3-2
Reference Generator, V
ROUT
and V
RIN
The HI5905 has an internal reference voltage generator,
therefore no external reference voltage is required. The eval
board, however, offers the ability to use the internal or an
external reference. V
ROUT
must be connected to V
RIN
when
using the internal reference. Internal to the converter, two
reference voltages of 1.3V and 3.3V are generated making
for a fully differential analog input signal range of
±
2V.
The HI5905 can be used with an external reference. The
converter requires only one external reference voltage
connected to the V
RIN
pin with V
ROUT
left open. The
evaluation board is configured with V
ROUT
connected to V
RIN
through a 0
Ω
resistor, R4. If it is desired to evaluate the
performance of the converter utilizing an externally provided
reference voltage, R4 can be removed and the alternate
reference voltage can be brought in through twisted pair wire or
coaxial cable. The latter would be the recommended method
since it would provide the greatest immunity to externally
coupled noise voltages. In order to minimize overall converter
noise it is recommended that adequate high frequency
decoupling be provided at the reference input pin, V
RIN
.
Analog Input
The fully differential analog input of the HI5905 A/D can be
configured in various ways depending on the signal source
and the required level of performance.
Differential Analog Input Configuration
A fully differential connection (Figure 1) will yield the best
performance from the HI5905 A/D converter. Since the HI5905
is powered off a 5V supply, the analog input must be
biased so it lies within the analog input common mode voltage
range of 1.0V to 4.0V. Figure 2 illustrates the differential analog
input common mode voltage range that the converter will
accommodate. The performance of the ADC does not change
significantly with the value of the common mode voltage.
A 2.3V DC bias voltage source, V
DC
, half way between the top
and bottom internally generated reference voltages, is made
available to the user to help simplify circuit design when using a
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. The DC voltage source has a
temperature coefficient of about +200ppm/
o
C.
The difference between the converter's two internally
generated voltage references is 2V. For the AC coupled
differential input (Figure 1), if V
IN
is a 2V
P-P
sinewave 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
+ 1V and the
V
IN
- input is at V
DC
- 1V (V
IN
+ - V
IN
- = +2V). Conversely, the
ADC will be at negative full scale when the V
IN
+ input is equal
to V
DC
- 1V and V
IN
- is at V
DC
+ 1V (V
IN
+ - V
IN
- = -2V).
It should be noted that overdriving the analog input beyond
the
±
2.0V 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.
Evaluation Board Layout and
Power Supplies
The HI5905 evaluation board is a four layer board with a
layout optimized for the best performance of the ADC. This
application note includes an electrical schematic of the
evaluation board, a component parts list, a component
placement layout drawing and reproductions of the various
board layers used in the board stack-up. The user should
feel free to copy the layout in their application. Refer to the
component layout and the evaluation board electrical
schematic for the following discussions.
The HI5905 monolithic A/D converter has been designed
with separate analog and digital supply and ground pins to
keep digital noise out of the analog signal path. The
evaluation board provides separate low impedance analog
and digital ground planes on layer 2. Since the analog and
digital ground planes are connected together at a single
point where the power supplies enter the board, DO NOT tie
them together back at the power supplies.
V
IN
+
V
DC
V
IN
-
HI5905
V
IN
-V
IN
FIGURE 1. AC COUPLED DIFFERENTIAL INPUT
FIGURE 2A.
FIGURE 2B.
FIGURE 2C.
FIGURE 2. DIFFERENTIAL ANALOG INPUT COMMON MODE
VOLTAGE RANGE
V
IN
+
V
IN
-
2.0V
P-P
VDC = 4.0V
+5V
+5V
V
IN
+
V
IN
-
2.0V
P-P
1.0V < VDC < 4.0V
0V
V
IN
+
V
IN
-
2.0V
P-P
VDC = 1.0V
0V
Application Note 9785
