Datasheet

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SBAS312B − JANUARY 2005 − REVISED AUGUST 2007
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13
GENERAL DESCRIPTION
The ADS1205 is a two-channel, 2nd-order, CMOS device
with two delta-sigma modulators, designed for medium- to
high-resolution A/D signal conversions from DC to 39kHz
(filter response −3dB) if an oversampling ratio (OSR) of 64
is chosen. The output of the converter (OUT
X
) provides a
stream of digital ones and zeros. The time average of this
serial output is proportional to the analog input voltage.
The modulator shifts the quantization noise to high
frequencies. A low-pass digital filter should be used at the
output of the delta-sigma modulator. The filter serves two
functions. First, it filters out high-frequency noise. Second,
the filter converts the 1-bit data stream at a high sampling
rate into a higher-bit data word at a lower rate (decimation).
An application-specific integrated circuit (ASIC) or
field-programmable gate array (FPGA) could be used to
implement the digital filter. Figure 28 and Figure 29 show
typical application circuits with the ADS1205 connected to
an FPGA or ASIC.
The overall performance (that is, speed and accuracy)
depends on the selection of an appropriate OSR and filter
type. A higher OSR produces greater output accuracy
while operating at a lower refresh rate. Alternatively, a
lower OSR produces lower output accuracy, but operates
at a higher refresh rate. This system allows flexibility with
the digital filter design and is capable of A/D conversion
results that have a dynamic range exceeding 98dB with an
OSR = 256.
27
Ω
5kΩ
5k
Ω
2k
Ω
2k
Ω
+5V
0.1nF
+3V
+3V
0.1
µ
F
0.1
µ
F
0.1
µ
F
±
5V
OPA2350
0.1
µ
F
27Ω
5k
Ω
5kΩ 2kΩ
2k
Ω
+5V
0.1nF
±
5V
OPA23500.1
µ
F
2nd−Order
∆Σ
Modulator
FPGA
or
ASIC
CH A+
AV
DD
CH A−
Output
Interface
Circuit
RC
Oscillator
20MHz
Out EN
Clock
Select
Divider
REFIN A
Reference
Voltage
2.5V
REFOUT
OUTA
OUT B
CLKIN
AGNDAGNDAGNDAGND
BV
DD
CLKOUT
BV
DD
BGND
+5V
0.1
µ
F
+5V
0.1µF
+5V
0.1
µ
F
+5V
0.1µF
CLKSEL
AV
DD
AV
DD
AV
DD
AV
DD
2nd−Order
∆Σ
Modulator
CH B+
CH B
−
REFIN B
+5V
OPA336
Figure 28. Single-Ended Connection Diagram for the ADS1205 Delta-Sigma Modulator