I changed the heater circuit described in this elog to a current sink. The new and old circuits are shown in the attachment. The heater is and is currently 24Ω; the sense resistor is currently 6Ω. The op-amp is still an OP27 and the MOSFET is still an IRF630.
The current through the old circuit was saturating because the gate voltage on the MOSFET was saturating at the op-amp supply rails. This is because the source voltage is relatively high: .
In the new circuit the source voltage is lower and the op-amp can thus drive a large enough to draw more current (until the power supply saturates at 25V/30Ω = 0.8A in this case). The source and DAC voltages are equal in this case and so the current is . Since this is the same current through the heater, the drain voltage is . I observed this behavior in this circuit until the power supply saturated at 0.8A. Note that when this happens and the gate voltage saturates at the supply rails in an attempt to supply the necessary current.