AD8324

Rev. A | Page 12 of 16

The BYP pin is used to decouple the output stage to ground.

Typically, for normal DOCSIS operation, the BYP pin should be

decoupled to ground with a 0.1 μF capacitor. However, in

applications that may require transient on/off times faster than

2 μs, smaller capacitors may be used, but it should be noted that

the BYP pin should always be decoupled to ground.

POWER SAVING FEATURES

The AD8324 incorporates three distinct methods of reducing

power consumption: transmit disable and sleep modes for

between-burst and shutdown modes, as well as gain dependent

quiescent current for transmit enable mode.

The asynchronous TXEN pin is used to place the AD8324 into

between-burst mode. In this reduced current state, the 75 Ω

output impedance is maintained. Applying Logic 0 to the TXEN

pin deactivates the on-chip amplifier, providing a 98.8% reduc-

tion in consumed power. For 3.3 V operation, the supply

current is typically reduced from 207 mA to 2.5 mA. In this

mode of operation, between-burst noise is minimized and high

input to output isolation is achieved. In addition to the TXEN

pin, the AD8324 also incorporates an asynchronous SLEEP pin,

which may be used to further reduce the supply current to

approximately 30 μA. Applying Logic 0 to the SLEEP pin places

the amplifier into SLEEP mode. Transitioning into or out of

SLEEP mode may result in a transient voltage at the output of

the amplifier.

In addition to the sleep and transmit disable functions, the

AD8324 provides yet another means of reducing system power

consumption. While in the transmit enable state, the AD8324

incorporates supply current scaling, which allows for lower

power consumption at lower gain codes. Figure 20 shows the

typical relationship between supply current and gain code.

DISTORTION, ADJACENT CHANNEL POWER, AND

DOCSIS

To deliver the DOCSIS required 58 dBmV of QPSK signal and

55 dBmV of 16 QAM signal, the PA is required to deliver up to

61 dBmV. This added power is required to compensate for

losses associated with the diplex filter or other passive compo-

nents that may be included in the upstream path of cable

modems or set-top boxes. It should be noted that the AD8324

was characterized with a differential input signal. Figures 7 to

10 show the AD8324 second and third harmonic distortion

performance versus the fundamental frequency for various

output power levels. These figures are useful for determining

the in-band harmonic levels from 5 MHz to 65 MHz.

Harmonics higher in frequency (above 42 MHz for DOCSIS

and above 65 MHz for Euro-DOCSIS) will be sharply

attenuated by the low-pass filter function of the diplexer.

Another measure of signal integrity is adjacent channel power,

commonly referred to as ACP. DOCSIS 2.0, section 6.2.21.1.1

states, “Spurious emissions from a transmitted carrier may

occur in an adjacent channel that could be occupied by a carrier

of the same or different symbol rates.” Figure 13 shows the

typical ACP for a 61 dBmV (approximately 12 dBm) QPSK

signal taken at the output of the AD8324 evaluation board. The

transmit channel width and adjacent channel width in Figure 13

correspond to the symbol rates of 160 kSym/s. Table 7 shows

the ACP results for the AD8324 driving a QPSK, 61 dBmV

signal for all conditions in DOCSIS Table 6-9, Adjacent

Channel Spurious Emissions.

UTILIZING DIPLEX FILTERS

The AD8324 was designed to drive 61 dBmV without any

external filtering and still meet DOCSIS spurious emissions and

distortion requirements. However, in most upstream CATV

applications, a diplex filter is used to separate the upstream and

downstream signal paths from one another. The diplex filter

does have insertion loss that the upstream driver needs to over-

come, but it also provides a low-pass filter. The addition of this

low-pass filter to the signal chain can greatly attenuate second

harmonic products of channels above 21 MHz and third

harmonic products of channels at or above 14 MHz up for

diplexers with a 42 MHz upstream cutoff. Similar performance

gains can be achieved using European-specified diplexers to

filter second harmonics for channels above 33 MHz and third

harmonics for channels above 22 MHz (65 MHz upstream

cutoff). This filtering allows the AD8324 to drive up to

63 dBmV of QPSK (this level can vary by application and

modulation type).

NOISE AND DOCSIS

At minimum gain, the AD8324 output noise spectral density is

1.3 nV/√Hz measured at 10 MHz. DOCSIS Table 6-10,

Spurious Emissions in 5 MHz to 42 MHz, specifies the output

noise for various symbol rates. The calculated noise power in

dBmV for 160 kSym/s is

Comparing the computed noise power of –65.7 dBmV to the

+8 dBmV signal yields –73.7 dBc, which meets the required

level set forth in DOCSIS Table 6-10. As the AD8324 gain is

increased above this minimum value, the output signal

increases at a faster rate than the noise, resulting in a signal-to-

noise ratio that improves with gain. In transmit disable mode,

the output noise spectral density is 1.1 nV/√Hz, which results in

–67 dBmV when computed over 160 kSym/s. The noise power

was measured directly at the AD8324AR-EVAL’s output.