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
1MΩ, 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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