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Here is the data sheet of an AE101 amplifier http://www.hbm.com.pl/pdf/a0114.pdf

At page 46 there is a line as such:

Common mode rejection

0... 300 Hz dB >100

300 Hz dB >85

Do these mean for AE101 the common mode voltage will be rejected more than 100 dB or %100 for the frequencies more than 300 Hz?

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2 Answers 2

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The Amplifier has a common mode rejection of at least 100dB for frequencies below 300Hz and a for frequencies above 300Hz the rejection is at least 85 dB.

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  • \$\begingroup\$ does that mean if we apply 1V CMV to both inputs it will act as 1/100000 V applied as differential inputs below 300Hz frequencies? \$\endgroup\$
    – user16307
    Nov 25, 2013 at 10:59
  • \$\begingroup\$ 1/100000 V or less. CMRR is an upper limit, the amp may do better than that. (However you need to pay VERY careful attention to detail or errors in ground or power supply can introduce far worse errors than the amp itself) \$\endgroup\$
    – user16324
    Nov 25, 2013 at 11:02
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It's a weakly defined spec. Even for the modest op-amp I'd expect to see a graph such as this: -

enter image description here

This happens to be for an OP4177 op-amp and it tells you what you can expect all the way up to 10MHz.

The AE101 spec in the question only goes as high as 300 Hz and this is not good-enough in my opinion. You can expect, at frequencies beyond 1kHz, for this to get worse so I'd contact the supplier and get them to confirm what really happens.

Also, I wouldn't be very happy with the quoted noise voltages on page 47 - these are telling me that the bridge excitation supply voltage is the biggest contributor to noise and this shouldn't be so - there is little information about the noise and how it is shaped across the bandwidth of the device.

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  • \$\begingroup\$ Its a wheatstone bridge amp. Excitation voltage is often the biggest source of noise that you can do anything about. I wouldn't expect this to read like an op amp datasheet. \$\endgroup\$ Nov 25, 2013 at 11:37
  • \$\begingroup\$ do u guys mean that this is a shitty amplifier? could u advise me a good one for force transducers, strain gauges ect? \$\endgroup\$
    – user16307
    Nov 25, 2013 at 11:44
  • \$\begingroup\$ @user16307 I can't advice anything other than design your own (because it would be better IMHO) BUT I have to do this day in and day out on multi channel systems and looking for an off-the-shelf solution is not viable for me. I'd recommend talking to the supplier to try and understand what you will be getting. \$\endgroup\$
    – Andy aka
    Nov 25, 2013 at 11:56
  • \$\begingroup\$ its a huge undertaking to build an IA even such as this crap one \$\endgroup\$
    – user16307
    Nov 25, 2013 at 12:01
  • \$\begingroup\$ @user16307 I understand. I've told you my side of the story to justify why I do it my way. My recommendation is that you speak with the supplier to understand better the CMRR issues and the noise issues. It may be that these will be OK for your job. They wouldn't for my jobs but of course I have first hand knowledge of my jobs and virtually no knowledge of what your spec is. \$\endgroup\$
    – Andy aka
    Nov 25, 2013 at 12:50

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