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560

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2

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40.2

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PCM4220

SBAS407A – DECEMBER 2006 – REVISED MAY 2007

film or C0G ceramic capacitors; both types perform with equivalent results in this application. Surface-mount

devices are utilized throughout because they provide superior performance when combined with a wideband

amplifier such as the OPA1632. The DGN package version of the OPA1632 is utilized; this package includes a

thermal pad on the bottom side. The thermal pad must be soldered to the PCB ground plane for heat sink and

mechanical support purposes.

Figure 45. Differential Input Buffer Circuit Utilizing the OPA1632

Figure 46 demonstrates the same circuit topology of Figure 45, while using standard single or dual op amps.

The noise level of this circuit is adequate for obtaining the typical A-weighted dynamic range performance for the

PCM4220. However, unweighted performance may suffer, depending upon the op amp noise specifications.

Near-typical THD+N can be achieved with this configuration, although this performance also depends on the op

amps used for the application. The NE5534A and OPA227 (the lower cost 'A' version) are good candidates from

a noise and distortion perspective, and are reasonably priced. More expensive lower-noise models, such as the

OPA211, should also work well for this configuration. Feedback and input resistor values may be changed to

alter circuit gain. However, it is recommmended that all circuit changes be simulated and then tested on the

bench using a working prototype to verify performance.

Figure 47 illustrates a differential input circuit that employs a noninverting architecture. The total noise and

distortion is expected to be higher than that measured for Figure 45 and Figure 46. As with Figure 46, the

NE5534A and OPA227 are good candidates for this circuit, although similar op amps should yield equivalent

results.

A useful tool for simulating the circuits shown here is TINA-TI, a free schematic capture and SPICE-based

simulator program available from the Texas Instruments web site. This tool includes macro models for many TI

and Burr-Brown branded amplifiers and analog integrated circuits. TINA-TI runs on personal computers using

Microsoft Windows

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