Datasheet

ADS1110
SBAS276A − MARCH 2003 − REVISED NOVEMBER 2003
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7
For example, the ideal expression for output codes with a
data rate of 16SPS and PGA = 2 is:
Output Code + 16384 2
(V
IN)
) * (V
IN*
)
2.048V
The ADS1110 outputs all codes right-justified and
sign-extended. This makes it possible to perform
averaging on the higher data rate codes using only a 16-bit
accumulator.
Table 2 shows the output codes for various input levels.
SELF-CALIBRATION
The previous expressions for the ADS1110’s output code
do not account for the gain and offset errors in the
modulator. To compensate for these, the ADS1110
incorporates self-calibration circuitry.
The self-calibration system operates continuously and
requires no user intervention. No adjustments can be
made to the self-calibration system, and none need to be
made. The self-calibration system cannot be deactivated.
The offset and gain error figures shown in the Electrical
Characteristics include the effects of calibration.
CLOCK OSCILLATOR
The ADS1110 features an onboard clock oscillator, which
drives the operation of the modulator and digital filter. The
Typical Characteristics show variations in data rate over
supply voltage and temperature.
It is not possible to operate the ADS1110 with an external
system clock.
INPUT IMPEDANCE
The ADS1110 uses a switched-capacitor input stage. To
external circuitry, it looks roughly like a resistance. The
resistance value depends on the capacitor values and the
rate at which they are switched. The switching frequency
is the same as the modulator frequency; the capacitor
values depend on the PGA setting. The switching clock is
generated by the onboard clock oscillator, so its frequency,
nominally 275kHz, is dependent on supply voltage and
temperature.
The common-mode and differential input impedances are
different. For a gain setting of PGA, the differential input
impedance is typically:
2.8MΩ/PGA
The common-mode impedance also depends on the PGA
setting. See the Electrical Characteristics for details.
The typical value of the input impedance often cannot be
neglected. Unless the input source has a low impedance,
the ADS1110’s input impedance may affect the
measurement accuracy. For sources with high output
impedance, buffering may be necessary. Bear in mind,
however, that active buffers introduce noise, and also
introduce offset and gain errors. All of these factors should
be considered in high-accuracy applications.
Because the clock oscillator frequency drifts slightly with
temperature, the input impedances will also drift. For many
applications, this input impedance drift can be neglected,
and the expression given above for typical input
impedance can be used.
ALIASING
If frequencies are input to the ADS1110 that exceed half
the data rate, aliasing will occur. To prevent aliasing, the
input signal must be bandlimited. Some signals are
inherently bandlimited. For example, a thermocouple’s
output, which has a limited rate of change, may
nevertheless contain noise and interference components.
These can fold back into the sampling band just as any
other signal can.
The ADS1110’s digital filter provides some attenuation of
high-frequency noise, but the digital filter’s Sinc
1
frequency response cannot completely replace an
anti-aliasing filter. For a few applications, some external
filtering may be needed; in such applications, a simple RC
filter will suffice.
When designing an input filter circuit, remember to take
into account the interaction between the filter network and
the input impedance of the ADS1110.
DATA RATE
DIFFERENTIAL INPUT SIGNAL
DATA RATE
−2.048V
(1)
−1LSB ZERO +1LSB +2.048V
15SPS 8000
H
FFFF
H
0000
H
0001
H
7FFF
H
30SPS C000
H
FFFF
H
0000
H
0001
H
3FFF
H
60SPS E000
H
FFFF
H
0000
H
0001
H
1FFF
H
240SPS F800
H
FFFF
H
0000
H
0001
H
07FF
H
(1)
Differential input only; do not drive the ADS1110’s inputs below −200mV.
Table 2. Output Codes for Different Input Signals