| ??? 03/20/07 05:24 Read: times |
#135349 - Artefacts Responding to: ???'s previous message |
David said:
I'm eager for your views on how AC excitation can remove gain noise that I assume is being introduced by the ADC (which is back in the DC domain). When using DC excitation you suffer from rather high very low frequency noise of analog front-end stages. When using AC excitation in combination with lock-in methode, on the other hand, only the spectral noise density at the chopping frequency contributes to the noise, which is considerably smaller than the very low frequency noise. More, when extreme low noise and extreme high linearity is desired, a mixed analog digital chip like these AD7730 and similar should not be used. The internal high frequency clock signals and noise is injected into the sensitive analog section via on-die stray capacitances and becomes demodulated on certain non-linearities of analog stages, resulting in artefacts like drifting DC offsets and gain noise. The only remedy is to use a purely analog front end circuitry, which sees the digital world only after the signal(s) have been dramatically amplified. Kai |
