Very low distortion
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From: Vladimir Vassilevsky on 17 May 2010 11:35 I need to make a test source with THD ~ 0.0001% at the frequencies below 1kHz. Leaving many details aside, the biggest problem seems to be an opamp. The typical THD vs frequency plot for a good opamp looks like this: ADTL082 http://www.analog.com/static/imported-files/data_sheets/ADTL082_084.pdf It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz. Perhaps the limit is set by Audio Precision; it can't measure THD below ~0.008% in direct way. But, even if 0.001% is the artifact of the measurement, there is still not enough of open loop gain to get to 0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost the open loop gain and improve distortion? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: David Eather on 17 May 2010 11:41 On 18/05/2010 1:35 AM, Vladimir Vassilevsky wrote: > > I need to make a test source with THD ~ 0.0001% at the frequencies below > 1kHz. Leaving many details aside, the biggest problem seems to be an opamp. > > The typical THD vs frequency plot for a good opamp looks like this: > > ADTL082 > http://www.analog.com/static/imported-files/data_sheets/ADTL082_084.pdf > > It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According > to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz. > > Perhaps the limit is set by Audio Precision; it can't measure THD below > ~0.008% in direct way. But, even if 0.001% is the artifact of the > measurement, there is still not enough of open loop gain to get to > 0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost > the open loop gain and improve distortion? > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultant > http://www.abvolt.com This might be better: http://www.national.com/pf/LM/LME49990.html#Overview
From: George Herold on 17 May 2010 11:46 On May 17, 11:35 am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > I need to make a test source with THD ~ 0.0001% at the frequencies below > 1kHz. Leaving many details aside, the biggest problem seems to be an opamp. > > The typical THD vs frequency plot for a good opamp looks like this: > > ADTL082http://www.analog.com/static/imported-files/data_sheets/ADTL082_084.pdf > > It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According > to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz. > > Perhaps the limit is set by Audio Precision; it can't measure THD below > ~0.008% in direct way. But, even if 0.001% is the artifact of the > measurement, there is still not enough of open loop gain to get to > 0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost > the open loop gain and improve distortion? > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.com How about a different opamp? The opa134 seems to have enough zero's in the THD+noise spec. (0.00008%) Though my eyes start to boggle with so many zeros in a row. George H.
From: John Larkin on 17 May 2010 12:15 On Mon, 17 May 2010 10:35:12 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > >I need to make a test source with THD ~ 0.0001% at the frequencies below >1kHz. Leaving many details aside, the biggest problem seems to be an opamp. > >The typical THD vs frequency plot for a good opamp looks like this: > >ADTL082 >http://www.analog.com/static/imported-files/data_sheets/ADTL082_084.pdf > >It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According >to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz. > >Perhaps the limit is set by Audio Precision; it can't measure THD below >~0.008% in direct way. But, even if 0.001% is the artifact of the >measurement, there is still not enough of open loop gain to get to >0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost >the open loop gain and improve distortion? > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com Follow Jim Williams' rule: always invert. Add JL's rule: use really good resistors. John
From: Mike on 17 May 2010 14:28
Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > Would it be possible to make a cascaded amp to boost > the open loop gain and improve distortion? > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultant > http://www.abvolt.com Yes. See AN67, Linear Technology Magazine Circuit Collection, Volume III An Ultrapure Oscillator By Dale Eagar. Http://cds.linear.com/docs/Application%20Note/an67f.pdf Article starts on page 62 Schematic is Figure 96 on page 65 |