AN1082 Intersil Corporation, AN1082 Datasheet - Page 2

• Manufacturer Part Number: AN1082
• Description: Using the ISL6401 RSLIC PWM Controller Evaluation Board
• Manufacturer: Intersil Corporation

High Frequency Switching
When considering a switching element for RF or video type
information, two factors must be watched: attenuation vs.
frequency characteristics of an ON switch, and feedthrough
(isolation) vs. frequency characteristics of the OFF switch.
Optimizing the first characteristic requires a low R
product, and the second a low value of C
One approach is to use the 30 switch types of the
Hl-5040 series.
Figure 4 illustrates three circuit configurations; (a) is a simple
series switch, (b) is a series-shunt configuration to reduce
feedthrough, and (c) is an SPDT selector configuration with
series-shunt elements. A 1k load is illustrated, which might
be the input impedance of a buffer amplifier; a lower load
resistance would improve the response characteristics, but
would create greater losses in the switch and would tend to
distort high level signals.
Figure 5 shows ON and OFF frequency response for each of
the above configurations. Arbitrarily, we will define useful
frequency response as the region where ON losses are less
than 3dB and OFF isolation is greater than -40dB.
SIG. GEN.
SIG. GEN.
(a)
(b)
(c)
SIG. GEN.
FIGURE 4. CIRCUIT CONFIGURATIONS
SIG. GEN.
V
1K
Z
0
Z
0
Z
Z
0
0
0V
HI-5046A
0V
HI-5046A
HI-5046A
2
+5V
+5V
HI-5046A
1K
DS (OFF)
1K
V
0V
V
Application Note 1034
ON
.
+5V
x C
D
The simple configuration (a) has excellent ON response, but
OFF isolation limits the useful range to about 1MHz (the data
sheet indicates -80dB isolation at 100kHz, but this is
measured with 100
difference).
The circuit in (b) shows a good improvement in isolation
produced by the low impedance of the shunt switch. The
useful range is about 10MHz, which could also be achieved
in a simple SPDT 2-switch selector if source impedances are
very low.
The selector switch in (c) has excellent characteristics, both
ON and OFF curves indicating 40MHz useful response.
Additional switches connected to the same point would
reduce the ON response because of added shunt
capacitance, but this could be eliminated by feeding
separate summing amplifier inputs.
For many applications, a better approach is to use the
Hl-524 monolithic wideband CMOS multiplexer. This device
utilizes a series-shunt multiple switching network to achieve
low crosstalk without sacrificing or compromising other
operational parameters. As shown in Figure 6, each channel
comprises three CMOS FET switch gates, with two in series
and the third shunted to ground. The two series switches
ensure both a high off isolation and low feed-through
capacitance. The shunt grounding switch, closed
automatically by the control logic when its corresponding
series pair are open, shunts nonselected channels to
ground, thus minimizing crosstalk. With this circuit topology,
crosstalk is typically -60dB at 10MHz.
A buffer amplifier is used with the HI-524 for high frequency
applications, due to its higher ON resistance, and should
offer sufficient bandwidth and slew rate to avoid degradation
of the anticipated signals. For video switching, the HA-5033
and HA-2542 offer good performance plus 100mA output
FIGURE 5. ON AND OFF FREQUENCY RESPONSES
-10
-20
-30
-40
-50
-60
0
1
SWITCH OPEN
SWITCH CLOSED
load, which acounts for the 20dB
FREQUENCY (MHz)
10
(b)
(c)
(a)
100
(a)
(b)
(c)