Actions
Forum Thread Details
  • State Not Answered
  • Replies 3 replies
  • Subscribers 302 subscribers
  • Views 669 views
  • Users 0 members are here
Related
See a helpful answer?

Be sure to click 'more' and select 'suggest as answer'!

If you're the thread creator, be sure to click 'more' then 'Verify as Answer'!

Common mode LCR Filter

johnsmalley

How to correctly assign the values to the filter.

 

The (very poorly drawn!) circuit is the crux of the matter. It is a mirror-image 2 pole RC filter driven antiphase by the split secondary of a 1:1 microphone transformer. At the output side is an In-Amp. There is no load resistance to GND on either leg at the In-Amp input: I can put a pair in if needs be.

 

Having attempted to calculate the values then build the circuit, the response is miles away from what the numbers say. Then I realised I’d not taken the coil into consideration. Therein lies the difficulty. I cannot, as yet, find a formula that takes it into account when the coil is before the first (left hand) resistor. I don’t even know if the coil should be taken into account as there are no resistors to ground, except the very high Zin of each amplifier leg.

 

The basic plan is to produce a an audio circuit that is as flat as possible over the audio range.

 

As a side issue I have selected all components from batches to give the In-Amp the best chance to reduce CM noise.

 

Wisdom, help, etc, greatly appreciated.

 

Thanks.

 

John

Attachments:
image
  • 0

    Hi John,

     

    I'm not an audio expert, so I'm sure I've missed something, but I don't think those resistors and capacitors are needed. Your transformer will already have a high frequency roll-off.

    You'd have to examine the instrumentation amp datasheet to see what resistance is needed at the inputs for it to correctly operate.

    Also, won't the transformer remove most of the common-mode noise? In which case the instrumentation amp is unnecessary, and an op-amp could be used.

    Audio transformers (e.g. from Jensen) have huge CMRR at audio frequencies (>80dB at 3kHz), but >110dB at the mains-hum frequency. Combined with a decent mic cable, wouldn't that meet your needs?

  • 0 in reply to shabaz

    Hello, and thanks. image

     

    My understanding is that the audio transformer doesn’t act as a CM device as such but, as always, I could be wrong, image

     

    The amp is biased through the transformer windings, the centre tap of which connects to 0V. This method is one of the ways recommended by Analog Devices for their chips, so I feel comfortable with that.

     

    Their tutorial, MT-070 shows the how to protect an In-Amp with Rs and Cs in the way I have done, but not using two pairs of R and C per leg. Their reasoning for this scheme is protecting In-Amps from RFI. (They also suggest a CM choke in series with the In-Amp as another way of achieving same). Yes, there is a formula or two, but none that considers a coil driving the filter.

     

    [Part of the reason for the the things I do with electronics is to learn. To learn more. Being retired and self taught in this field (and others) there comes a point beyond which I seem unable to go with Internet and reading investigations.]

     

    Thanks again for your contribution. Much appreciated. imageimage

     

    John

  • 0 in reply to johnsmalley

    Hi John,

     

    I see.. I wasn't sure what the RC filter was for. For RFI suppression, a ferrite is a nice option, it is suggested in that PDF too. They suggest a part, but a better way is to buy a small ferrite bead, and loop both wires through it a couple of times (not too many times, otherwise it will affect the frequency response). You could try any ferrite bead/core (there are variants, designed for different frequencies, but there's variance in the variants too : ) so sometimes it is easier just to try one and see if it suppresses any noise), and they are low-cost. Something like this can work: Ferrite CoreFerrite Core