High impedance vs frequency

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  • #26
sophiecentaur
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Why are you bothered about DC and RF noise if your bridge receiver is so well filtered?
Did you not take my point about SNR only being relevant once you have specified the bandwidth of the system and the noise components around your probe frequency?
If you are concerned by the 80dB loss then you need to do some tuning of the cable impedance. But a bridge looks for a null, doesn't it? So you would expect to be looking for a point at which the unbalance signal is (near) zero.

You never said why your bridge resistance needs to be what it is and have not commented on the notion of balancing out (or tuning out) the unwanted reactance apart from dismissing it as not being an option; this tuning would also have the effect of providing some selectivity against out of band noise. But, in fact you don't seem to want to use the bridge as a bridge at all. I can't understand that either.

As for the use of the term "stiff", why not just refer to source impedance? Stiffness is normally used as a mechanical term, I thought.
I hope the new amplifier helps but I am still sure that performance could be improved in other ways.
 
  • #27
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Why are you bothered about DC and RF noise if your bridge receiver is so well filtered?
The lock in is a filter, pretty much a bandpass as sharp as I you can make it, if the AD chips don't clip.

Did you not take my point about SNR only being relevant once you have specified the bandwidth of the system and the noise components around your probe frequency?
If the input doesn't clip, everything is fine. The only purpose of the filtering is to prevent the clipping.

If you are concerned by the 80dB loss then you need to do some tuning of the cable impedance. But a bridge looks for a null, doesn't it?
In my case the bridge doesn't look for a null, it just shifts the signal so you measure deviations instead of absolute value. This is also commonly done for strain gauges.

You never said why your bridge resistance needs to be what it is and have not commented on the notion of balancing out (or tuning out) the unwanted reactance apart from dismissing it as not being an option; this tuning would also have the effect of providing some selectivity against out of band noise. But, in fact you don't seem to want to use the bridge as a bridge at all. I can't understand that either.

OK, so enlighten me. I attached a sketch of my bridge, including the parasitic capacitance of the signal cable. What would you put in to "balance" it, so it does not low pass. I need to measure the small deviation "e". The bridge is driven with at least 50KHz.

As for the use of the term "stiff", why not just refer to source impedance? Stiffness is normally used as a mechanical term, I thought.
It is also used in electronics for sources that don't change under load.
 

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  • #28
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Due to a forum bug or equally likely due to temporary insanity on my part I didn't see Adjustor's post until today. I didn't know it was called bootstrapping, but I knew it as guarding. I gave it some thought, but I don't think it would have worked because my signal is comparable to the output offset voltage of a buffer amp.

I didn't know that you could choose cables by signal speed, and always thought it was much lower. I heard somewhere that metals have an index of refraction of 8 or so, so I assumed the signal speed would be limited by that. Interesting...

Propagation velocities in cables are more typically two-thirds that in free space. They depend on the construction, and particularly on the dielectric used, because that is where the fields (mostly) are. The speed is therefore more linked to the refractive index of the dielectric than that of the conductors.
 
  • #29
sophiecentaur
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This is interesting. I have two observations.
Firstly,it would have helped if you had actually stated the layout more explicitly. The bridge, according to your diagram, is mounted remotely from the detector (not clear uless your setup was in view). I was assuming that the only remote component would be the variable resistor and you were assuming that I knew what you meant. A diagram, much earlier on, would have been very useful. From the position of the 400pF, in your diagram, it appears that is the capacitative loading between the bridge and the detector amplifier. It makes more sense now. I was assuming that you would have a remote resistor and would compensate for the lead length by having a similar lead in the reference arm. I mentioned this several times but, of course, I must have appeared 'loony' in the context of the circuit which was quite clear to you.:wink:
Secondly it strikes me there is a' generation gap', here. When I say "filter" I imply a passive filter with linear components - when you read "filter" you assume a digitally implemented filter.

Digital implementation of any analogue quantity always requires an analogue anti aliasing filter, at the very least (explicitly or implicitly); you don't want to sample any signals that you don't want actually to measure or which will embarrass your DSP. To get the best out of your system you will need a suitable analogue filter. A fairly narrow band pass filter (b/w suitable for the rate of change of R that you want to monitor, centred on your probe frequency. If you design / select a filter design which includes the (estimated) 400pF in it and which will reject DC and out of band components and not attenuate the 50kHz signal then you will improve matters considerably. The simplest filter would involve a large C in series (to reject DC and low frequencies and a shunt L, to resonate at 50kHz. You just don't need to be losing all those dB's which your present system introduces. Play with Spice and put in some suitable values - which you can easily work out for yourself. Good luck.
 

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