Datasheet

Modulator
DIGITAL OUTPUT
V
REF
X
4
X
6
Integrator2
Comparator
f
CLK
DATA
D/AConverter
X
3
X
2
X(t)
f
S
Integrator1
ModulatorOutput
AnalogInput
+FS(AnalogInput)
-FS(AnalogInput)
ADS1204
SBAS301C – OCTOBER 2003 – REVISED FEBRUARY 2009 ........................................................................................................................................... www.ti.com
clock pulse by changing its analog output voltage at
X6, causing the integrators to progress in the
The ADS1204 can be operated in two modes. When
opposite direction. The feedback of the modulator to
CKLSEL = 1, the four modulators operate using the
the front end of the integrators forces the value of the
internal clock, which is fixed at 20MHz. When
integrator output to track the average of the input.
CKLSEL = 0, the modulators operate using an
external clock . In both modes, the clock is divided by
two internally and functions as the modulator clock.
The frequency of the external clock can vary from
A differential input signal of 0V will ideally produce a
1MHz to 32MHz to adjust for the clock requirements
stream of ones and zeros that are high 50% of the
of the application.
time and low 50% of the time. A differential input of
+2V produces a stream of ones and zeros that are
The modulator topology is fundamentally a
high 80% of the time. A differential input of – 2V
second-order, switched-capacitor, Δ Σ modulator,
produces a stream of ones and zeros that are high
such as the one conceptualized in Figure 29 . The
20% of the time. The input voltage versus the output
analog input voltage and the output of the 1-bit
modulator signal is shown in Figure 30 .
digital-to-analog converter (DAC) are differentiated,
providing analog voltages at X2 and X3. The voltages
at X2 and X3 are presented to their individual
integrators. The output of these integrators
progresses in a negative or positive direction. When
the value of the signal at X4 equals the comparator
reference voltage, the output of the comparator
switches from negative to positive, or positive to
negative, depending on its original state. When the
output value of the comparator switches from high to
low or vice versa, the 1-bit DAC responds on the next
Figure 29. Block Diagram of the Second-Order Modulator
Figure 30. Analog Input vs Modulator Output of the ADS1204
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