Datasheet

ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
SBAS459I –JANUARY 2010–REVISED JANUARY 2012 www.ti.com
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
0 X External clock Input: external clock
1 0 Internal clock oscillator 3-state
1 1 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 V
REF
/(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, V
IN
(AINP – AINN) IDEAL OUTPUT CODE
(3)
≥ V
REF
7FFFFFh
+V
REF
/(2
23
– 1) 000001h
0 000000h
–V
REF
/(2
23
– 1) FFFFFFh
≤ –V
REF
(2
23
/2
23
– 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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