THS1040_15 Datasheet, PDF (25/33 Pages) Texas Instruments – CMOS ANALOG-TO-DIGITAL CONVERTER

Not Recommended For New Designs
THS1040
AVDD
SLAS290C − OCTOBER 2001 − REVISED OCTOBER 2004
APPLICATION INFORMATION
AIN
AGND
R2 = 1/CS fCLK
C1 ^IIN
8 pF
+
_
VCM = AIN+/AIN− Common Mode Voltage
Figure 33. Equivalent Circuit for the AIN Switched Capacitor Input
AIN input damping
The charging current pulses into AIN+ and AIN− can make the signal sources jump or ring, especially if the
sources are slightly inductive at high frequencies. Inserting a small series resistor of 20 Ω or less and a small
capacitor to ground of 20 pF or less in the input path can damp source ringing (see Figure 34). The resistor and
capacitor values can be made larger than 20 Ω and 20 pF if reduced input bandwidth and a slight gain error (due
to potential division between the external resistors and the AIN equivalent resistors) are acceptable.
Note that the capacitors should be soldered to a clean analog ground with a common ground point to prevent
any voltage drops in the ground plane appearing as a differential voltage at the ADC inputs.
R < 20 Ω _AIN
VS
C < 20 pF
Figure 34. Damping Source Ringing Using a Small Resistor and Capacitor
driving the VREF pin
Figure 35 shows the equivalent load on the VREF pin when driving the ADC internal references buffer via this
pin (MODE = AV_DD/2 or AV_DD and REFSENSE = AV_DD).
AVDD
VREF
RIN
10 kΩ
MODE = AVDD
AGND
REFSENSE = AVDD,
MODE = AVDD/2 or AVDD
+
_
(AVDD + VREF) /4
Figure 35. Equivalent Circuit of VREF
The nominal input current I_REF is given by:
IREF
+
3
VREF
4
* AVDD
RIN
(13)
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