AN207 Datasheet, PDF (7/12 Pages) Vishay Siliconix – 12-ns DG611 Switch Family Combines Benefits of CMOS and DMOS Technologies

Analog Input
Aperature Error
AN207
Vishay Siliconix
Analog Output
Slew Rate
Control
Aperature Uncertainty (Jitter)
Hold Step
Droop Rate = dV/dt
Aquisition
Time
Sample
Feedthrough
Aperature Delay
Hold
FIGURE 11. Error Sources in Sample-and-Hold Circuits
Because of non-ideal component characteristics, there are many
sources of error in a sample-and-hold circuit. Figure 11 illustrates
the most important ones. Table 1 lists these error sources and
shows how the DG611 characteristics reduce those errors and
improve overall circuit performance.
Figure 12 shows a closed loop sample-and-hold circuit. When
SW1 and SW2 are closed, the output buffer is forced to track
the analog input with a high degree of accuracy. SW3 improves
acquisition time by letting the input buffer follow the input signal
even during the hold periods. This circuit achieves high speed
and high accuracy.
Switched-Capacitor Filters
Switched-capacitor filters (SCFs) offer high accuracy and
excellent temperature stability. In addition, their cutoff
frequencies are programmable over a wide range by simply
changing the clock frequency.
Highly efficient audio frequency SCFs have been
commercially available for several years, but since their
sampling clocks need to be 25 to 100 times the signal
frequency, they have been limited to applications in the 30- to
50-kHz range.
Programmable Low-Pass Filter
Many active filter implementations can be made
programmable by using analog switches to select different
component values into the circuit. Figure 13 illustrates a
low-pass filter where a single DG613 is used to change the
corner frequencies from 300 kHz to 400 kHz by selecting one
of two resistor pairs. Keeping the input and feedback resistor
values equal maintains unity gain. The DG613s low parasitic
capacitances improve frequency response and permit
operation at higher frequencies.
Document Number: 70605
03-Aug-99
Error Source
Acquisition Time
Aperature Delay
Aperature
Uncertainty
Hold Step
Feed Through
Droop Rate
TABLE 1
DG611
Parameter
r_DS(on) = 18 W
t_(on) = 12 ns
t_(off) = 8 ns
t_(off) = 8 ns
DQ = 3 pC
Off Isolation = 74 dB @
5 MHz
I_D(off) = 0.25 nA
Benefit
Faster sampling,settling
times
Minimal turn-off delay
Consistent propagation
delay
Minimal switching glitches
Low capacitive coupling
Guaranteed low
leakages
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