3-9
analog sample is taken (see the timing diagram in Figure 1).
This time delay is specified as the data latency. After the
data latency time, the digital data representing each
succeeding analog sample is output during the following
clock cycle. The digital output data is provided in offset
binary format (see Table 1, A/D Code Table).
Internal Reference Voltage Output, V
ROUT
The ISL5640 is equipped with an internal 1.25V bandgap
reference voltage generator, therefore, no external reference
voltage is required. V
ROUT
should be connected to V
RIN
when using the internal reference voltage. An external, user-
supplied, 0.1
μ
F capacitor may be connected from the V
ROUT
output pin to filter any stray board noise.
Reference Voltage Inputs, I/Q V
REFIN
The ISL5640 is designed to accept a 1.25V reference
voltage source at the V
RIN
input pins for the I and Q
channels. Typical operation of the converter requires V
RIN
to
be set at 1.25V. The ISL5640 is tested with V
RIN
connected
to V
ROUT
yielding a fully differential analog input voltage
range of
±
0.5V.
The user does have the option of supplying an external 1.25V
reference voltage. As a result of the high input impedance
presented at the V
RIN
input pin, M
typically, the external
reference voltage being used is only required to source small
amount of reference input current.
In order to minimize overall converter noise it is
recommended that adequate high frequency decoupling be
provided at the reference voltage input pin, V
RIN
.
Analog Input, Differential Connection
The analog input of the ISL5640 is a differential input that
can be configured in various ways depending on the signal
source and the required level of performance. A fully
differential connection (Figure 16 and Figure 17) will deliver
the best performance from the converter.
Since the ISL5640 is powered by a single +3V analog
supply, the analog input is limited to be between ground and
+3V. For the differential input connection this implies the
analog input common mode voltage can range from 0.25V to
2.75V. The performance of the ADC does not change
significantly with the value of the analog input common
mode voltage.
For the AC coupled differential input (Figure 16) and with V
RIN
connected to V
ROUT
, full scale is achieved when the V
IN
and
-V
IN
input signals are 0.5V
P-P
, with -V
IN
being 180 degrees
out of phase with V
IN
. The converter will be at positive full
scale when the I/Q
IN
+ input is at I/Q
VRIN
+ 0.25V and the
I/Q
IN
- input is at I/Q
VRIN
- 0.25V (I/Q
IN
+ - I/Q
IN
- = +0.5V).
Conversely, the converter will be at negative full scale when
the I/Q
IN
+ input is equal to I/Q
VRIN
- 0.25V and I/Q
IN
- is at
I/Q
VRIN
+ 0.25V (I/Q
IN
+ - I/Q
IN
- = -0.5V).
The analog input can be DC coupled (Figure 17) as long as
the inputs are within the analog input common mode voltage
range (0.25V
≤
VDC
≤
2.75V).
The resistors, R, in Figure 17 are not absolutely necessary
but may be used as load setting resistors. A capacitor, C,
connected from I/Q
IN
+ to I/Q
IN
- will help filter any high
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.
Analog Input, Single-Ended Connection
The configuration shown in Figure 18 may be used with a
single ended AC coupled input.
Again, with V
RIN
connected to V
ROUT
, if V
IN
is a 1V
P-P
sinewave, then I/Q
IN
+ is a 1.0V
P-P
sinewave riding on a
positive voltage equal to V
DC
. The converter will be at
positive full scale when I/Q
IN
+ is at V
DC
+ 0.5V (I/Q
IN
+ -
I/Q
IN
+
I/QV
RIN
I/Q
IN
-
ISL5640
V
IN
-V
IN
R
R
FIGURE 4. AC COUPLED DIFFERENTIAL INPUT
I/Q
IN
+
I/QV
RIN
I/Q
IN
-
ISL5640
V
IN
-V
IN
R
R
C
V
DC
V
DC
FIGURE 5. DC COUPLED DIFFERENTIAL INPUT
I/Q
IN
+
I/Q
IN
-
ISL5640
V
IN
V
DC
R
FIGURE 6. AC COUPLED SINGLE ENDED INPUT
ISL5640