recommended for the output. Since C
GD
< C
GS
this causes less charge injection noise on the
load.
As can be seen from Figures 3a and 3b, the
body-source and body-drain pn junctions should
be kept reverse biased at all times-otherwise,
signal clipping and even device damage may
occur if unlimited currents are allowed to flow.
Body biasing is conveniently set, in most cases,
by connecting the substrate to V-.
The circuit shown in Figure 4 exhibits the r
DS(on)
vs. analog signal voltage relationship shown in
Figure 5.
When the analog signal excursion is large (for
example ±10 V) the channel on-resistance
changes as a function of signal level. To
achieve minimum distortion, this channel on-
resistance modulation should be kept in mind,
and the amount of resistance in series with the
switch should be properly sized. For instance, if
the switch resistance varies between 20
Ω
and
30
Ω
over the signal range and the switch is in
series with a 200
Ω
load, the result will be a
total
∆R
= 4.5 %. Whereas, if the load is 100
kΩ,
∆R
will only be 0.01 %.
200
r
DS(on)
(Ω)
160
120
80
40
0
-10
-5
0
(b)
(a)
(c)
20 V = On
-10 V = Off
S
R
GEN
Switch Input
V
S
= ±10 V
Control
Input
G
D
R
L
B
-10 V
Switch
Output
V
O
C
L
Figure 4. Normal Switch Configuration for a ±10
V Analog Switch
5
V
S
(V)
10
15
Main Switch Characteristics
r
DS(on)
Channel on-resistance is controlled by the
electric field present across and along the
channel. Channel resistance is mainly
determined by the gate-to-source voltage
difference. When V
GS
exceeds the threshold
voltage (V
GS(th)
), the FET starts to turn on.
Numerous applications call for switching a point
to ground. In these cases the source and
substrate are connected to ground and a gate
voltage of 3 to 4 V is sufficient to ensure
switching action.
With a V
GS
in excess of +5 V, a low resistance
path exists between the source and the drain.
(a)
(b)
(c)
V
BODY
= -10 V, V
GATE
= 20 V
V
BODY
= -10 V, V
GATE
= 15 V
V
BODY
= 0 V, V
GATE
= 20 V
Figure 5. On Resistance Characteristics
Threshold Voltage
The threshold voltage (V
GS(th)
) is a parameter
used to describe how much voltage is needed to
initiate channel conduction. Figure 6 shows the
applicable test configuration. In this circuit, it is
worth noting, for instance, that if the device has
a V
GS(th)
= 0.5 V, when V+ = 0.5 V, the channel
resistance will be:
R
CHANNEL
=
0.5V
=
500kΩ
1
µ
A
3
Linear Integrated Systems, Inc.
●
4042 Clipper Ct.
●
Fremont, CA 94538
●
Tel: 510 490-9160
●
Fax: 510 353-0261
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
