Other Parts Discussed in Thread: PCM4222
I have a customer who wants to use one of our input transformers, model JT-10KB-DPC, to drive PCM4222 analog inputs. The data sheet is rather sketchy about just what these inputs are in terms of an equivalent circuit. It says the "input impedance" of "each pin" (presumably with respect to ground) is 2.8 kΩ (which tends to define common-mode impedances). May I safely assume that the differential input impedance is 5.6 kΩ and that each input pin is internally through a 2.8 kΩ resistor to Vcom? Then there's the matter of the 2.7 nF capacitor shown across the differential input pins. May I presume that this is intended to be a reservoir since the inputs of the PCM4222 do charge transfer via internal switches to sample the input? The effective source impedance of the transformer driving the input pins is about 230 Ω differential. Since both input pins seem to be DC biased to Vcom internally ... and the transformer secondary "floats" (no DC path to ground), do I need to concern myself with any external biasing? It would be great if the data sheet could treat the analog inputs with the same rigor afforded all the digital specs. For example, digital inputs have a specified input capacitance (3 pF) yet this spec is conspicuous by its absence for the analog inputs. As a separate issue, the input buffer circuit used on the EVM and shown on the data sheet is a truly horrible design ... its common-mode input impedances are way, way too low ... so low that even source impedance imbalance of a fraction of an ohm will destroy the expected CMRR. I've written a number of papers about this all-too-often overlooked design pitfall (AES Journal, EDN magazine, etc.) - Bill Whitlock, chief engineer, Jensen Transformers, Inc., AES Life Fellow - IEEE Life Senior
