puts codes in binary two’s complement format, so the abso-
lute values of the minima and maxima are not the same; the
maximum n-bit code is 2n-1 – 1, while the minimum n-bit code
THEORY OF OPERATION
is –1 • 2n-1
.
The ADS1100 is a fully differential, 16-bit, self-calibrating,
delta-sigma A/D converter. Extremely easy to design with
and configure, the ADS1100 allows you to take high-quality
measurements with a minimum of effort.
For example, the ideal expression for output codes with a
data rate of 16SPS and PGA = 2 is:
V
– V
IN
–
(
)
(
)
IN
+
The ADS1100 consists of a delta-sigma A/D converter core
with adjustable gain, a clock generator, and an I2C interface.
Each of these blocks are described in detail in the sections
that follow.
Output Code = 16384 • 2 •
VDD
The ADS1100 outputs all codes right-justified and sign-
extended. This arrangement makes it possible to perform
averaging on the higher data rate codes using only a 16-bit
accumulator.
ANALOG-TO-DIGITAL CONVERTER
The ADS1100’s A/D converter core consists of a differential
switched-capacitor delta-sigma modulator followed by a digi-
tal filter. The modulator measures the difference between the
positive and negative analog inputs and compares this to a
reference voltage, which, in the ADS1100, is the power
supply. The digital filter receives a high-speed bitstream from
the modulator and outputs a code, which is a number
proportional to the input voltage.
Output codes for various input levels are shown in Table II.
SELF-CALIBRATION
The previous expressions for the ADS1100’s output code do
not account for the gain and offset errors in the modulator. To
compensate for these, the ADS1100 incorporates self-cali-
bration circuitry.
The self-calibration system operates continuously, and re-
quires 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.
OUTPUT CODE CALCULATION
The output code is a scalar value which is (except for clipping)
proportional to the voltage difference between the two analog
inputs. The output code is confined to a finite range of numbers;
this range depends on the number of bits needed to represent
the code. The number of bits needed to represent the output
code for the ADS1100 depends on the data rate, as shown in
Table I.
The offset and gain error figures shown in the specifications
table include the effects of calibration.
CLOCK GENERATOR
The ADS1100 features an onboard clock generator, which
drives the operation of the modulator and digital filter. The
Typical Characteristics show varieties in data rate over
supply voltage and temperature.
Data rate
Number of Bits Minimum Code Maximum Code
8SPS
16SPS
32SPS
128SPS
16
15
14
12
–32768
–16384
–8192
32767
16383
8191
It is not possible to operate the ADS1100 with an external
modulator clock.
–2048
2047
TABLE I. Minimum and Maximum Codes.
INPUT IMPEDANCE
For a minimum output code of Min Code, gain setting of
PGA, positive and negative input voltages of VIN+ and VIN-
and power supply of VDD, the output code is given by the
expression:
The ADS1100 uses a switched-capacitor input stage. To
external circuitry, it looks roughly like a resistance. The
resistance value, as with all switched-capacitor circuits, de-
pends on the capacitor values and the rate at which they are
switched. The switching frequency is the same as the modu-
lator frequency; the capacitor values depend on the PGA
setting. The switching clock is generated by the onboard
clock generator, so its frequency, nominally 275 kHz, is
somewhat dependent on supply voltage and temperature.
,
V
– V
IN
–
(
)
(
)
IN
+
Output Code = –1•Min Code •PGA •
VDD
In the above expression, it is important to note that the
negated minimum output code is used. The ADS1100 out-
Input Signal
Data Rate
Negative Full-Scale
–1 LSB
Zero
+1 LSB
Positive Full-Scale
8 SPS
16 SPS
32 SPS
128 SPS
8000H
C000H
E000H
F800H
FFFFH
FFFFH
FFFFH
FFFFH
0000H
0000H
0000H
0000H
0001H
0001H
0001H
0001H
7FFFH
3FFFH
1FFFH
07FFH
TABLE II. Output Codes for Different Input Signals.
ADS1100
7
SBAS239
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