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Audio Converters Forum
PCM5100 output filter
In the datasheet for the PCM5100, the recommended output filter is a 470 ohm resistor and a 2.2nF (.0022uF) cap. This has an approximate -3dB filter point of 153 kHz. However, the datasheet for the PCM510xEVM-U eval board uses 100 ohms and a 22 nF (.022 uF) cap. That combo has a -3dB point of about 72 kHz. Why the discrepancy?
Because different people write the d/s versus design the EVM.
Thanks for pointing that out. We will try to fix them to be consistent.
Audio Applications Engineering Manager
Dallas, TX USA
So which is the actual preferred filter?
Sorry for the delay. I'd go with what's on the EVM as we have thoroughly tested that combination.
I have a follow-on question to this thread - what type of output stage does the PCM5100 have - is it a push-pull, class A, etc? Its not specified in the datasheet anywhere I can find.
Thanks - Bruce
TI has recommended fc=54kHz as 2nd order LPF in the PCM DAC datasheets.Why fc=54kHz? I think as follows.The frequency response of N-order Butterworth LPF is;1/sqrt((f/fc)^(2*N)+1)N=2, 20kHz, fc=54kHz-10*log10((20/54)^4+1) =-0.081 dB
For the response at 20kHz within -0.1dB,fc =f/(10^(2*0.1/20)-1)^(2*N)) =20000/(10^(2*0.1/20)-1)^4) =51.195 kHz
In case of 1st order LPF,fc =20000/(10^(2*0.1/20)-1)^2) =131.04 kHz
In case of the value of PCM5100 datasheet,N=1, 20kHz, fc=153kHz-10*log10((20/153)^2+1) =-0.074 dB
The value of datasheets may be designed to keep the frequency response within 0.1dB.
I am still looking for an answer to this - what type of electrical output stage does the PCM5100 have?
Sorry we missed your post. Let me check tomorrow and get back to you.
I'm curious -- why is this information needed? How does it help you in your design or comparison to a competitor's part? I'm kind of new to the codec world; in fact, my boss like to tease me and call me an AmpHead.
I had asked Bruce to find me the answer. So can I drive it directly to an op amp inverting amplifier adjust gains for increasing or lowering volume? I am not familiar with PWM (class D) circuits and wanted to make sure this isn't that.
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