OPAX134 datasheet(6/15 Pages) TI

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OPA1632

SBOS286A − DECEMBER 2003 − REVISED SEPTEMBER 2006

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APPLICATIONS INFORMATION

Figure 1 shows the OPA1632 used as a differential-output

driver for the PCM1804 high-performance audio ADC.

Supply voltages of

±15V are commonly used for the

OPA1632. The relatively low input voltage swing required

by the ADC allows use of lower power-supply voltage, if

desired. Power supplies as low as

±8V can be used in this

application with excellent performance. This reduces

power dissipation and heat rise. Power supplies should be

bypassed with 10

µF tantalum capacitors in parallel with

0.1

µF ceramic capacitors to avoid possible oscillations

and instability.

The VCOM reference voltage output on the PCM1804 ADC

provides the proper input common-mode reference

voltage (2.5V). This VCOM voltage is buffered with op amp

A2 and drives the output common-mode voltage pin of the

OPA1632. This biases the average output voltage of the

OPA1632 to 2.5V.

The signal gain of the circuit is generally set to

approximately 0.25 to be compatible with commonly-used

audio line levels. Gain can be adjusted, if necessary, by

changing the values of R1 and R2. The feedback resistor

values (R3 and R4) should be kept relatively low, as

indicated, for best noise performance.

R5, R6, and C3 provide an input filter and charge glitch

reservoir for the ADC. The values shown are generally

satisfactory. Some adjustment of the values may help

optimize performance with different ADCs.

It is important to maintain accurate resistor matching on

R1/R2 and R3/R4 to achieve good differential signal

balance. Use 1% resistors for highest performance. When

connected

for

single-ended inputs (inverting input

grounded, as shown in Figure 1), the source impedance

must be low. Differential input sources must have

well-balanced or low source impedance.

Capacitors C1, C2, and C3 should be chosen carefully for

good distortion performance. Polystyrene, polypropylene,

NPO ceramic, and mica types are generally excellent.

Polyester and high-K ceramic types such as Z5U can

create distortion.

Ω

270

Ω

270

Ω

1nF

0.1

µF

0.1

µF

V+

+8V to +16V

−8V to −16V

−

1/2

PCM1804

OCM

2.7nF

COM

(2.5V)

Enable(1)

Balanced or

Single−Ended

Input

OPA1632

OPA134

µF

0.1

µF

−

NOTE: (1) Leave open to enable.

Logic signals referenced to V

− supply.

See the Shutdown Function section.

Figure 1. ADC Driver for Professional Audio

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