AD1938 datasheet(13/32 Pages) AD

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AD1938

Rev. 0 | Page 13 of 32

THEORY OF OPERATION

ANALOG-TO-DIGITAL CONVERTERS (ADCS)

There are four analog-to-digital converter (ADC) channels in

the AD1938 configured as two stereo pairs with differential

inputs. The ADCs can operate at a nominal sample rate of

48 kHz, 96 kHz, or 192 kHz. The ADCs include on-board

digital antialiasing filters with 79 dB stop-band attenuation and

linear phase response, operating at an oversampling ratio of 128

(48 kHz, 96 kHz, and 192 kHz modes). Digital outputs are

supplied through two serial data output pins (one for each

stereo pair) and a common frame (ALRCLK) and bit (ABCLK)

clock. Alternatively, one of the TDM modes can be used to

access up to 16 channels on a single TDM data line.

The ADCs must be driven from a differential signal source for

best performance. The input pins of the ADCs connect to inter-

nal switched capacitors. To isolate the external driving op amp

from the glitches caused by the internal switched capacitors,

each input pin should be isolated by using a series connected,

external, 100 Ω resistor together with a 1 nF capacitor connected

from each input to ground. This capacitor must be of high quality,

for example, ceramic NPO or polypropylene film.

The differential inputs have a nominal common-mode voltage

of 1.5 V. The voltage at the common-mode reference pin (CM)

can be used to bias external op amps to buffer the input signals

(see the Power Supply and Voltage Reference section). The

inputs can also be ac-coupled and do not need an external dc

bias to CM.

A digital high-pass filter can be switched in line with the ADCs

under serial control to remove residual dc offsets. It has a

1.4 Hz, 6 dB per octave cutoff at a 48 kHz sample rate. The

cutoff frequency scales directly with sample frequency.

DIGITAL-TO-ANALOG CONVERTERS (DACS)

The AD1938 digital-to-analog converter (DAC) channels are

arranged as single-ended, four stereo pairs giving eight analog

outputs for minimum external components. The DACs include

on-board digital reconstruction filters with 70 dB stop band

attenuation and linear phase response, operating at an oversam-

pling ratio of 4 (48 kHz or 96 kHz modes) or 2 (192 kHz mode).

Each channel has its own independently programmable

attenuator, adjustable in 255 steps in increments of 0.375 dB.

Digital inputs are supplied through four serial data input pins

(one for each stereo pair) and a common frame (DLRCLK) and

bit (DBCLK) clock. Alternatively, one of the TDM modes can

be used to access up to 16 channels on a single TDM data line.

Each output pin has a nominal common-mode dc level of 1.5 V

and swings ±1.27 V for a 0 dBFS digital input signal. A single op

amp, third-order, external, low-pass filter is recommended to

remove high frequency noise present on the output pins. The

use of op amps with low slew rate or low bandwidth can cause

high frequency noise and tones to fold down into the audio

band; therefore, exercise care in selecting these components.

The voltage at CM, the common-mode reference pin, can be

used to bias the external op amps that buffer the output signals

(see the Power Supply and Voltage Reference section).

CLOCK SIGNALS

The on-chip phase locked loop (PLL) can be selected to

reference the input sample rate from either of the LRCLK pins

or 256, 384, 512, or 768 times the sample rate, referenced to the

48 kHz mode from the MCLKI pin. The default at power-up is

256 × fS from MCLKI. In 96 kHz mode, the master clock fre-

quency stays at the same absolute frequency; therefore, the

actual multiplication rate is divided by 2. In 192 kHz mode,

the actual multiplication rate is divided by 4. For example, if a

device in the AD193x family is programmed in 256 × fS mode, the

frequency of the master clock input is 256 × 48 kHz = 12.288 MHz.

If the AD193x is then switched to 96 kHz operation (by writing

to the SPI or I2C port), the frequency of the master clock should

remain at 12.288 MHz, which is now 128 × fS. In 192 kHz mode,

this becomes 64 × fS.

The internal clock for the ADCs is 256 × fS for all clock modes.

The internal clock for the DACs varies by mode: 512 × fS (48 kHz

mode), 256 × fS (96 kHz mode), or 128 × fS (192 kHz mode). By

default, the on-board PLL generates this internal master clock

from an external clock. A direct 512 × fS (referenced to 48 kHz

mode) master clock can be used for either the ADCs or DACs if

selected in PLL and Clock Control Register 1.

Note that it is not possible to use a direct clock for the ADCs set

to the 192 kHz mode. It is required that the on-chip PLL be

used in this mode.

The PLL can be powered down in PLL and Clock Control

modes, or if the reference clock is unstable at power-on, power

down the PLL and then power it back up when the reference

clock has stabilized.

The internal master clock (MCLK) can be disabled in PLL and

Clock Control Register 0 to reduce power dissipation when the

AD1938 is idle. The clock should be stable before it is enabled.

Unless a standalone mode is selected (see the Serial Control

Port section), the clock is disabled by reset and must be enabled

by writing to the SPI or I2C port for normal operation.

To maintain the highest performance possible, it is recommended

that the clock jitter of the internal master clock signal be limited

to less than 300 ps rms TIE (time interval error). Even at these

levels, extra noise or tones can appear in the DAC outputs if the

jitter spectrum contains large spectral peaks. If the internal PLL

is not being used, it is highly recommended that an independent

crystal oscillator generate the master clock. In addition, it is

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