JAJSMD4D December 2003 – March 2022 OPA1632
Supply voltages of ±15 V 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 ±8 V 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 (2.5 V). 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 2.5 V.
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 8-1), 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.