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Application Note 9822
External Reference Voltage Generator,
V REFOUT and V REFIN
The HI5767 has an internal reference voltage generator,
therefore no external reference voltage is required. The
evaluation board, however, offers the ability to use the
converters’ internal reference voltage, V REFOUT , or the on
board external variable reference voltage generator.
V IN
-V IN
V IN +
V DC
V IN -
HI5767
The external variable reference voltage circuitry is
implemented using the Intersil ICL8069 low voltage, 1.2V,
bandgap reference (D1) sourcing a non-inverting variable
FIGURE 1. AC COUPLED DIFFERENTIAL INPUT
gain operational ampli?er circuit based on the Intersil
HA5127 ultra-low noise precision operational ampli?er (U1).
Potentiometer VR1 is used to adjust the output voltage level
+5V
V IN +
0.5V P-P
V IN -
VDC = 4.75V
+5V
of this external voltage reference. With this the user is able
to observe the effects of reference voltage variations on the
converters performance. Turning VR1 in a clockwise (CW)
FIGURE 2A.
direction will decrease the external reference voltage while
turning VR1 in a counterclockwise (CCW) direction will
decrease the external reference voltage.
V IN +
0.5V P-P
V IN -
0.25V < VDC < 4.75V
Selection of the reference voltage to be used by the
converter is accomplished by placing the P3 header jumper
across the appropriate pins. The converters’ internal
FIGURE 2B.
reference voltage generator, V REFOUT , must be connected
to V REFIN when using the converters internal reference and
is selected by placing the P3 header jumper across P3-2 and
P3-3. Alternately, if it is desired to use the on board external
0V
V IN +
0.5V P-P
V IN -
VDC = 0.25V
0V
variable reference voltage generator, selection of this option
is done by placing the P3 header jumper across P3-1 and
P3-2. See Appendix A, Board Layout for the location of the
P3 reference voltage selection header.
Analog Input
The fully differential analog input of the HI5767 A/D can be
con?gured in various ways depending on the signal source
and the required level of performance.
Differential Analog Input Con?guration
A fully differential connection (Figure 1) will yield the best
performance from the HI5767 A/D converter. Since the
HI5767 is powered off a single +5V supply, the analog input
must be biased so it lies within the analog input common
mode voltage range of 0.25V to 4.75V. Figure 2 illustrates
the differential analog input common mode voltage, VDC,
range that the converter will accommodate. The
performance of the converter does not change signi?cantly
with the value of the analog input common mode voltage.
FIGURE 2C.
FIGURE 2. DIFFERENTIAL ANALOG INPUT COMMON MODE
VOLTAGE RANGE
A DC bias voltage source, V DC , equal to 3.0V (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 DC bias source and stays well within
the analog input common mode voltage range over
temperature.
For the AC coupled differential input (Figure 1) and with
V REFIN connected to V REFOUT , 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 V IN + input is at V DC + 0.25V and
the V IN - input is at V DC - 0.25V (V IN + - V IN - = +0.5V).
Conversely, the converter will be at negative full scale when
the V IN + input is equal to V DC - 0.25V and V IN - is at
V DC + 0.25V (V IN + - V IN - = -0.5V).
It should be noted that overdriving the analog input beyond
the ± 0.5V fullscale input voltage range will not damage the
converter as long as the overdrive voltage stays within the
converters analog supply voltages. In the event of an
overdrive condition the converter will recover within one
sample clock cycle.
3-2
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