1
Analog Switch and Multiplexer Applications
High Current Switching
Analog switches are sometimes required to conduct
appreciable amounts of current, either continuous, or
instantaneous - such as charging or discharging a capacitor.
For best reliability, it is recommended that instantaneous
current be limited to less than 80mA peak and that average
power over any 100 millisecond period be limited to
I
2
x R
ON
(absolute maximum derated power- quiescent
power). Note that R
ON
increases at high current levels,
which is characteristic of any FET switch. Switching
elements may be connected in parallel to reduce R
ON
.
Op Amp Switching Applications
When analog switches are used either to select an op amp
input, or to change op amp gain, minor circuit
rearrangements can frequently enhance accuracy. In
Figure 1, R
ON
of the input selector switch adds to R
1
,
reducing gain and allowing gain to change with temperature.
By switching into a non inverting amplifier (b), gain change
becomes negligible. Similarly, in a gain switching circuit,
R
ON
is part of the gain determining network in (c), but has
neglible effect in (d).
Switching Spikes And Charge Injection
Transient effects when turning a switch off or on are of
concern in certain applications. Short duration spikes are
generated (Figure 2 (a)) as a result of capacitive coupling
between digital signals and the analog output. These have
the effect of creating an acquisition time interval during
which the output level is invalid even when little or no steady
state level change is involved. The total net energy (charge
injection) coupled to the analog circuit is of concern when
switching the voltage on a capacitor, since the injected
charge will change the capacitor voltage at the instant the
switch is opened (Figure 2 (b)).
Charge injection is measured in picocoulombs; the voltage
transferred to the capacitor is computed by:
V = Charge (pC)
Capacitance (pF)
Both of these effects are, in general, considerably less for
CMOS switches than for equivalent resistance JFET or
PMOS devices, since the gate drive signals for the two
switching transistors are of opposite polarity. However,
complete cancellation is not possible, since the N and P
channel switches do not receive gate signals quite
simultaneously, and their geometries are necessarily
different to achieve the desired DC resistance match.
In applications where transients create a problem, it is
frequently possible to minimize the effect by cancellation in a
differential circuit, similar to Figure 3.
Among the analog switches, the Hl-201 is the best from the
transient standpoint, having turn-on spikes of about 100mV
peak, 50ns width at the 50% point, and charge injection at
turn-off of about 20 picocoulombs. Transients of the HI-504X
series are several times higher.
FIGURE 1. OP AMP SWITCHING APPLICATIONS
+
-
(a) LOW ACCURACY
-
+
(b) HIGH ACCURACY
-
+
(c) LOW ACCURACY
-
+
(d) HIGH ACCURACY
FIGURE 2. SWITCHING SPIKES AND CHARGE INJECTION
SCOPE
1M
, 20pF
(a)
(b)
SW
CLOSED
OPEN
V =Q
C
FIGURE 3. DIFFERENTIAL CIRCUIT
R
S
SIGNAL
R =
R
S
DIFFERENTIAL
AMPLIFIER
Application Note
August 2002
AN1034
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Copyright Intersil Americas Inc. 2002. All Rights Reserved
2
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
ON
x C
D
product, and the second a low value of C
DS (OFF)
.
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.
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
load, which acounts for the 20dB
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
(a)
(b)
FIGURE 4. CIRCUIT CONFIGURATIONS
SIG. GEN.
V
1K
Z
0
+5V
0V
HI-5046A
SIG. GEN.
V
1K
Z
0
+5V
0V
HI-5046A
SIG. GEN.
V
1K
Z
0
+5V
0V
HI-5046A
HI-5046A
Z
0
SIG. GEN.
(c)
FIGURE 5. ON AND OFF FREQUENCY RESPONSES
(a)
(b)
(c)
INPUT T
O
O
U
TP
U
T
TR
ANSMIS
S
ION dB
-60
-50
-40
-30
-20
-10
0
1
10
100
FREQUENCY (MHz)
(a)
(b)
(c)
SWITCH CLOSED
SWITCH OPEN
Application Note 1034
3
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Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
current for driving coaxial cables. For general wideband
applications, the HA-2541 offers the convenience of unity
gain stability plus 90ns settling (to
0.1%) and
10V output
swing. Also, the HI-524 includes a feedback resistance for
use with the HA-2541. This resistance matches and tracks
the channel ON resistance, to minimize offset voltage due to
the buffer's bias currents.
Careful layout is, of course, important for high frequency
switching applications to avoid feedthrough paths or
excessive load capacitance.
FIGURE 6. HI-524 MONOLITHIC WIDEBAND CMOS
MULTIPLEXER
1 OF 4
DECODER
OUTPUT
17
8
1
11
10
9
SIG GND
SIG GND
SIG GND
IN 4
IN 3
SIG GND
15
7
6
5
4
3
FB(OUT)
2
16
18
FB(IN)
12
13
14
IN 2
IN 1
SIG GND
-15V GND +15V EN
A
0
A
1
Application Note 1034
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