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Old 6th November 2012, 08:41 AM   #831
KSTR is offline KSTR  Germany
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CMRR at 110dB and a pole at 3kHz looks good indeed. Likewise, PSRR also looks to be significantly better than on a, say AD797.
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Old 6th November 2012, 11:37 AM   #832
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Note that common-mode distortion is not necessarily well correlated to high CMRR. Common-mode distortion is a function of common-mode voltage-dependence of CMRR and common-mode input impedance. If CMRR and common-mode input impedance stay very constant over the relevant common-mode swing, absolute CMRR does not need to be high.

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Old 6th November 2012, 01:50 PM   #833
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Originally Posted by KSTR View Post
CMRR at 110dB and a pole at 3kHz looks good indeed. Likewise, PSRR also looks to be significantly better than on a, say AD797.
The LME49990 has better specs.
http://www.ti.com/lit/ds/symlink/lme49990.pdf
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Old 6th November 2012, 02:41 PM   #834
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Quote:
Originally Posted by Samuel Groner View Post
Note that common-mode distortion is not necessarily well correlated to high CMRR. Common-mode distortion is a function of common-mode voltage-dependence of CMRR and common-mode input impedance. If CMRR and common-mode input impedance stay very constant over the relevant common-mode swing, absolute CMRR does not need to be high.

Samuel
Even more surprising IIRC LT uses JFET's with back gate to substrate capacitance. The input impedances are out of phase at the harmonics so this component is not cancelled.
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Old 6th November 2012, 06:24 PM   #835
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Maybe the LT1468 has exceptional common mode rejection?
It is the 1468-2 uncompensated version used.

I haven't been too successful using this one.

It have to be operated at gains above 2 to be stable and even then....
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Old 7th November 2012, 11:34 AM   #836
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Originally Posted by dirkwright View Post
The LME49990 has better specs.
http://www.ti.com/lit/ds/symlink/lme49990.pdf
thd is high -- dc specs are very good. Neds to be included within gnfb loop.
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Old 8th November 2012, 04:19 AM   #837
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thd is high -- dc specs are very good. Neds to be included within gnfb loop.
LME49990 THD high? 0.00001% (.1 ppm)? What do you want?
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Old 8th November 2012, 04:30 AM   #838
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LME49990 THD high? 0.00001% (.1 ppm)? What do you want?
More zeros I guess.
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Old 8th November 2012, 01:39 PM   #839
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LME49990 THD high? 0.00001% (.1 ppm)? What do you want?
Yeah, the LME49710 is 0.00003% THD (at unity gain...).

The only difficulty with using the LME49990 is the number of bypass capacitors required at the power supply pins. The datasheet says 3 capacitors per pin are required for stability. As I see it, the only way to fit everything in there is to use the back side of the board as well as the front (top). This can get really fiddly for DIY.
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Old 8th November 2012, 02:00 PM   #840
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Dirk,

Putting bypass caps on the opposite side of where the part lies is not recommended for good high freq decoupling. Vias create problems as they have inductance, something that you do not want for good de-coupling. Maybe the larger bulk decoupler (1uF) may go on the opposite side, but the smaller ones should be on the same side as the IC. The layout rules usually goes to put the smaller cap closest to the pin followed by the larger ones. Say 1nF,100nF,1uF. Using TH caps is not recommended as they take up a lot more space than a SM caps do, leading to more trace inductance. Choose X7R dielectric for the big ones and COG for the smaller ones, is usually recommended.The trace is to flow through the caps hi side, then finally to the IC pad/lead, with minimal trace length, in other words, no stubs. 0805/0603 are great for this. I agree, not that easy for a bread board design or discrete wiring prototype.

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