Kai Klaas wrote:
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Sorry you feel so upset, but there are dozens of AC bridge circuits, not just the most basic and simple "Wheatstone" arrangements. AC bridges are BETTER than DC bridges. I want to enlighten you about ac bridges. Are you familiar with Schering bridges, Wein bridges, kelvin's double bridge, Heaviside-Campbell bridges ? No ? Not forgetting various RF bridges.

Erik is not without a hell of a lot of experience in Analogue electronics.......

Synchronous rectification / phase sensitive detection is a very powerful technique, but why do you assume that the bridge signal will be submerged in noise ? It rather misses the point about the technique. The nice thing is that it can extract phase and quadrature signals from the bridge at balance.

Alright, under limited circumstances you are "right", in your simple case, but to essentially dismiss AC techniques, as a result of your very simple analysis, is, well, wrong. I don't want your copious calculations to be used by anyone else as a reason for not considering the AC Bridge "because Kai said so".


I don't think you are looking in the right places ! And because people have forgotten the techniques, or never knew them in the first place. Try "Alternating Bridge Methods" by Hague (1942), or "Coaxial AC bridges" by Kibble and Rayner (usually in print). Also look for "Blumlein Bridges"

Some of these methods will extract femto-farads of signal in hundreds of picofarads of strays and parasitics, or sub micro-ohms in Kilo-Ohms. Others will work with ground tied loads, or floating loads.

ZERO chips are needed. It can all be done with simple discrete circuits. They DO need wound components, but basic multi-filar windings can be done on simple ferrite cores, rather than the best "MuMetal" cores.

Remember transformer bridges are THE method for measurement of fundamental quantities in national standard labs.



My illustration was to point out that your dismissal of your bridge because it was only capable of 13 bit resolution was well wide of the mark.

Erik actually said
If one used AC, all problems re offset, drift and such would be eliminated.
And he is quite right.

Many dismiss the old methods as somehow obsolete, but there are many truths still in them.

Again, sorry you feel offended, but copious calculation and analysis is not a replacement for fundamental insight.

Steve