THS1030_13 Datasheet, PDF (24/39 Pages) Texas Instruments – 3-V TO 5.5-V, 10-BIT, 30MSPS CMOS ANALOG-TO-DIGITAL CONVERTER

THS1030
3ĆV TO 5.5ĆV, 10ĆBIT, 30 MSPS
CMOS ANALOGĆTOĆDIGITAL CONVERTER
SLAS243E − NOVEMBER 1999 − REVISED DECEMBER 2003
PRINCIPLES OF OPERATION
AIN input current and input load modeling
When CLK goes low, the source driving AIN must charge the total switched capacitance C_S = C_SAMPLE + C_P2.
The total charge transferred depends on the voltage at AIN and is given by:
QCHARGING + (AIN * VLAST) CS.
(7)
For a fixed voltage at AIN, so that AIN and V_LAST do not change between samples, the maximum amount of
charge transfer occurs at AIN = FS− (charging current flows out of THS1030) and AIN = FS+ (current flows into
THS1030). If AIN is held at the voltage FS+, V_LAST = [(FS+) + VM]/2, giving a maximum transferred charge:
Q(FS)
+
(FS
))
*
[(FS
2
))
)
VM]
[(FS )) * VM]
CS +
2
CS
(8)
+ (1ń4 of the input voltage span) CS
If the input voltage changes between samples, then the maximum possible charge transfer is
Q(max) + 3 Q(FS)
(9)
which occurs for a full-scale input change (FS+ to FS− or FS− to FS+) between samples.
The charging current pulses can make the AIN source jump or ring, especially if the source is slightly inductive
at high frequencies. Inserting a small series resistor of 20 Ω or less in the input path can damp source ringing
(see Figure 31). This resistor can be made larger than 20 Ω if reduced input bandwidth or distortion performance
is acceptable.
R < 20 Ω _AIN
VS
Figure 28. Damping Source Ringing Using a Small Resistor
24
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265