the linearity of the device is ensured only when the analog
voltage applied to either input resides within the range defined
by –320mV and +320mV.
ANALOG INPUT STAGE
Analog Input
The input design topology of the ADS1202 is based on a fully
differential switched-capacitor architecture. This input stage
provides the mechanism to achieve low system noise, high
common-mode rejection (90dB), and excellent power-supply
rejection. The input impedance of the analog input is depen-
dant on the input capacitor and modulator clock frequency
(MCLK), which is also the sampling frequency of the modu-
lator. Figure 2 shows the basic input structure of the ADS1202.
The relationship between the input impedance of the ADS1202
and the modulator clock frequency is:
Modulator
The modulator sampling frequency (CLK) can be operated
over a range of a few MHz to 12MHz in mode 3. The
frequency of MCLK can be decreased to adjust for the clock
requirements of the application. The external MCLK must
have double the modulator frequency.
The modulator topology is fundamentally a 2nd-order, charge-
balancing A/D converter, as the one conceptualized in Figure 3.
The analog input voltage and the output of the 1-bit Digital-to-
Analog Converter (DAC) are differentiated, providing an analog
voltage at X2 and X3. The voltage at X2 and X3 are presented
to their individual integrators. The output of these integrators
progress 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 clock pulse by
changing its analog output voltage at X6, causing the integrators
to progress in the opposite direction. The feedback of the
modulator to the front end of the integrators forces the value of
the integrator output to track the average of the input.
1012
AIN Ω =
(
)
(1)
7 • fMCLK(MHz)
The input impedance becomes a consideration in designs
where the source impedance of the input signal is HIGH. In
this case, it is possible for a portion of the signal to be lost
across this external source impedance. The importance of this
effect depends on the desired system performance. There are
two restrictions on the analog input signal to the ADS1202.
Under no conditions should the current into or out of the
analog inputs exceed 10mA. The absolute input voltage range
must stay in the range GND – 0.4V to VDD + 0.3V. If either of
the inputs exceed these limits, the input protection diodes on
the front end of the converter will begin to turn on. In addition,
RSW
350kΩ (typ)
High
Impedance
> 1GΩ
AIN
+
CINT
7pF (typ)
1.5pF
1.5pF
Switching Frequency
= CLK
VCM
CINT
7pF (typ)
RSW
350kΩ (typ)
High
Impedance
> 1GΩ
AIN
–
FIGURE 2. Input Impedance of the ADS1202.
fCLK
X2
X3
X4
X(t)
Integrator 1
Integrator 2
DATA
fS
VREF
Comparator
X6
D/A Converter
FIGURE 3. Block Diagram of the 2nd-Order Modulator.
ADS1202
11
SBAS275
www.ti.com
BB [ BURR-BROWN CORPORATION ]
