PGA2505: Capacitor and resistor value query

Hello Anthony,

I was unable to find any information whatsoever about what the R and C values should be for the circuit shown in the PGA2505, Figure 15 diagram. Therefore, I had to derive values which I have tested using simulation. The actual solution would have to be tested with the PCM ADC.

Here are the assumptions that used to derive the values:

1. The PGA2505 can drive a 600 ohm load
2. The shunt resistor would be 2x the value of the two series resistors. There sum total would be equal to 600 ohms
3. Since this is intended for a microphone application fL was set to 22 Hz and fH to 22 kHz
4. The fH cutoff frequency establishes the -3 dB bandwidth of the anti-alias filter

The two series resistors in the filter are 150 ohms and the 2x shunt resistor is 300 ohms, for a total of 600 ohms. My calculations indicated that 22 uF capacitors set the zero at 22 Hz for the fL -3 dB cutoff, and a 47 nF sets the pole for the 22 kHz fH -3 dB cutoff. You can see the filter circuit below in the TINA Spice schematic. Note that there isn't a simulation model for the PGA2505 so I used a pair of TLV2770 op amps to drive the filter differentially. The actual PGA2505 outputs would be used to drive the filter which would be followed by the differential PCM ADC inputs.

Please let me know if this works for you.

Regards, Thomas

Precision Amplifiers Applications Engineering