Datasheet

ESD
Diodes
ESD
Diodes
3pF
R
eff
=40kW
(f =16MHz)
CLK
AVDD
AVSS
VREFP
VREFN
AVSS 100mV<(VREFPorVREFN)<AVDD+100mV-
ADS1158
SBAS429D –JUNE 2008– REVISED MARCH 2011 www.ti.com
MULTIPLEXER INPUTS The load presented by the switched capacitor can be
modeled with an effective resistance (R
eff
) of 40kΩ for
A simplified diagram of the input multiplexer is
f
CLK
= 16MHz. Note that the effective impedance of
illustrated in Figure 22. The multiplexer connects one
the reference inputs loads an external reference with
of 16 single-ended external inputs, one of eight
a non-zero source impedance.
differential external inputs, or one of the on-chip
internal variables to the ADC inputs. The output of the
channel multiplexer can be routed to external pins
and then to the input of the ADC. This flexibility
allows for use of external signal conditioning. See the
External Multiplexer Loop section.
Electrostatic discharge (ESD) diodes protect the
analog inputs. To keep these diodes from turning on,
make sure the voltages on the input pins do not go
below AVSS by more than 100mV, and likewise do
not exceed AVDD by more than 100mV:
AVSS – 100mV < (Analog Inputs) < AVDD + 100mV.
Overdriving the multiplexer inputs may affect the
conversions of other channels. See the Input
Overload Protection description in the Hardware
Considerations segment of the Applications section.
The converter supports two modes of channel access
through the multiplexer: the Auto-Scan mode and the
Figure 21. Simplified Reference Input Circuit
Fixed-Channel mode. These modes are selected by
ESD diodes protect the reference inputs. To keep
the MUXMOD bit of register CONFIG0. The
these diodes from turning on, make sure the voltages
Auto-Scan mode scans through the selected
on the reference pins do not go below AVSS by more
channels automatically, with break-before-make
than 100mV, and likewise do not exceed AVDD by
switching. The Fixed-Channel mode requires the user
100mV:
to set the channel address for each channel
measured.
A high-quality reference voltage is essential to
VOLTAGE REFERENCE INPUTS
achieve the best performance from the ADS1158.
(VREFP, VREFN)
Noise and drift on the reference degrade overall
The voltage reference for the ADS1158 ADC is the
system performance. It is especially critical that
differential voltage between VREFP and VREFN:
special care be given to the circuitry that generates
V
REF
= VREFP – VREFN. The reference inputs use a
the reference voltages and the layout when operating
structure similar to that of the analog inputs with the
in the low-noise settings (that is, with low data rates)
circuitry on the reference inputs shown in Figure 21.
to prevent the voltage reference from limiting
performance. See the Reference Inputs description in
the Hardware Considerations segment of the
Applications section.
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