Source Characteristics
•
The source impedance unbalance
will produce offset,
common-mode and channel-to-channel gain-scatter er-
rors. Use sources which do not have large impedance
unbalances if at all possible.
Keep source impedances as low as possible
to minimize
resistive loading errors.
Minimize ground loops.
If signal lines are shielded,
ground all shields to a common point at the system
analog common.
R
SA
Node A
C
SA
Source
C
SB
C
CMS
R
SB
Rd
A
Cd
A
Z
CM
R
CMS
•
•
MPC509A Load
Channel
Rd
B
Node B
Cd
B
If the MPC509A is used for multiplexing high-level signals
of
±1V
to
±10V
full-scale ranges, the foregoing precautions
should still be taken, but the parameters are not as critical as
for low-level signal applications.
DYNAMIC CHARACTERISTICS
Settling Time
The gate-to-source and gate-to-drain capacitance of the CMOS
FET switches, the RC time constants of the source and the
load determine the settling time of the multiplexer.
Governed by the charge transfer relation i = C (dV/dt), the
charge currents transferred to both load and source by the
analog switches are determined by the amplitude and rise
time of the signal driving the CMOS FET switches and the
gate-to-drain and gate-to-source junction capacitances as
shown in Figures 3 and 4. Using this relationship, one can see
that the amplitude of the switching transients, seen at the
source and load, decrease proportionally as the capacitance
of the load and source increase. The trade-off for reduced
switching transient amplitude is increased settling time. In
effect, the amplitude of the transients seen at the source and
load are:
dV
L
= (i/C) dt
where i = C (dV/dt) of the CMOS FET switches
C = load or source capacitance
The source must then redistribute this charge, and the effect
of source resistance on settling time is shown in the Typical
Performance Curves. This graph shows the settling time for
a 20V step change on the input. The settling time for smaller
step changes on the input will be less than that shown in the
curve.
MPC508A Channel
Source
Load
FIGURE 4. Settling and Common-Mode-Effects—
MPC509A
Switching Time
This is the time required for the CMOS FET to turn ON after
a new digital code has been applied to the Channel Address
inputs. It is measured from the 50 percent point of the address
input signal to the 90 percent point of the analog signal seen
at the output for a 10V signal change between channels.
Crosstalk
Crosstalk is the amount of signal feedthrough from the three
(MPC509A) or seven (MPC508A) OFF channels appearing
at the multiplexer output. Crosstalk is caused by the voltage
divider effect of the OFF channel, OFF resistance and junc-
tion capacitances in series with the R
ON
and R
S
impedances
of the ON channel. Crosstalk is measured with a 20Vp-p
1kHz sine wave applied to all OFF channels. The crosstalk
for these multiplexers is shown in the Typical Performance
Curves.
Common-Mode Rejection (MPC509A Only)
The matching properties of the load, multiplexer and source
affect the common-mode rejection (CMR) capability of a
differentially multiplexed system. CMR is the ability of the
multiplexer and input amplifier to reject signals that are
common to both inputs, and to pass on only the signal
difference to the output. For the MPC509A, protection is
provided for common-mode signals of
±20V
above the
power supply voltages with no damage to the analog switches.
The CMR of the MPC509A and Burr-Brown’s INA110
instrumentation amplifier is 110dB at DC to 10Hz (G = 100)
with a 6dB/octave roll off to 70dB at 1000Hz. This measure-
ment of CMR is shown in the Typical Performance Curves
and is made with a Burr-Brown model INA110 instrumenta-
tion amplifier connected for gains of 10, 100, and 500.
Node A
R
S
C
S
C
L
R
L
FIGURE 3. Settling Time Effects—MPC508A
6
MPC508A, MPC509A
www.ti.com
SBFS019A
BURR-BROWN [ BURR-BROWN CORPORATION ]
