THS1031 Datasheet, PDF (22/26 Pages) Texas Instruments – 2.7 V . 5.5 V, 10-BIT, 30 MSPS CMOS ANALOG-TO-DIGITAL CONVERTER

THS1031
2.7 V – 5.5 V, 10-BIT, 30 MSPS
CMOS ANALOG-TO-DIGITAL CONVERTER
SLAS242A – NOVEMBER 1999 – REVISED JANUARY 2000
PRINCIPLES OF OPERATION
clamp operation (continued)
D Clamp and Droop Analysis
CLAMPIN
CLAMP
C1
R1
AIN
+
SW1
–
SW2
16–Bit DAC
Control
Register
S/H
Figure 22. Clamp Operation
D Clamp Acquisition Time: Figure 22 shows the basic operation of the clamp circuit in which the ac input
signal is passed through an RC coupler.
The acquisition time when the switch is closed will equal a: T(acq) = C_i.R_i ln(Vc/Ve)(Eq.1)
In case of composite video, typical input Ri = 20 Ω. In a video clamping application, the droop is a critical
parameter and thus the input capacitor should be sized to allow sufficient acquisition time of clamp voltage at
AIN within the CLAMP interval, but also to minimize droop between clamping intervals. Typically, C_i = 1µF
By applying equation 1 above, the following examples apply to an NTSC composite video signal:
D The acquisition time needed to clamp 1-V input level to black level (0.340 Vdc ) is about 130 µs.
D The acquisition time needed to clamp 2-V input level to the white level (1 Vdc) is about 140 µs.
D The acquisition time needed to clamp 3-V input level to the sync level (0.288 Vdc) is about 160 µs.
droop
The voltage droop is the voltage change across the input capacitor C_i by the bias current as follows:
+ ^ǒ ń ^Ǔ dV I_bias Ci (t)
where t = elapsed time between clamping intervals
The bias current depends on the sampling rate. For a sampling rate of 30 MSPS and a typical input capacitance
of 1 pF, the input resistance is
Rs = 1/(Cs.Fs) = 1/(1 pFx30 MHz) = 33 kΩ
For 1-V input range and clamping period = 64 µs, the max bias current will equal I_bias = 0.5 V/33 kΩ = 15 µA:
dV = (15 µA/1 µF)(64 µs) = 0.96 mV
For 1-V input range and clamping period = 64 µs, the max bias current will equal I_bias = 0.5 V/33 kΩ = 15 µA:
dV = (15 µA/1 µF)(64 µs) = 0.96 mV
For 2-V input range and clamping period = 64 µs, the max bias current will equal I_bias = 1.0 V/33 kΩ = 30 µA
dV = (30 µA/1 µF)(64 µs) = 1.9 mV
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