ISL5740
I/Q
IN
- = +0.5V) and will be at negative full scale when I/Q
IN
+
is equal to V
DC
- 0.5V (I/Q
IN
+ - I/Q
IN
- = -0.5V). Suf鏗乧ient
headroom must be provided such that the input voltage
never goes above +3V or below AGND. In this case, V
DC
could range between 0.5V and 2.5V without a signi鏗乧ant
change in ADC performance. The simplest way to produce
VDC is to use the I/Q
VRIN
bias source, I/QV
DC
, output of
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.
V
IN
V
DC
R
C
ISL5740
I/Q
IN
+
OPERATIONAL MODES
S1
0
0
1
1
S2
0
1
0
1
MODE
Standby I and Q Channels.
I channel operates normally with Q Channel in
standby mode.
I and Q Channels operating with I/Q output data in
phase.
I and Q Channels operating with Q data 180 degrees
out of phase.
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%
鹵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 鏗乺st applied to the converter, a maximum of 20
cycles at a sample rate above 1MSPS must be performed
before valid data is available.
V
DC
I/Q
IN
-
FIGURE 7. DC COUPLED SINGLE ENDED INPUT
The resistor, R, in Figure 19 is not absolutely necessary but
may be used as a load setting resistor. A capacitor, C,
connected from I/Q
IN
+ to I/Q
IN
- will help 鏗乴ter any high
frequency noise on the inputs, also improving performance.
Values around 20pF are suf鏗乧ient 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 鏗乺st converted to differential before
driving the ISL5740.
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 鈥淯sing Intersil High Speed A/D
Converters鈥?(AN9214) for additional considerations when
using high speed converters.
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.
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