6

If M-ary FSK is required (more than two frequencies), the user

will have to continually reprogram the center frequency register.

The maximum write rate to the same parallel register is the

every register updated. The maximum possible rate occurs if

the user only needs to change eight bits (one register). For M-

ary FSK, the output frequency rate of change is

M-ary FSK Rate = WR / REG,

where REG = quantity of registers being written and WR =

write rate.

PSK Modulation

Binary or quadrature phase shift keying (PSK) can be done

by using the phase pins, PH0 and PH1. The change in

phase can be pipelined such that the PH pins can be toggled

at a rate up to

Quadrature Local Oscillators

Two ISL5314s can be used as sine/cosine generators for

quadrature local oscillator applications. It is important to note

that the phase accumulator feedback needs to be zeroed in

both devices if it is desired that both DDSs restart with a

known phase, which is determined by the use of the phase

control pins, PH1 and PH0. To zero the phase accumulator,

pull bit 5 of address 13 low and then high again at the same

time in both devices.

Squarewave Clock Source

The on-chip comparator can be used to generate a square

wave. The analog output is filtered and then fed into the

comparator input. Because the analog output is a sampled-

waveform, a high DAC output frequency (relative to the clock

rate) creates large amplitude steps in the sampled

waveform. These steps have to be smoothed with a lowpass

filter in order for the comparator to operate properly,

otherwise the zero-order hold nature of the sampled analog

output could possibly hold at the comparator’s trigger point

temporarily causing the comparator to toggle unexpectedly.

For this reason, it is very important that a lowpass filter be

used on the analog output prior to the input of the

comparator. The user can set one input to the comparator at

a DC reference point (typically the mid-point of the filtered

signal) and feed the filtered analog output into the other

input. See Figure 2 for an example of a square wave circuit

using this method. Since IOUTA and IOUTB are differential,

the mid-point between the 10k resistors will always be the

average value of each signal. The large resistors have to be

used so that the parallel resistance of the intended load and

the extra load of the averaging circuit yields a negligible

effect on the intended load. The average value is used as the

reference voltage for one input to the comparator, with a

capacitor to filter off any high frequency noise. The other

comparator input is connected to the lowpass filter output. It

is important that both IOUTA and IOUTB are equally loaded

so that each generates the same amplitude and therefore

has the same average value.

The user can filter both IOUTA and IOUTB and feed them

differentially into the comparator. It is difficult to perfectly

match the differential option, so the single-ended option is

recommended. The jitter of the comparator is typically 500ps

peak to peak. The actual jitter achieved is partially

dependent on the quality of the signal at the comparator

input, which is dictated by the amount of oversampling of the

analog output and the quality of the lowpass filter.

The user also has the option to evaluate the comparator

circuit in Figure 2 with lower output current in order to save

power consumption in the ISL5314. The DAC output current

can be set to 5 or 10mA instead of 20mA and evaluated to

determine if the comparator performance is still suitable for

the application. Since the output current is derived from the

+5V analog supply, reducing the output from 20mA to 10mA

saves approximately 50mW of power. The recommended

minimum amplitude of the comparator input is 100mV, so

operation of the analog outputs with less than 20mA of

output current should be possible with appropriate resistive

loading (for example, 5mA into a 50

Ω load provides 250mV

of amplitude).

If needed, series resistance on the comparator output can

be used to reduce overshoot and/or ringing. The comparator

can be used to drive a 50

Ω load.

PIN 18

PIN 17

>10k

Ω

ISL5314

100

Ω

50

Ω

FIGURE 2. SQUAREWAVE GENERATION USING THE

ON-CHIP COMPARATOR

IOUTA

IOUTB

>10k

Ω

100

Ω

LPF (100

Ω)

PIN 23

PIN 22

IN+

IN-

PIN 10

COMPOUT

>1nF

COMPARATOR INPUTS

(TYP 20-40MHz)

ISL5314