Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsData collecting ICsData acquisition IC - AD converter (ADC) External TSSOP 20

Clock

Detect

Internal

Oscillator

MUX

SystemClock

SEL

S0 S1

ENB

XTAL1/CLKIN

XTAL2

100

120

140

Magnitude(dB)

0.9

1.8 2.8 3.7 4.6 7.4

Frequency(MHz)

5.5 6.5

XTAL1/CLKIN

XTAL2

ADS1259

SBAS424D –JUNE 2009– REVISED AUGUST 2011

www.ti.com

ALIASING CLOCK SOURCE

The low-pass characteristic of the digital filter repeats There are three ways to provide the ADS1259 clock:

at multiples of the modulator rate (f

MOD

= f

CLK

/8). the internal oscillator, an external clock, or an

Figure 43 shows the responses plotted out to external crystal/ceramic resonator. The ADS1259

7.3728MHz at the data rate of 14.4kSPS. Notice how selects the clock source automatically. Figure 44

the responses near dc, 0.9216MHz, 1.8432MHz, shows the clock select block. If either external clock

2.7698MHz, etc, are the same as given by f = sources are present, the internal oscillator is disabled

MOD

± f

DATA

where N = 0, 1, 2, etc. The digital filter and the external clock source is selected. If no

attenuates high-frequency noise on the ADS1259 external clock is present, the internal oscillator is

inputs up to the frequency where the response selected. The ADS1259 continuously monitors the

repeats. However, noise or frequency components clock source. The clock source can be polled by the

existing in the signal where the response repeats EXTCLK bit (bit 6 of register CONFIG2), 0 = internal

alias into the passband. Often, a simple RC antialias oscillator, 1 = external clock.

filter is sufficient to reject these input frequencies.

The data rate and corresponding filter notches scale

by the accuracy of clock frequency. Consideration

should be given to the clock accuracy and the

corresponding effect to the notch frequency locations.

Figure 44. Equivalent Circuitry of the Clock

Figure 43. Frequency Response to 7.3728MHz

Source

(Data Rate = 14400SPS)

Internal Oscillator

Figure 45 shows the internal oscillator connection.

XTAL1/CLKIN is grounded and XTAL2 is not

connected (floating). The internal oscillator draws

approximately 40μA from the DVDD supply. Note that

the internal oscillator has ±2% accuracy over

temperature. The oscillator accuracy has a

corresponding effect on line-cycle notch frequency

locations.

Figure 45. Internal Oscillator Connection