EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: SArepairman on September 03, 2014, 02:03:54 am
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A dual op amp current source (buffered high impedance howland current pump) is driving a wire with a current going to a sensor. This wire runs in parallel with audio frequency (of 100mAish rms power) lines for about a meter.
Does it make sense to put a sallen key filter in the feedback path after the howland current pump feedback buffer?
(http://s17.postimg.org/pyt31w4cf/32432342.png)
I saw this in an app note before. Will this sort of current source turn coupled noise into current?
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"Audio frequency" = < 20kHz BW, or not many times more than that.
1m of most any kind of cable isn't going to be more than ~300pF, or more than 26kohms reactance (or not many times below that).
So, if you current source is much better than 26kohms, you're wasting your time -- or alternately, if the sensor can't tolerate under 26kohms of compliance or crosstalk. (Which means, for example, a Howland CCS built with 1M 5% resistors will still have a worst case good enough for present purposes, if I'm estimating that right.)
What is this, current bias for a voltage on the sensor?
Tim
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Howland can be a poor choice for a regulated unipolar current source - a fet or a bjt (with base current compensation if needed) can have higher output Z to higher frequency, only one op amp needed for adding accuracy to the transistor ccs
and you shouldn't in general "add a filter" to a feedback connection - feedback loop performance has to be designed for