Datasheet
t
SAMPLE
ON
OFF
S
1
S
2
OFF
ON
Equivalent
Circuit
f = 250 kHz
CLK
Z
CM
Z
DIFF
Z
CM
AIN
N
AIN
P
0.7 V
0.7 V
S
1
S
1
C
A1
C
B
C
A2
S
2
S
2
0.7 V
0.7 V
AIN
N
AIN
P
ADS1018
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SBAS526B –NOVEMBER 2012–REVISED OCTOBER 2013
ANALOG INPUTS
The ADS1018 uses a switched-capacitor input stage in which capacitors are continuously charged and then
discharged to measure the voltage between AIN
P
and AIN
N
. The capacitors used are small, and to external
circuitry, the average loading appears resistive. This structure is shown in Figure 11. The resistance is set by the
capacitor values and the rate at which they are switched. Figure 12 shows the on/off setting of the switches in
Figure 11. During the sampling phase, all S
1
switches are closed. This event charges C
A1
to AIN
P
, C
A2
to AIN
N
,
and C
B
to (AIN
P
– AIN
N
). During the discharge phase, S
1
is first opened and then S
2
is closed. Both C
A1
and C
A2
then discharge to approximately 0.7 V, and C
B
discharges to 0 V. This charging draws a very small transient
current from the source driving the ADS1018 analog inputs. The average value of this current can be used to
calculate the effective impedance (R
eff
), where R
eff
= V
IN
/ I
AVERAGE
.
Figure 11. Simplified Analog Input Circuit
Figure 12. S
1
and S
2
Switch Timing for Figure 11
The common-mode input impedance is measured by applying a common-mode signal to the shorted AIN
P
and
AIN
N
inputs and measuring the average current consumed by each pin. The common-mode input impedance
changes depending on the PGA gain setting, but is approximately 6 MΩ for the default PGA gain setting. In
Figure 11, the common-mode input impedance is Z
CM
.
The differential input impedance is measured by applying a differential signal to AIN
P
and AIN
N
inputs where one
input is held at 0.7 V. The current that flows through the pin connected to 0.7 V is the differential current and
scales with the PGA gain setting. In Figure 11, the differential input impedance is Z
DIFF
. Table 1 describes the
typical differential input impedance.
Table 1. Differential Input Impedance
FS (V) DIFFERENTIAL INPUT IMPEDANCE
±6.144
(1)
22 MΩ
±4.096
(1)
15 MΩ
±2.048 4.9 MΩ
±1.024 2.4 MΩ
±0.512 710 kΩ
±0.256 710 kΩ
(1) This parameter expresses the full-scale range of the ADC scaling. In
no event should more than VDD + 0.3 V be applied to this device.
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