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500
W
500
W
R
LOAD
Input 1
Input 2
ADS1282
AVSS + 0.7V < (AINN or AINP) < AVDD
1.25V
-
T
otalHarmonicDistortion(dB)
R
( )
W
LOAD
0
-
-
-
-
-
-
-
20
40
60
80
100
120
140
0.1k
1k
10k
10M
100k
1M
PGA = 1
PGA = 2
PGA = 64
PGA = 32
PGA = 16
PGA = 8
PGA = 4
S
1
S
2
S
3
S
4
S
5
S
6
AINP1
AINP2
AINN2
AINN1
S
7
(+)
(
-
)
To PGA
AVDD + AVSS
2
400
W
400
W
AVSS
AVDD
AVDD
AVSS
ESD Diodes
ESD Diodes
SBAS418I – SEPTEMBER 2007 – REVISED MARCH 2015
9.5 Analog Inputs and Multiplexer
Analog Inputs and Multiplexer (continued)
A diagram of the input multiplexer is shown in
Table 2. Multiplexer Modes
MUX[2:0]
SWITCHES
DESCRIPTION
ESD diodes protect the multiplexer inputs. If either
AINP1 and AINN1 connected to
000
S
1
, S
5
preamplifier
input is taken below AVSS – 0.3V or above AVDD +
AINP2 and AINN2 connected to
0.3V, the ESD protection diodes may turn on. If these
001
S
2
, S
6
preamplifier
conditions are possible, external Schottky clamp
Preamplifier inputs shorted together
diodes and/or series resistors may be required to limit
010
S
3
, S
4
through 400
Ω
internal resistors
the input current to safe values (see the
AINP1, AINN1 and AINP2, AINN2
011
S
1
, S
5
, S
2
, S
6
table).
connected together and to the preamplifier
External short, preamplifier inputs shorted
Also, overdriving one unused input may affect the
100
S
6
, S
7
to AINN2 (common-mode test)
conversions of the other input. If overdriven inputs
are possible, it is recommended to clamp the signal
The typical on-resistance (R
ON
) of the multiplexer
with external Schottky diodes.
switch is 30
Ω
. When the multiplexer is used to drive
an external load on one input by a signal generator
on the other input, on-resistance and on-resistance
amplitude dependency can lead to measurement
errors.
shows THD versus load resistance
and amplitude. Note that THD improves with high-
impedance loads and with lower amplitude drive
signals. The data are measured with the circuit from
with MUX[2:0] = 011.
Figure 26. Analog Inputs and Multiplexer
Figure 27. THD Versus External Load and Signal
Magnitude (PGA) (see
)
The specified input operating range of the PGA is
shown in
(2)
Absolute input levels (input signal level and common-
mode level) should be maintained within these limits
for best operation.
The multiplexer connects one of the two external
differential inputs to the preamplifier inputs, in
addition to internal connections for various self-test
modes.
summarizes
the
multiplexer
configurations for
Figure 28. Driving an External Load Through the
Mux
Copyright © 2007–2015, Texas Instruments Incorporated
13
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