Datasheet

Z =
IN
100kW
f /10MHz
MOD
650W
650W
SwitchingFrequency=CLK
High
Impedance
>1GW
1.2pF
V
CM
A
IN
+
1.2pF
0.4pF
0.4pF
A
IN
-
High
Impedance
>1GW
ADS1209
www.ti.com SBAS491 –FEBRUARY 2010
THEORY OF OPERATION The input impedance of the analog input depends on
the modulator clock frequency (f
MOD
). Figure 22
The differential analog input of the ADS1209 is
shows the basic input structure of one channel of the
implemented with a switched-capacitor circuit. This
ADS1209. The relationship between the input
circuit implements a second-order modulator stage,
impedance of the ADS1209 and the modulator clock
which digitizes the analog input signal into a 1-bit
frequency is:
output stream. The clock source can be internal as
well as external. Every analog input signal is
continuously sampled by the modulator and
(1)
compared to a reference voltage that is applied to the
The input impedance becomes a consideration in
REFINx pin. A digital stream that represents the
designs where the source impedance of the input
analog input voltage over time appears at the output
signal is high. This high impedance may cause
of the corresponding converter.
degradation in gain, linearity, and THD. The
importance of this effect depends on the desired
ANALOG INPUT STAGE
system performance. There are two restrictions on
the analog input signals, CHx+ and CHx–. If the input
Analog Input
voltage exceeds the range (AGND – 0.3V) to (AVDD
The topology of the analog inputs of ADS1209 is + 0.3V), the input current must be limited to 10mA
based on fully differential switched-capacitor because the input protection diodes on the front end
architecture. This input stage provides the of the converter begin to turn on. In addition, the
mechanism to achieve low system noise, high linearity and noise performance of the device meet
common-mode rejection, and excellent power-supply the stored specifications only when the differential
rejection. analog voltage resides within ±2.3V (with V
REFIN
as a
midpoint); however, the FSR input voltage is ±2.5V.
BLANKSPACE
Figure 22. Input Impedance of the ADS1209
Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): ADS1209