Electronic Design

Low-Power 24-Bit Stereo ADCs Boast 124-dB SNR

For professional digital audio recording and processing, Texas Instruments' PCM4220 and PCM4222 24-bit, 216-kHz delta-sigma analog-to-digital converters (ADCs) provide up to 124dB signal-to-noise ratio (SNR). While sampling at 48 kHz, these two-channel ADCs consume 305 mW, permitting designs that power computer audio interfaces entirely through a USB or Firewire bus.

The 6-bit output from the delta-sigma modulators in the PCM4220 is routed to a digital decimation filter, whose output provides linear pulse-code modulation (PCM) data. This data then is available at the audio serial port interface. In the PCM4222, the modulator output bypasses the decimation filter so designers can implement the digital filter in an FPGA for end-product differentiation.

To provide more power-supply rejection than ADCs that use the power-supply rails as the modulator reference, these chips employ a bandgap reference circuit. The audio serial port supports common audio data formats at sampling rates from 8 to 216 kHz. The 24-bit PCM data can be reduced to 20-, 18-, or 16bit word lengths, if needed.

The PCM4220's multistage finite impulse response (FIR) digital decimation filter provides two filter-response options: "classic" and "low group delay." The classic response will be familiar to most audio engineers. The low-group-delay filter response provides lower latency. Specifically, the group delay for the classic filter is 39 output sample periods, while the low-group-delay filter provides a 21-output sample period group delay.

Digital high-pass filtering can be turned on or off independently. Also, individual overflow outputs indicate the overflow status for each channel, allowing designers to optimize for studio or live sound.

The PCM4222 and PCM4220 come in 48-pin thin quad flat-pack (TQFP) packages. The PCM4222 costs $14.95 and the PCM4220 costs $9.95, both in quantities of 1000 units.

Texas Instruments

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