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100mF

+

10-100nF

+

10-100nF

+

100mF

AGND

AGND

+

VREFR

REFGNDR

REFGNDL

VREFL

46

47

14

15

PCM4220

COMMON-MODE VOLTAGE OUTPUTS

VCOML

or

VCOMR

PCM4220

Precision,Low-NoiseOpAmp

(OPA227 orequivalent)

To

BiasNodes

R

(Optional)

DirectConnectto

High-ZBiasNode

(Z >10M )

W

L

100nFto1mF

ClosetoICpins

PCM4220

SBAS407A – DECEMBER 2006 – REVISED MAY 2007

PRODUCT OVERVIEW (continued)

Figure 36. Recommended Reference Capacitor Connections and Values

The PCM4220 includes two dc common-mode voltage outputs, VCOML (pin 13) and VCOMR (pin 48), which

correspond to the left and right input channels, respectively. The common-mode voltage is utilized to bias

internal op amps within the modulator section of the PCM4220, and may be used to bias external input circuitry

when proper design guidelines are followed. The common-mode voltages are derived from the VCC1 and VCC2

analog power supplies using internal voltage dividers. The voltage divider outputs are buffered and then routed

to internal circuitry and the VCOML and VCOMR outputs.

The common-mode output voltage is nominally equal to (0.4875

× VCC1) for VCOMR and (0.4875 × VCC2) for

VCOML. Given an analog supply voltage of +4.0V connected to both VCC1 and VCC2, the resulting

common-mode voltages are +1.95V.

The common-mode voltage outputs have limited drive capability. If multiple bias points are to be driven, or the

external bias nodes are not sufficiently high impedance, an external output buffer is recommended. Figure 37

shows a typical buffer configuration using the OPA227. The op amp utilized in the buffer circuit should exhibit

low dc offset and drift characteristics, as well as low output noise.

Figure 37. Common-Mode Output Connections

17

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