AD1878*

The AD1879 is a two-channel, 18-bit oversampling ADC based on technology and intended primarily for digital audio applications.The AD1878 is identical to the 18-bit AD1879 except that it outputs 16-bit data words. Statements in this data sheet should be read as applying to both parts unless otherwise noted.

Each input channel of these ADCs is fully differential. Each data conversion channel consists of a fifth order one-bit noise shaping modulator and a digital decimation filter. An on-chip voltage reference provides a voltage source to both channels stable over temperature and time. Digital output data from both channels is time-multiplexed to a single, flexible serial interface.The AD1878/AD1879 accepts a 256 *F_S input master clock.

Input signals are sampled at 64 * F_S on switched-capacitors,eliminating external sample-and-hold amplifiers and minimizing the requirements for antialias filtering at the input. With simplified antialiasing, linear phase can be preserved across the passband.The AD1878/AD1879's proprietary fifth-order differential switched-capacitor modulator architecture shapes the one-bit comparator's quantization noise out of the audio passband. The high order of the modulator randomizes the modulator output,reducing idle tones in the AD1878/AD1879 to very low levels. The AD1878/AD1879's differential architecture provides increased dynamic range and excellent common-mode rejection characteristics. Because its modulator is single-bit, AD1878/AD1879 is inherently monotonic and has no mechanism for producing differential linearity errors.

The digital decimation filters are single-stage, 4095-tap finite impulse response filters for filtering the modulator's high frequency quantization noise and reducing the 64 * F_S single-bit output data rate to a F_S word rate. They provide linear phase and a narrow transition band that permits the digitization of 20 kHz signals while preventing aliasing into the passband even when using a 44.1 kHz sampling frequency. Passband ripple is less the 0.001 dB, and stopband attenuation exceeds 115 dB.

The flexible serial output port produces data in twos-complement,MSB-first format. Input and output signals are to TTL and CMOS-compatible logic levels. The port is configured by pin selections. The AD1878/AD1879 can operate in either master or slave mode. Each 16-/18-bit output word of a stereo pair can be formatted within a 32-bit field as either right-justified, I²Scompatible,or at user-selected positions. The output can also be truncated to 16-bits by formatting into a 16-bit field.The AD1878/AD1879 consists of two integrated circuits in a single ceramic 28-pin DIP package. The modulators and reference are fabricated in a BiCMOS process; the decimator and output port, in a 1.0 mm CMOS process. Separating these functions reduces digital crosstalk to the analog circuitry. Analog and digital supply connections are separated to further isolate the analog circuitry from the digital supplies.

The AD1878/AD1879 operates from ±5 V power supplies over the temperature range of 25°C to +70°C.