THS1041 Datasheet, PDF (29/37 Pages) Texas Instruments – 10-Bit, 40-MSPS ANALOG-TO-DIGITAL CONVERTER WITH PGA AND CLAMP

THS1041
AVDD
SLAS289B – OCTOBER 2001 – REVISED FEBRUARY 2002
APPLICATION INFORMATION
AIN
AGND
R2 = 1/CS fCLK
C1 ^IIN
8 pF
+
_
VCM = AIN+/AIN– Common Mode Voltage
Figure 43. 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 44). 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 44. Damping Source Ringing Using a Small Resistor and Capacitor
driving the VREF pin
Figure 45 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 45. Equivalent Circuit of VREF
The nominal input current I_REF is given by:
IREF
+
3
VREF
4
* AVDD
RIN
(15)
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