Datasheet ADS1015

t
SAMPLE
ON
OFF
S
1
S
2
OFF
ON
Equivalent
Circuit
f
MOD
= 250 kHz
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
12
ADS1013 , ADS1014 , ADS1015
SBAS473E –MAY 2009–REVISED JANUARY 2018 www.ti.com
Product Folder Links: ADS1013 ADS1014 ADS1015
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Feature Description (continued)
8.3.2 Analog Inputs
The ADS101x use a switched-capacitor input stage where capacitors are continuously charged and then
discharged to measure the voltage between AIN
P
and AIN
N
. The frequency at which the input signal is sampled
is called the sampling frequency or the modulator frequency (f
MOD
). The ADS101x has a 1-MHz internal oscillator
that is further divided by a factor of 4 to generate f
MOD
at 250 kHz. The capacitors used in this input stage are
small, and to external circuitry, the average loading appears resistive. Figure 11 shows this structure. The
capacitor values set the resistance and switching rate. Figure 12 shows the timing for the switches in Figure 11.
During the sampling phase, switches S
1
are closed. This event charges C
A1
to V
(AINP)
, C
A2
to V
(AINN)
, and C
B
to
(V
(AINP)
– V
(AINN)
). 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 ADS101x analog inputs. The average value of this current can be used to calculate
the effective impedance (Z
eff
), where Z
eff
= V
IN
/ I
AVERAGE
.
Figure 11. Simplified Analog Input Circuit
Figure 12. S
1
and S
2
Switch Timing
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 full-scale range, but is approximately 6 MΩ for the default full-scale range. 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 full-scale range. In Figure 11, the differential input impedance is Z
DIFF
.
Make sure to consider the typical value of the input impedance. Unless the input source has a low impedance,
the ADS101x input impedance may affect the measurement accuracy. For sources with high-output impedance,
buffering may be necessary. Active buffers introduce noise, and also introduce offset and gain errors. Consider
all of these factors in high-accuracy applications.
The clock oscillator frequency drifts slightly with temperature; therefore, the input impedances also drift. For most
applications, this input impedance drift is negligible, and can be ignored.