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PCM1690: Allowable Output Capacitance for passive Reconstruction Filter

Part Number: PCM1690
Other Parts Discussed in Thread: TPA3245

Hello all, i'm designing a PCB with the PCM1690 directly driving a TPA3245. Due to size and cost, i'm willing to sacrifice some 'dbs stopband attenuation by only using a passive reconstruction filter on the output. 

The PCM per Datasheet can only drive a maximum of 50pF at the Analoge Outputs, the passive Filter will have magnitudes larger capacitances than this. The Caps do have a series resistor in line to form an RC-Filter, yet the R is very small to avoid building up a voltage divider with the TPA Input Resistance. 

My question now would be, is the filter structure shown below OK for the PCM? Or will this result in problems due to large output capacitances?

Thanks,

best regards,

Lukas

  • Hello Lukas, It is always recommended to stick with the limits presented in the datasheet. In this case, 50pF is the max allowable analog load capacitance so you should stay within this limit. Having said that, we occasionally go outside the limits knowing some degradation is expected but going too far will degrade the performance substantially .

    I don't  have access to internal schematic but (for example)  if it has an opamp at the output, with  output resistance, Ro, ,then  in conjunction with a capacitive load, it can form an additional pole in the amplifier's transfer function and depending on the value of CL ( and thus the location of the pole) and you can get many unwanted effects such as oscillation. 

    I do not recommend to use loads  larger than the limits presented by datasheet.

    Regards,

    Arash