adc1204802 National Semiconductor Corporation, adc1204802 Datasheet - Page 27

• Manufacturer Part Number: adc1204802
• Description: 12-bit Plus Sign 216khz 8-channel Sampling Analog-to-digital Converter
• Manufacturer: National Semiconductor Corporation

Features and Operating Modes
WR signal until the rising edge of the SYNC pulse, at which
time the signal will be held and conversion begins. The RDY
signal will go low when the conversion is done. A new MUX
channel and/or operational command may be written into the
Configuration register at this time, if needed. Two consecu-
tive read cycles are required to retrieve the entire 13-bit
conversion result from the ADC12048’s data register. The
first read will place the lower byte of the conversion result
contained in the Data register on the data bus. The second
read will place the upper byte of the conversion result stored
in the Data register on the data bus. With the START com-
mand in the configuration register, the rising edge of the
second read pulse will raise the RDY signal high and begin
a conversion cycle following a rising edge on the SYNC pin.
13-bit mode: The MUX channel should be selected, the
SYNC bit should be set and the START command issued
with a single write to the ADC12048. A rising edge on the
SYNC pin or on the RD pin will force the RDY signal high. It
is recommended that the action of reading from the
ADC12048 (not the rising edge of the SYNC signal) be used
to raise the RDY signal. This will ensure that the conversion
result is read during the acquisition period of the next con-
version cycle, eliminating a read from the ADC12048 while it
is performing a conversion. Noise generated by accessing
the ADC12048 while it is converting may degrade the con-
version result. In the SYNC-IN mode, only the rising edge of
the SYNC signal will begin a conversion cycle. The RDY
signal will go low when the conversion cycle is done. The
acquisition time is controlled by the SYNC signal. The acqui-
sition period begins following a write cycle containing MUX
channel information. The selected MUX channel is sampled
after the rising edge of the WR signal until the rising edge of
the SYNC pulse, at which time the signal will be held and
conversion begins. A new MUX channel and/or operational
command may be written into the Configuration register at
this time, if needed. With the START command in the Con-
figuration register, a read from the ADC12048 will place the
entire conversion result stored in the Data register on the
data bus and the rising edge of the read pulse will force the
RDY signal high. The selected MUX channel will be sampled
until a rising edge appears on the SYNC pin, at which the
time sampled signal will be held and a conversion cycle
started.
STANDBY COMMAND
8-bit mode: The first byte written to the ADC12048 should set
the HB bit in the Configuration register (bit b
byte must issue the Standby command (bits b
0, 0).
13-bit mode: The Standby command must be issued to the
ADC12048 in single write (bits b
RESET
The RESET command places the ADC12048 into a ready
state and forces the RDY signal low. The RESET command
can be used to interrupt the ADC12048 while it is performing
(Continued)
11
, b
10
, b
9
= 0, 0, 0).
7
11
). The second
, b
10
, b
9
= 0,
27
a conversion, full-calibration or auto-zero cycle. It can also
be used to get the ADC12048 out of the standby mode.
Analog Application Information
REFERENCE VOLTAGE
The ADC12048 has two reference inputs, V
They define the zero to full-scale range of the analog input
signals over which 4095 positive and 4096 negative codes
exist. The reference inputs can be connected to span the
entire supply voltage range (V
or they can be connected to different voltages when other
input spans are required. The reference inputs of the
ADC12048 have transient capacitive switching currents. The
voltage sources driving V
low output impedence and noise and must be adequately
bypassed. The circuit in Figure 20 is an example of a very
stable reference source.
The ADC12048 can be used in either ratiometric or absolute
reference appplications. In ratiometric systems, the analog
input voltage is proportional to the voltage used for the
ADC’s reference voltage. This technique relaxes the system
reference requirements because the analog input voltage
moves with the ADC’s reference. The system power supply
can be used as the reference voltage by connecting the
V
accuracy, where the analog input voltage varies between
very specific voltage limits, a time and temperature stable
voltage source can be connected to the reference inputs.
Typically, the reference voltage’s magnitude will require an
initial adjustment to null reference voltage induced full-scale
errors.
The reference voltage inputs are not fully differential. The
ADC12048 will not generate correct conversions if
(V
able relationship between V
REF
REF
+ pin to V
+) – (V
FIGURE 19. V
REF
A
+ and the V
−) is below 1V. Figure 19 shows the allow-
REF
REF
REF
REF
+ and V
REF
Operating Range
− pin to AGND. For absolute
+ and V
− = AGND, V
REF
REF
− must have very
REF
−.
+ and V
REF
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01238737
+ = V
REF
A
+)
−.