switched attenuator to define the ranges.

So on the lowest range, the mdac is set to 2 lsb's out of 1024. Dac error specs are in terms of lsb error relative to fullscale. So what's the mdac error *relative to output* when you set it to 2? Technically, it's +-0.5 LSB, which is up to +-25% gain error. If you look at the inverted r-2r ladder architecture, you could guess that it's actually a lot better. And it is. But the cmos switches aren't perfect, and the switches have a bad tc, so at low codes the gain is higher than theoretical and the excess gain has a nasty positive tc that's code dependent.

But we have a temperature sensor on the board, and a uP...

John

Hey !

Some can do both. ;-)

Graham

No! The input bias current of the opamp continually charges [C], so the (Vos+8mV) across it must be renewed immediately before each (quick) ADC reading of Vin.

--
Tony Williams.

So tell me the model of your analog PC then. ;-)

Good day!

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Vannevar Bush, 1945 Model...

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

The basic processor type is the 741 -)-)-) Of course we also use more advanced types, but that is propriatary info.......