3-11

headroom must be provided such that the input voltage

could range between 0.5V and 2.5V without a significant

change in ADC performance. The simplest way to produce

the ISL5740.

The single ended analog input can be DC coupled

(Figure 19) as long as the input is within the analog input

common mode voltage range.

The resistor, R, in Figure 19 is not absolutely necessary but

may be used as a load setting resistor. A capacitor, C,

frequency noise on the inputs, also improving performance.

Values around 20pF are sufficient and can be used on AC

coupled inputs as well. Note, however, that the value of

capacitor C chosen must take into account the highest

frequency component of the analog input signal.

A single ended source may give better overall system

performance if it is first converted to differential before

driving the ISL5740.

Operational Mode

The ISL5740 contains several operational modes including a

normal two channel operation, placing one or both channels

in standby and delaying the Q channel data 1/2 clock cycle.

The operational mode is selected via the S1 and S2 pins and

is asynchronous to either clock. When either channel is

placed in standby, the output data is stalled and not high

impedance. When recovering from standby, valid data is

available after 20 clock cycles.

The delay mode can be used to set the Q channel 180

degrees out phase of the I channel if the same clock is

driving both channels. If separate, inverted clocks are used

for the I and Q channels, this feature can be used to align the

data.

Sampling Clock Requirements

The ISL5740 sampling clock input provides a standard high-

speed interface to external TTL/CMOS logic families.

In order to ensure rated performance of the ISL5740, the

duty cycle of the clock should be held at 50%

±5%. It must

also have low jitter and operate at standard TTL/CMOS

levels.

Performance of the ISL5740 will only be guaranteed at

conversion rates above 1MSPS (Typ). This ensures proper

performance of the internal dynamic circuits. Similarly, when

power is first applied to the converter, a maximum of 20

cycles at a sample rate above 1MSPS must be performed

before valid data is available.

Supply and Ground Considerations

The ISL5740 has separate analog and digital supply and

ground pins to keep digital noise out of the analog signal

path. The part should be mounted on a board that provides

separate low impedance connections for the analog and

digital supplies and grounds. For best performance, the

supplies to the ISL5740 should be driven by clean, linear

regulated supplies. The board should also have good high

frequency decoupling capacitors mounted as close as

possible to the converter. If the part is powered off a single

supply then the analog supply can be isolated by a ferrite

bead from the digital supply.

Refer to the application note “Using Intersil High Speed A/D

Converters” (AN9214) for additional considerations when

using high speed converters.

ISL5740

R

C

FIGURE 7. DC COUPLED SINGLE ENDED INPUT

OPERATIONAL MODES

S1

S2

MODE

0

0

Standby I and Q Channels.

0

1

I channel operates normally with Q Channel in

standby mode.

1

0

I and Q Channels operating with I/Q output data in

phase.

1

1

I and Q Channels operating with Q data 180 degrees

out of phase.

ISL5740