FORMAT0
DIN
Modulator Clock Output
IOVDD
SCLK
FORMAT1
FORMAT2
Modulator Data Channel 2
DOUT2
Modulator Data Channel 1
DOUT1
Modulator Data Channel 4/8
(1)
DOUT4/8
(1)
DRDY
(SPI Protocol)
DOUT
(Frame-Sync Protocol)
Internal Reset
CLK
3V nom
(1)
AVDD
1V nom
(1)
IOVDD
1V nom
(1)
DVDD
Valid Data
2
18
f
CLK
129 (max)
t
DATA
SBAS367F
–
JUNE 2007
–
REVISED FEBRUARY 2011
POWER SUPPLIES
MODULATOR OUTPUT
The ADS1274/78 has three power supplies: AVDD,
The ADS1274/78 incorporates a 6th-order, single-bit,
DVDD, and IOVDD. AVDD is the analog supply that
chopper-stabilized
modulator
followed
by
a
powers the modulator, DVDD is the digital supply that
multi-stage digital filter that yields the conversion
powers the digital core, and IOVDD is the digital I/O
results. The data stream output of the modulator is
power supply. The IOVDD and DVDD power supplies
available directly, bypassing the internal digital filter.
can be tied together if desired (+1.8V). To achieve
The digital filter is disabled, reducing the DVDD
rated performance, it is critical that the power
current, as shown in
. In this mode, an
supplies
are
bypassed
with
0.1
μ
F
and
10
μ
F
external digital filter implemented in an ASIC, FPGA,
capacitors placed as close as possible to the supply
or similar device is required. To invoke the modulator
pins. A single 10
μ
F ceramic capacitor may be
output, tie FORMAT[2:0], as shown in
substituted in place of the two capacitors.
DOUT[4:1]/[8:1] then becomes the modulator data
stream outputs for each channel and SCLK becomes
shows the start-up sequence of the
the modulator clock output. The DRDY/FSYNC pin
ADS1274/78. At power-on, bring up the DVDD supply
becomes an unused output and can be ignored. The
first, followed by IOVDD and then AVDD. Check the
normal
operation
of
the
Frame-Sync
and
SPI
power-supply sequence for proper order, including
interfaces is disabled, and the functionality of SCLK
the ramp rate of each supply. DVDD and IOVDD may
changes from an input to an output, as shown in
be sequenced at the same time (for example, if the
supplies are tied together). Each supply has an
internal reset circuit whose outputs are summed
Table 16. Modulator Output Clock Frequencies
together to generate a global power-on reset. After
MODULATOR
the supplies have exceeded the reset thresholds, 2
18
CLOCK
ADS1274
ADS1278
f
CLK
cycles are counted before the converter initiates
MODE
OUTPUT
DVDD
DVDD
the conversion process. Following the CLK cycles,
[1:0]
CLKDIV
(SCLK)
(mA)
(mA)
the data for 129 conversions are suppressed by the
00
1
f
CLK
/4
4.5
8
ADS1274/78 to allow output of fully-settled data. In
01
1
f
CLK
/4
4.0
7
SPI protocol, DRDY is held high during this interval.
1
f
CLK
/8
2.5
4
In frame-sync protocol, DOUT is forced to zero. The
10
power supplies should be applied before any analog
0
f
CLK
/4
2.5
4
or digital pin is driven. For consistent performance,
1
f
CLK
/40
1.0
1
11
assert SYNC after device power-on when data first
0
f
CLK
/8
0.5
1
appear.
(1) The ADS1274 has four channels; the ADS1278 has eight
channels.
Figure 85. Modulator Output
(1) The power-supply reset thresholds are approximate.
Figure 84. Start-Up Sequence
©
2007
–
2011, Texas Instruments Incorporated
35
Product Folder Link(s):