Dear Alberto, LF Group,
I think Johan's "PWM-ed PWM" is a better idea, since you should be more or
less guaranteed a monotonic Vout vs. input code transfer function for the
DAC.
I don't think op-amp noise in the filter will be a major problem - the noise
voltage is proportional to sqrt(bandwidth), and the bandwidth is only a
small fraction of a Hz. The high resistor values needed for the filter and
their associated thermal noise mean that an op-amp with a low input noise
voltage is pointless (1Meg has about 130nV/rootHz)- it is more important to
minimize the input current noise, ie. use a FET input device. The active
filter op-amp driving the EFC will have wide-band noise in its output, but
most of this could be filtered out with a simple RC section (say
1kohm/100uF) between the op-amp and the EFC pin.
In some work on an electrometer circuit I did some years back, I found the
newer types of CMOS op-amp, like the LMC660/LMC662 were cheap and good in
this sort of application. Usually, the noise and drift of the op-amp was
less significant than thermal effects, leakage, vibration, cross-talk from
the other parts of the circuit and all sorts of peculiar effects that plague
DC circuits at the uV level.
Cheers, Jim Moritz
73 de M0BMU
.
