Question
The spice model on the web page
http://www.analog.com/en/products/amplifiers/operational-amplifiers/zero-drift-a
mplifiers/ad8638.html#product-overview appears to be much more sensitive to
temperature than I would expect. It is more sensitive than a MAX4122 or an
AD8031 in a high gain inverting amplifier circuit.
Answer
I have to say that the temperature of our IC’s is not well modelled in SPICE,
because at different temperatures a lot of things change in the transistors.
Our SPICE models are tested for 25°C and are behavioural models and not
transistor models. This is the reason that for our IC SPICE models some
temperature behaviour is implemented, but it is limited to exactly this.
Therefore accurate simulation with our SPICE models for every circuit is not
expected over different temperatures.
Therefore I would recommend to look at the temperature behaviour in the
datasheet and evaluate it from there. This is specific to your circuit, because
different parameters have different effects in the circuits.
The two main parameters for a high gain inverting amplifier circuit, which are
critical when changing the temperature are Offset voltage and Input Bias
Current. Both parameters are input referred in the datasheets. If you want to
calculate the complete effect on the output you have to consider the gain and
the impedance of the circuit. The problem with Input Bias Current is, that it
does not change linearly with temperature for the AD8638. Therefore it is quite
hard to give a output voltage change per °C.
A good way to estimate the effect on the temperature is to take the Offset
Voltage and Input Bias Current at a certain temperature and take the
resistances into account. Kindly find a how to calculate the Operational
Amplifier total offset voltage below.
Kindly note that this is only a model and several minor factors (CMRR etc.) are
not taken into account, but this should not affect the output heavily.
I had a look at the other amplifiers you were comparing the AD8638 (http://www.analog.com/AD8638_datasheet) to and I expect the AD8638 to have by
far the best Output Voltage change per degree Celsius, because of the
significantly better specifications.