ADS1294 datasheet(31/88 Pages) TI

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ADS1294, ADS1294R

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SBAS459H

– JANUARY 2010 – REVISED OCTOBER 2011

CLOCK

The ADS129x provide two different methods for device clocking: internal and external. Internal clocking is ideally

suited for low-power, battery-powered systems. The internal oscillator is trimmed for accuracy at room

temperature. Over the specified temperature range the accuracy varies; see the Electrical Characteristics. Clock

selection is controlled by the CLKSEL pin and the CLK_EN register bit.

The CLKSEL pin selects either the internal or external clock. The CLK_EN bit in the CONFIG1 register enables

and disables the oscillator clock to be output in the CLK pin. A truth table for these two pins is shown in Table 7.

The CLK_EN bit is useful when multiple devices are used in a daisy-chain configuration. It is recommended that

during power-down the external clock be shut down to save power.

Table 7. CLKSEL Pin and CLK_EN Bit

CONFIG1.CLK_EN

CLKSEL PIN

BIT

CLOCK SOURCE

CLK PIN STATUS

External clock

Input: external clock

Internal clock oscillator

3-state

Internal clock oscillator

Output: internal clock oscillator

DATA FORMAT

The ADS129x output 24 bits of data per channel in binary twos complement format, MSB first. The LSB has a

weight of VREF/(2

23 – 1). A positive full-scale input produces an output code of 7FFFFFh and the negative

full-scale input produces an output code of 800000h. The output clips at these codes for signals exceeding

full-scale. Table 8 summarizes the ideal output codes for different input signals. Note that for DR[2:0] = 000 and

001, the device has only 17 and 19 bits of resolution, respectively. The last seven (in 17-bit mode) or five (in

19-bit mode) bits can be ignored.

Table 8. Ideal Output Code versus Input Signal(1)(2)

INPUT SIGNAL, VIN

(AINP

– AINN)

IDEAL OUTPUT CODE(3)

≥ VREF

7FFFFFh

+VREF/(2

23 – 1)

000001h

000000h

–VREF/(2

23 – 1)

FFFFFFh

≤ –VREF (2

23/223 – 1)

800000h

(1)

Only valid for 24-bit resolution data rates.

(2)

Assumes gain = 1.

(3)

Excludes effects of noise, linearity, offset, and gain error.

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