Datasheet

V
REF
X
4
X
6
Integrator2
Comparator
f
CLK
DATA
DAC
X
3
X
2
X(t)
f
S
Integrator1
ModulatorOutput
AnalogInput
+FS(AnalogInput)
-FS(AnalogInput)
ADS1209
SBAS491 –FEBRUARY 2010 www.ti.com
Modulator
comparator switches from low to high, or vice versa,
depending on its original state. When the output
The ADS1209 can be operated in two modes. When
value of the comparator switches direction, the 1-bit
CKLSEL = 1, the two modulators operate using the
DAC responds on the next clock pulse by changing
internal clock, which is fixed at 20MHz. When
its analog output voltage at X
6
, causing the
CKLSEL = 0, the modulators operate using an
integrators to progress in the opposite direction. The
external clock. In both modes, the clock is internally
feedback of the modulator to the front end of the
divided by two and functions as the modulator clock.
integrators forces the value of the integrator output to
The frequency of the external clock can vary from
track the average of the input.
1MHz to 24MHz to adjust for the clock requirements
of the application.
DIGITAL OUTPUT
The modulator topology is a second-order,
A differential input signal of 0V ideally produces a
switched-capacitor, ΔΣ modulator, such as the one
stream of ones and zeros that are high 50% of the
conceptualized in Figure 23. The analog input voltage
time and low 50% of the time. A differential input of
and the output of the 1-bit digital-to-analog converter
+2.3V produces a stream of ones and zeros that are
(DAC) are differentiated, providing analog voltages at
high 92% of the time. A differential input of –2.3V
X
2
and X
3
. The voltages at X
2
and X
3
are presented
produces a stream of ones and zeros that are high
to the respective individual integrators. The output of
8% of the time. The input voltage versus the output
these integrators progresses in a negative or positive
modulator signal is shown in Figure 24.
direction. When the value of the signal at X
4
equals
the comparator reference voltage, the output of the
Figure 23. Block Diagram of the Second-Order Modulator
Figure 24. Analog Input vs Modulator Output of the ADS1209
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