Why are my DAC and ADC responses drooping?: Page 4 of 5

May 14, 2012 // By Kendall Castor-Perry
Why are my DAC and ADC responses drooping?
The Filter Wizard aka Kendall Castor-Perry turns his focus on to the troublesome problem of unexpected droop when dealing your DAC and ADC responses.

the zero-order hold we just looked at.  In fact, usually the droop in response is two to four times worse at any given frequency.  This is because the filters used are usually, effectively at least,  the cascade of two to four averaging filters.  In the case of the lovely delsig ADC in Cypress's PSoC3 and PSoC5 devices (OK, so I'm biased) the ADC's decimation filter has four stages (over most of its range) and therefore has a sinc^4() response.  At any given signal frequency it therefore has four times (expressed in dB), the droop shown in Figure 1. 

In other words it's -12 dB at 0.443 times Fs.  Such a gross departure from flatness of frequency response is not of consequence when what you really want is just the weight of bananas on a scale.  But for most audio, communications and vibration measurement systems, it’s just plain terrible.  And that’s before you add on the extra droop you’ll get if you feed the signal into a DAC.     

There’s a good side to this response effect though.  Figure 1 showed that the sinc() response 'bounces back up' to only about -13.3 dB at about 1.43 times Fs.  This is a reminder that the simple averager is not a very good filter for getting rid of high frequency variations. But if you put four of them in series to set sinc^4(), you now have a stopband response that only bounces back to around -53 dB, shown in Figure 4.  That’s quite useful filtering, usually more than enough for precision measurements in the time domain where there’s not too much high frequency interference.

Figure 4: The sinc^4() response of the PSoC3 ADC's decimation filter

So what can you do when you need a flat frequency response and you really have to use a 'held' DAC, an instrumentation delsig ADC or both?  You should expect by now that a solution proposed in this

Design category: 

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