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Application Note 9707
Reference Generator, V ROUT and V RIN
The HI5805 has an internal reference voltage generator,
therefore no external reference voltage is required. 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 HI5805 can be used with an external reference. The
converter requires only one external reference voltage
The difference between the converter's two internally
generated voltage references is 2V. For the AC coupled
differential input, (Figure 3), 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).
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 ? resistor, R 15 . If it is desired to evaluate the
+5V
V IN +
2.0V P-P
V IN -
VDC = 4.0V
+5V
performance of the converter utilizing an externally provided
reference voltage, R 15 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 .
0V
V IN +
V IN +
2.0V P-P
2.0V P-P
V IN -
1.0V < VDC < 4.0V
V IN -
VDC = 1.0V
0V
Analog Input
The fully differential analog input of the HI5805 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 3) will yield the best
performance from the HI5805 A/D converter. Since the
HI5805 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 1.0V to 4.0V. Figure 4 illustrates the
differential analog input common mode voltage range that
the converter will accommodate. The performance of the
ADC does not change signi?cantly with the value of the
common mode voltage.
FIGURE 4. DIFFERENTIAL ANALOG INPUT COMMON MODE
VOLTAGE RANGE
Evaluation Board Layout and Power Supplies
The HI5805 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 HI5805 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
V IN
-V IN
V IN +
V DC
V IN -
HI5805
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.
The analog and digital supplies are also kept separate on
the evaluation board and should be driven by clean linear
regulated supplies. The external power supplies can be
hooked up with wires to the plated through holes marked
FIGURE 3. AC COUPLED DIFFERENTIAL INPUT
A 2.3V DC bias voltage source, VDC, 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.
3-4
+5VAIN, +5VAIN1, -5VAIN, +5VDIN, +5VD1IN, +5VD2IN,
-5VDIN, AGND and DGND near the analog prototyping area.
+5VDIN, +5VD1IN, +5VD2IN and -5VDIN are digital
supplies and should be returned to DGND. +5VAIN,
+5VAIN1 and -5VAIN are the analog supplies and should be
returned to AGND. Table 1 lists the operational supply
voltages, typical current consumption and the evaluation
board circuit function being powered. Single supply
operation of the converter is possible but the overall
performance of the converter may degrade.
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