ADS1208
www.ti.com
SBAS348A–MARCH 2005–REVISED MARCH 2005
APPLICATION INFORMATION
should be used at the output of the delta-sigma
modulator. The primary purpose of the digital filter is
to remove high-frequency noise. The secondary pur-
pose is to convert the 1-bit data stream at a high
sampling rate into a higher-bit data word at a lower
rate (that is, decimation). A digital signal processor
(DSP), microcontroller (µC), or field programmable
gate array (FPGA) could be used to implement the
digital filter.
GENERAL DESCRIPTION
The ADS1208 is a 2nd-order delta-sigma modulator,
which is implemented with a switched capacitor
circuit. The analog input signal is continuously
sampled by the modulator and compared to an
internal voltage reference. A digital bit stream, which
accurately represents the analog input voltage over
time, appears at the output of the converter.
Analog Inputs
The ADS1208 is optimized for Hall sensors and
similar applications. As a result, the full-scale input
range is ±VREFIN/20, which is typically ±125mV. How-
ever, to achieve good noise and linearity, only 80% of
this range should be used (±100mV). The analog
input pins (VIN+ and VIN-) are internally buffered with
two low-noise, high bandwidth, low offset amplifiers.
The internal sampling capacitors present a very
significant load that needs to be recharged within
50ns. The ADS1208 provides two input buffers to
decouple the sampling capacitors from the pins (VIN+
VIN–). These buffers provide high bandwidth
(typically, 50MHz) at a low noise and low offset. This
configuration improves the system performance sig-
nificantly, if the input source has a high impedance in
the kΩ range. A source impedance in this range
without buffers would decrease THD and linearity
significantly, and would also cause a gain error that
changes with supply or temperature.
,
a
A current source is also integrated into the ADS1208
that can be used for biasing a Hall element or bridge
sensor. This current can be programmed with a
resistor that must be placed between AVDD and
IADJ.
Additionally, the ADS1208 includes a reference volt-
age source with a buffered output. A reference input
pin is provided as well. The voltage at the REFIN pin
sets the analog input range.
The input buffers have an auto zero function to
reduce the input offset. The auto zero switches of the
input buffers may apply a glitch of 10fC to 50fC to the
signal source in each clock cycle. For this reason,
placing a 1nF capacitor between the inputs is rec-
ommended, if the source impedance is larger than
500Ω. See Figure 35 for the equivalent input circuit,
including the protection diodes.
The device digital interface is fully compatible with the
ADS1202 and ADS1203. The ADS1208 also provides
inverted outputs of MCLK and MDATA (MCLK and
MDATA, respectively) to increase noise immunity for
the digital data transmission.
The clock source can be internal as well as external.
Different clock frequencies in combination with an
optional digital filter enable a variety of solutions and
signal bandwidths.
AVDD
AZ
VIN+
Figure 5 (page 8) shows the functional block diagram
with external circuitry. The Hall element is biased
from the internal current source. The current is set by
resistor RADJ. An offset compensation of the Hall
element is enabled by the optional resistors R1 to R4.
The analog inputs VIN+ and VIN– are directly connec-
ted with the Hall element outputs. The reference input
REFIN is connected to the reference output REFOUT
with an optional RC low-pass filter, for additional
noise filtering. For both power-supply pairs, AVDD
and BVDD, decoupling capacitors of 100nF and 10µF
(respectively) are recommended.
Delta−Sigma
Modulator
AZ
VIN
−
Figure 35. Equivalent Input Circuit
Internal Reference
The ADS1208 includes a 2.5V reference. The refer-
ence output is connected to the REFOUT pin via an
output buffer that can source 3mA. The sink current is
limited to 50µA. The output resistance of this buffer is
0.3Ω. The internal reference is also used to control
the current source at the IOUT pin.
ANALOG SECTION
Modulator
The 2nd-order modulator acts as a filter. The input
signal is low-passed while the quantization noise is
shifted to higher frequencies. A digital low-pass filter
The ADS1208 additionally provides a REFIN pin. The
applied voltage VREFIN sets the gain of the internal
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