HI5703 Datasheet, PDF (13/18 Pages) Intersil Corporation – 10-Bit, 40 MSPS A/D Converter

Reference Voltage Inputs, V_REF- and V_REF+
The HI5703 requires two reference voltages connected to the
V_REF pins. The HI5703 is tested with V_REF- equal to 2V and
V_REF+ equal to 3.25V for a fully differential input voltage
range of ±1.25V. V_REF+ and V_REF- can differ from the above
voltages as long as the reference common mode voltage,
((V_REF+ + V_REF-)/2), does not exceed 2.625V ±50mV and
the limits on V_REF+ and V_REF- are not exceeded.
In order to minimize overall converter noise it is recommended
that adequate high frequency decoupling be provided at the
reference voltage input pins, V_REF+ and V_REF-.
Analog Input, Differential Connection
The analog input to the HI5703 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 give the
best performance for the converter.
V_IN
V_IN+
R
HI5703
V_DC
R
-V_IN
V_IN-
FIGURE 16. AC COUPLED DIFFERENTIAL INPUT
Since the HI5703 is powered by a single +5V analog supply,
the analog input is limited to be between ground and +5V.
For the differential input connection this implies the analog
input common mode voltage can range from 0.625V to
4.375V. The performance of the ADC does not change
significantly with the value of the analog input common
mode voltage.
A DC voltage source, V_DC, equal to 2.8V (typical), is made
available to the user to help simplify circuit design when using
an AC coupled differential input. This low output impedance
voltage source is not designed to be a reference but makes an
excellent bias source and stays within the analog input
common mode voltage range over temperature. It has a
temperature coefficient of approximately +200ppm/^oC.
For the AC coupled differential input (Figure 16) assume the
difference between V_REF+, typically 3.25V, and V_REF-,
typically 2V, is 1.25V. Fullscale is achieved when the V_IN+
and V_IN- inputs are 1.25V_P-P, with V_IN- being 180 degrees
out of phase with V_IN+. The converter will be at positive
fullscale when the V_IN+ input is at V_DC + 0.625V and V_IN- is
at V_DC - 0.625V (V_IN+ - V_IN- = 1.25V). Conversely, the
converter will be at negative full scale when the V_IN+ input is
equal to V_DC - 0.625V and V_IN- is at V_DC + 0.625V (V_IN+ -
V_IN- = -1.25V).
The analog input can be DC coupled (Figure 17) as long as
the inputs are within the analog input common mode voltage
range (0.625V ≤ VDC ≤ 4.375V).
V_IN
VDC
-V_IN
VDC
V_IN+
R
C
HI5703
V_DC
R
V_IN-
FIGURE 17. DC COUPLED DIFFERENTIAL INPUT
The resistors, R, in Figure 17 are not absolutely necessary
but may be used as load setting resistors. A capacitor, C,
connected from V_IN+ to V_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.
V_IN
VDC
V_IN+
R
HI5703
V_IN-
FIGURE 18. AC COUPLED SINGLE ENDED INPUT
Again, assume the difference between V_REF+, typically
3.25V, and V_REF-, typically 2V, is 1.25V. If V_IN is a 2.5V_P-P
sinewave, then V_IN+ is a 2.5V_P-P sinewave riding on a
positive voltage equal to VDC. The converter will be at
positive fullscale when V_IN+ is at VDC + 1.25V and will be at
negative fullscale when V_IN+ is equal to VDC - 1.25V.
Sufficient headroom must be provided such that the input
voltage never goes above +5V or below AGND. In this case,
VDC could range between 1.25V and 3.75V without a
significant change in ADC performance. The simplest way to
produce VDC is to use the V_DC output of the HI5703.
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