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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?

user16307
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2 Answers2

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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.

Batuu
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  • 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? – user16307 Nov 25 '13 at 10:59
  • 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) –  Nov 25 '13 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.

Andy aka
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  • 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. – Scott Seidman Nov 25 '13 at 11:37
  • do u guys mean that this is a shitty amplifier? could u advise me a good one for force transducers, strain gauges ect? – user16307 Nov 25 '13 at 11:44
  • @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. – Andy aka Nov 25 '13 at 11:56
  • its a huge undertaking to build an IA even such as this crap one – user16307 Nov 25 '13 at 12:01
  • @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. – Andy aka Nov 25 '13 at 12:50
  • @ScottSeidman I reckon a well-designed system would have excitation noise well below the noise of the input amplifier. On the sort of jobs I do it's about the same but I use current excitation and bespoke circuitry for this. The IA's I use are the weaker link of the two by a small margin. – Andy aka Nov 25 '13 at 12:53
  • @Andyaka, the CMRR of a system coming from a Wheatstone bridge actually has excitation voltage prominently in the governing equations, and of course you can't do better than mathematically ideal. For many apps, particularly remote bridges, excitation voltage (both sides!) is controlled using a low-current excitation sense and feedback to keep the excitation voltage stable. You'll never do better than the performance of that feedback circuit. – Scott Seidman Nov 25 '13 at 13:40
  • @ScottSeidman I agree on CMRR but I've moved onto noise - the OP's spec on page 47 seems poor given that it's the basic residual imbalance of the bridge that determines both CMRR and noise. – Andy aka Nov 25 '13 at 13:57