AD1994 datasheet(18/24 Pages) AD

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AD1994

Rev. 0 | Page 18 of 24

Output Transistor Nonoverlap Time

Σ-Δ Modulator

As mentioned in the

section, the modulator has

a noise-shaping effect such that SNR is increased within the

audio band by shifting modulator quantization noise upward in

frequency. For external clock frequency of 12.288 MHz, the

modulator’s noise-shaping works in a manner that results in a

flat noise floor at the amplifier output for frequencies 20 kHz

and below. Above 20 kHz, the amplifier noise rises due to the

spectral shaping of the modulator quantization noise. At very

high frequencies, the noise floor levels off and decreases due to

poles in the modulator noise-transfer function and in the

external LC filter.

The AD1994 allows the user to select from one of eight different

nonoverlap times, as shown in Figure 46. Nonoverlap time

prevents or minimizes the period during which both the high-

side and low-side devices are on simultaneously due to propagation

delays and nonzero rise and fall times. If both the upper and

lower portions of a half-bridge conduct simultaneously, there is a

path directly from the power supply to ground and an induced

current flow known as shoot-through. However, introducing

this delay increases distortion by pushing the switching pattern

further from an ideal two-state waveform. Selecting the

nonoverlap delay requires a compromise between distortion

and efficiency. The logic levels on the three delay control pins,

DCTRL2, DCTRL1, and DCTRL0, set the nonoverlap time

according to

The clock frequency does not have to be exactly equal to

12.288 kHz and can vary by up to ±10%. For other rates, the

noise corner scales linearly with frequency. When the modulator

runs at a rate lower than nominal, the average power stage

switching frequency decreases, the efficiency increases slightly,

and the noise floor begins to rise at a slightly lower frequency.

Likewise, a faster clock gives slightly increased bandwidth and

slightly lower efficiency.

Table 12. The state of DCTRL[2:0] is read on the

rising edge of RESET and should not be changed while RESET

is logic high.

Table 12. Nonoverlap Time Settings

DCTRL2

DCTRL1

DCTRL0

Nonoverlap Time (ns)1

Using a Crystal Oscillator

The AD1994 can use a crystal connected to the CLKI and

CLKO pins as a master clock source, as shown in

Figure 47. The

CLKI and CLKO pins connect to an internal inverter to create a

full resonator. The typical values shown work in many applications,

but the crystal manufacturer should provide the exact type and

value of the capacitors and the resistor.

13.5

22pF

XTAL

47Ω

Values are typical and are not production tested.

HIGH-SIDE

GATE DRIVE

LOW-SIDE

GATE DRIVE

tNOL

Figure 47. Crystal Connection

Using an External Clock Source

Figure 46. Half-Bridge Nonoverlap Delay Timing

If a clock signal of the appropriate frequency already exists in

the application circuit, connect it directly to CLKI and leave

CLKO floating. The logic levels of the square wave should be

compatible with those defined in

The shortest setting (DCTRL[2:0] = 111) or the second shortest

setting (DCTRL[2:0] = 111) is recommended for most applications.

These two settings allow a small trade-off between efficiency

and distortion. Longer nonoverlap times generally increase

distortion while providing little or no decrease in shoot-

through current.

Specifications section.

Large amounts of jitter on the clock input degrade performance.

Whenever possible, avoid passing the clock signal though

programmable logic and other circuits with unknown or variable

propagation delay. In general, clock signals suitable for audio ADCs

or DACs are also appropriate for use with the AD1994.

CLOCKING

The AD1994 Σ-Δ modulator requires an external clock source

with a nominal frequency of 12.288 MHz. This clock can come

from a crystal or from an existing clock signal in the application

circuit. The discrete time portions of the modulator run internally

at 6.144 MHz, corresponding to 128 × f , where f

= 48 kHz.

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