http://www.national.com
8
Figure 9: System Settings window
4.6 Automatic Conversion Mode
The Automatic Conversion Mode configures the
ADC to continually perform conversions without
receiving “read” instructions from the I
2
C
controller. It is activated by writing a non-zero
value into the Cycle Time bits “D[7:5]” of the
Configuration Register. Various automatic
conversion rates are supported.
Refer to
Section 1.9
of the Datasheet for a
detailed description of the Automatic Conversion
Mode.
4.7 Power Supply Connections
The analog supply voltage (V
A
) can range from
+2.7V to +5.5V. The Evaluation system provides a
handful of power supply options via the JP5
header (
see Table 1
). V
A
can be driven by the
LM4050-4.1 Shunt Reference, the LM1117MPX-
3.3 LDO from the WV4 board, or an external
supply. National recommends the LM4050
precision reference for the ADC121C021.
NOTE: In Computer mode, if V
A
is driven with less
than 3.3V in Computer mode, an external 3.3V
pull-up supply is needed. This will ensure the
3.3V CMOS logic thresholds of the WaveVision4
board are met for SDA and SCL.
To use an external pull-up supply, remove the
jumper at JP2 and connect the supply to pin 2 of
JP2 or VIA6. An external pull-up supply will also
improve the dynamic performance of the ADC.
Typical supply currents when applying an external
supply to TP7 (VA_EXT) and performing ADC
conversions are as follows:
•
For V
A
= +3.3V, I = 8mA
•
For V
A
= +5.0V, I = 11mA
Note: A majority of this current is for powering
devices external to the ADC121C021 such as the
pull-up resistors and the input amplifier.
When operating in the computer mode, the supply
voltage for V
A
can be applied externally or
supplied directly by the WV4 board through J1.
The external supply voltage is selected by placing
a jumper across pins 9 & 10 of JP5, applying a
power source with a 300mA current limit to TP7,
and grounding TP6. To use the 5.0V supply
directly from the WV4 board, place a shorting
jumper across pins 3 & 4 of JP5. To use the 3.3V
supply directly from the WV4 board, place a
shorting jumper across pins 1 & 2 of JP5. To use
the LM4050 4.1V Shunt Reference, place a
shorting jumper across pins 7 & 8 of JP5. For the
best performance, use a clean external supply.
When operating in Stand Alone mode, always use
an external supply. Apply a +5.0V power source
with a current limit of 100mA to TP7, ground TP6,
and place a shorting jumper across pins 9 & 10 of
JP5.
Note: The +3.3V required to power the EEPROM
(U4) is obtained through J2 from the WV4 board.
The EEPROM is used to identify the type of Eval
Board connected to the WaveVision system in
Computer Mode.
5.0 Software Operation and Settings
The WaveVision 4 software is included with the
WV4 board and the latest version can be
downloaded for free from National's web site at
http://www.national.com/adc
. WaveVision4
software version 4.4 or later is required to
evaluate this device with the WaveVision system.
5.1 Software Quick Start
To install this software, follow the procedure in the
WV4 Board User's Guide. Once the software is
installed, run and set it up as follows:
1. Connect the WV4 board to the host computer
with a USB cable.
2. From the WaveVision main menu, go to
Settings and then Capture Settings to open
the System Settings window (
Figure 9
) and
select the following:
•
WaveVision
4
(USB)
•
Number of Samples: 2k to 128k, as
desired
•
Data Format: Binary