Datasheet

Input AINx
AVDD
BAT54SWTI
AVSS
10k
Ω
typ.
ADS1258
www.ti.com SBAS297G –JUNE 2005– REVISED MARCH 2011
APPLICATIONS
HARDWARE CONSIDERATIONS
The following summarizes the design and layout
considerations when using the ADS1258:
a. Power Supplies: The converter accepts a single
+5V supply (AVDD = +5V and AVSS = AGND) or
dual, bipolar supplies (typically AVDD = +2.5V,
AVSS = –2.5V). Dual supply operation
accommodates true bipolar input signals, within a
±2.5V range. Note that the maximum negative
input voltage to the multiplexer is limited to
Figure 62. Input Overload Protection
AVSS – 100mV, and the maximum positive input
d. ADC Inputs: The external multiplexer loop of the
voltage is limited to AVDD + 100mV. The range
ADS1258 allows for the inclusion of signal
for the digital power supply (DVDD) is 2.7V to
conditioning between the output of the multiplexer
5.25V. For all supplies, use a 10μF tantalum
and the input of the ADC. Typically, an amplifier
capacitor, bypassed with a 0.1μF ceramic
is used to provide gain, buffering, and/or filtering
capacitor, placed close to the device pins.
to the input signal. For best performance, the
Alternatively, a single 10μF ceramic capacitor can
ADC inputs should be driven differentially. A
be used. The supplies should be relatively free
differential in/differential out or a
from noise and should not be shared with devices
single-ended-to-differential driver is recom-
that produce voltage spikes (such as relays, LED
mended. If the driver uses higher supply voltages
display drivers, etc.). If a switching power supply
than the device itself (for example, ±15V),
is used, the voltage ripple should be low (< 2mV).
attention should be paid to power-supply
The analog and digital power supplies may be
sequencing and potential over-voltage fault
sequenced in any order.
conditions. Protection resistors and/or external
b. Analog (Multiplexer) Inputs: The 16-channel
clamp diodes may be used to protect the ADC
analog input multiplexer can accommodate 16
inputs. A 1nF or higher capacitor should be used
single-ended inputs, eight differential input pairs,
directly across the ADC inputs.
or combinations of either. These options permit
e. Reference Inputs: It is recommended to use a
freedom in choosing the input channels. The
10μF tantalum with a 0.1μF ceramic capacitor
channels do not have to be used consecutively.
directly across the reference pins, VREFP and
Unassigned channels are skipped by the device.
VREFN. The reference inputs should be driven
In the Fixed-Channel mode, any of the analog
by a low-impedance source. For rated
inputs (AIN0 to AIN15) can be addressed for the
performance, the reference should have less than
positive input and for the negative input. The
3μV
RMS
broadband noise. For references with
full-scale range of the device is 2.13V
REF
, but the
higher noise, external filtering may be necessary.
absolute analog input voltage is limited to 100mV
Note that when exiting the sleep mode, the
beyond the analog supply rails. Input signals
device begins to draw a small current through the
exceeding the analog supply rails (for example,
reference pins. Under this condition, the transient
±10V) must be divided prior to the multiplexer
response of the reference driver should be fast
inputs.
enough to settle completely before the first
c. Input Overload Protection: Overdriving the
reading is taken, or simply discard the first
multiplexer inputs may affect the conversions of
several readings.
other channels. In the case of input overload,
external Schottky diode clamps and series
resistor are recommended, as shown in Figure
61.
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Product Folder Link(s): ADS1258