Datasheet

Input AINx
AVDD
BAT54SWTI
AVSS
10k
Ω
typ.
ADS1258-EP
SBAS445D –MARCH 2009– REVISED MARCH 2011 www.ti.com
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 = 5 V and AVSS = AGND) or
dual, bipolar supplies (typically AVDD = 2.5 V,
AVSS = –2.5 V). Dual supply operation
accommodates true bipolar input signals, within a
±2.5-V range. Note that the maximum negative
input voltage to the multiplexer is limited to
Figure 63. Input Overload Protection
AVSS – 100 mV, and the maximum positive input
voltage is limited to AVDD + 100 mV. The range
d. ADC Inputs: The external multiplexer loop of the
for the digital power supply (DVDD) is 2.7 V to
ADS1258 allows for the inclusion of signal
5.25 V. For all supplies, use a 10-μF tantalum
conditioning between the output of the multiplexer
capacitor, bypassed with a 0.1-μF ceramic
and the input of the ADC. Typically, an amplifier
capacitor, placed close to the device pins.
is used to provide gain, buffering, and/or filtering
Alternatively, a single 10-μF ceramic capacitor
to the input signal. For best performance, the
can be used. The supplies should be relatively
ADC inputs should be driven differentially. A
free from noise and should not be shared with
differential in/differential out or a
devices that produce voltage spikes (such as
single-ended-to-differential driver is recom-
relays, LED display drivers, etc.). If a switching
mended. If the driver uses higher supply voltages
power supply is used, the voltage ripple should
than the device itself (for example, ±15V),
be low (< 2 mV). The analog and digital power
attention should be paid to power-supply
supplies may be sequenced in any order.
sequencing and potential over-voltage fault
b. Analog (Multiplexer) Inputs: The 16-channel
conditions. Protection resistors and/or external
analog input multiplexer can accommodate 16
clamp diodes may be used to protect the ADC
single-ended inputs, eight differential input pairs,
inputs. A 1-nF or higher capacitor should be used
or combinations of either. These options permit
directly across the ADC inputs.
freedom in choosing the input channels. The
e. Reference Inputs: It is recommended to use a
channels do not have to be used consecutively.
10-μF tantalum with a 0.1-μF ceramic capacitor
Unassigned channels are skipped by the device.
directly across the reference pins, VREFP and
In the Fixed-Channel mode, any of the analog
VREFN. The reference inputs should be driven
inputs (AIN0 to AIN15) can be addressed for the
by a low-impedance source. For rated
positive input and for the negative input. The
performance, the reference should have less than
full-scale range of the device is 2.13 V
REF
, but the
3μV
RMS
broadband noise. For references with
absolute analog input voltage is limited to 100 mV
higher noise, external filtering may be necessary.
beyond the analog supply rails. Input signals
Note that when exiting the sleep mode, the
exceeding the analog supply rails (for example,
device begins to draw a small current through the
±10 V) must be divided prior to the multiplexer
reference pins. Under this condition, the transient
inputs.
response of the reference driver should be fast
c. Input Overload Protection: Overdriving the
enough to settle completely before the first
multiplexer inputs may affect the conversions of
reading is taken, or simply discard the first
other channels. In the case of input overload,
several readings.
external Schottky diode clamps and series
resistor are recommended, as shown in Figure
61.
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