EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: NiHaoMike on July 18, 2015, 05:25:05 am
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I'm working on a HD DAC/headphone amplifier that uses a TPA6120A2 as the output amplifier. It has a few mV of offset which doesn't seem like much, but it's enough to make a very obvious click in the headphones when the output relay opens or closes.
Therefore, I want to use a low offset opamp like a LTC1051 as a DC servo to null out that offset. But how much noise do chopper opamps introduce to their supply rails? Is it likely enough to degrade the SNR of the system? (I'm going for a system SNR of well over 110dB, with the actual DAC chip being specified for 127dB.)
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Hello,
read datasheet page 6 above figure 1:
I is more likely that you will have the noise directly at the input of the LTC1051.
As charge injection from the clock to the input switches.
With best regards
Andreas
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There's going to be plenty of filtering where the servo opamp interfaces into the output amplifier. I just want to know how much filtering (if any) I should put on the supply rails.
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Your power driver should have enough PSRR to handle it, and the DC correction servo should have standard opamp decoupling on it's rails in any case, so the usual 100n ceramic and 10uF electrolytics from each rail to ground should work well enough to do it, probably there in the datasheet or app notes as part of the demo circuits.
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Noise would be more a problem through the input. A better choise would be a normal low Offset OP (e.g. LT1013) for the DC controll.
Normally a manual adjstment of the DC offset of the existing amplifier schould be enough, to keep below 0.1 mV.
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im gonna do some serious experiments on this when I finally get a DSA that works
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A normal precision OP easily suffices for this. Depending on the headphone impedance the audible offset varies and is especially low with high-efficiency 32 ? or 16 ? headphones. Better studio phones are much less sensitive and the acceptable offset is much higher.
With <1 mV of offset you should be on the safe side with almost any headphone.