Message-ID: <40A64A60.9030308@usa.net>
Date: Sat, 15 May 2004 18:50:40 +0200
From: Alberto di Bene <dibene@usa.net>
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Subject: LF: RE: VDC source with microvolt resolution
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Dear Jim,

  thanks for your further considerations. I will have a look at those 
opamps.
And yes, working at the uV level is a bit of challenge.  I wonder if 
reducing the
Hz/V sensitivity of the EFC with a passive attenuator could be a good 
strategy...
after all, I don't need an adjustment range of plus/minus 8 Hz. A tenth 
of this
should be enough...

73  Alberto  I2PHD
----------------------------------

James Moritz wrote:

>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
>
>
>
>.
>
>  
>