Datasheet

ESD
Diodes
ESD
Diodes
3pF
R
eff
= 40k
Ω
(f
CLK
= 16MHz)
AVDD
AVSS
VREFP
VREFN
AVSS * 100mV t
ǒ
VREFP or VREFN
Ǔ
t AVDD ) 100mV
ADS1258-EP
www.ti.com SBAS445D –MARCH 2009– REVISED MARCH 2011
MULTIPLEXER INPUTS
A simplified diagram of the input multiplexer is
illustrated in Figure 36. The multiplexer connects one
of 16 single-ended external inputs, one of eight
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.
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
100 mV, and likewise do not exceed AVDD by more
than 100 mV:
AVSS – 100mV < (Analog Inputs) < AVDD + 100 mV.
Overdriving the multiplexer inputs may affect the
conversions of other channels. See the Input
Overload Protection description in the Hardware
Figure 35. Simplified Reference Input Circuit
Considerations segment of the Applications section.
ESD diodes protect the reference inputs. To keep
The converter supports two modes of channel access
these diodes from turning on, make sure the voltages
through the multiplexer: the Auto-Scan mode and the
on the reference pins do not go below AVSS by more
Fixed-Channel mode. These modes are selected by
than 100mV, and likewise do not exceed AVDD by
the MUXMOD bit of register CONFIG0. The
100mV, as described in Equation 1:
Auto-Scan mode scans through the selected
channels automatically, with break-before-make
(1)
switching. The Fixed-Channel mode requires the user
A high-quality reference voltage is essential for
to set the channel address for each channel
achieving the best performance from the ADS1258.
measured.
Noise and drift on the reference degrade overall
system performance. It is especially critical that
VOLTAGE REFERENCE INPUTS
special care be given to the circuitry that generates
(VREFP, VREFN)
the reference voltages and the layout when operating
The voltage reference for the ADS1258 ADC is the
in the low-noise settings (that is, with low data rates)
differential voltage between VREFP and VREFN:
to prevent the voltage reference from limiting
V
REF
= VREFP – VREFN. The reference inputs use a
performance. See the Reference Inputs description in
structure similar to that of the analog inputs with the
the Hardware Considerations segment of the
circuitry on the reference inputs shown in Figure 35.
Applications section.
The load presented by the switched capacitor can be
modeled with an effective resistance (R
eff
) of 40 kΩ
for f
CLK
= 16 MHz. Note that the effective impedance
of the reference inputs loads an external reference
with a non-zero source impedance.
Copyright © 2009–2011, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Link(s): ADS1258-EP