ADS1278IPAPRG4 datasheet(35/51 Pages) TI1

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FORMAT0

DIN

ModulatorClockOutput

IOVDD

SCLK

FORMAT1

FORMAT2

ModulatorDataChannel2

DOUT2

ModulatorDataChannel1

DOUT1

ModulatorDataChannel4/8

(1)

DOUT4/8

(1)

DRDY

(SPIProtocol)

DOUT

(Frame-SyncProtocol)

InternalReset

CLK

3Vnom

(1)

AVDD

1Vnom

(1)

IOVDD

1Vnom

(1)

DVDD

ValidData

CLK

129(max)

DATA

ADS1274

ADS1278

www.ti.com

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

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 Table 16. In this mode, an

supplies

are

bypassed

with

0.1

μF

and

μF

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

μF ceramic capacitor may be

output, tie FORMAT[2:0], as shown in Figure 85.

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

Figure 84 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

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

Figure 85.

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, 218

CLOCK

ADS1274

ADS1278

fCLK cycles are counted before the converter initiates

MODE

OUTPUT

DVDD

the conversion process. Following the CLK cycles,

[1:0]

CLKDIV

(SCLK)

(mA)

the data for 129 conversions are suppressed by the

fCLK/4

4.5

ADS1274/78 to allow output of fully-settled data. In

fCLK/4

4.0

SPI protocol, DRDY is held high during this interval.

fCLK/8

2.5

In frame-sync protocol, DOUT is forced to zero. The

power supplies should be applied before any analog

fCLK/4

2.5

or digital pin is driven. For consistent performance,

fCLK/40

1.0

assert SYNC after device power-on when data first

fCLK/8

0.5

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

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