ISL5740 Datasheet, PDF (10/12 Pages) Intersil Corporation – 3V Dual 10-Bit, 20/40/60MSPS A/D Converter with Internal Voltage Reference

ISL5740
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 ISL5740 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, 0.1µF capacitor may be connected from the V_ROUT
output pin to filter any stray board noise.
Reference Voltage Inputs, I/Q V_REFIN
The ISL5740 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 ISL5740 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, MΩ 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 ISL5740 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.
V_IN
I/Q_IN+
R
ISL5740
I/QV_RIN
-V_IN
R
I/Q_IN-
FIGURE 4. AC COUPLED DIFFERENTIAL INPUT
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.
V_IN
V_DC
V_DC
-V_IN
I/Q_IN+
R
C
ISL5740
I/QV_RIN
R
I/Q_IN-
FIGURE 5. DC COUPLED DIFFERENTIAL INPUT
Analog Input, Single-Ended Connection
The configuration shown in Figure 18 may be used with a
single ended AC coupled input.
V_IN
V_DC
I/Q_IN+
R
ISL5740
I/Q_IN-
Since the ISL5740 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
FIGURE 6. AC COUPLED SINGLE ENDED 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+ -
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