Datasheet

REFN1REFP1
ADC
ADS1248 Only
REFN0REFP0
REFNREFP
VREFCOMVREFOUT
ReferenceMultiplexer
Internal
Voltage
Reference
ADS1246
ADS1247
ADS1248
www.ti.com SBAS426G –AUGUST 2008–REVISED OCTOBER 2011
ESD diodes protect the ADC inputs. To prevent these VOLTAGE REFERENCE INPUT
diodes from turning on, make sure the voltages on
The voltage reference for the ADS1246/7/8 is the
the input pins do not go below AVSS by more than
differential voltage between REFP and REFN:
100mV, and do not exceed AVDD by more than
V
REF
= V
REFP
– V
REFN
100mV, as shown in Equation 2. Note that the same
caution is true if the inputs are configured to be
In the case of the ADS1246, these pins are dedicated
GPIOs.
inputs. For the ADS1247 and ADS1248, there is a
AVSS – 100mV < (AINX) < AVDD + 100mV (2)
multiplexer that selects the reference inputs, as
shown in Figure 52. The reference input uses a buffer
Settling Time for Channel Multiplexing
to increase the input impedance.
The ADS1246/7/8 is a true single-cycle settling ΔΣ
As with the analog inputs, REFP0 and REFN0 can be
converter. The first data available after the start of a
configured as digital I/Os on the ADS1247/8.
conversion are fully settled and valid for use. The
time required to settle is roughly equal to the inverse
of the data rate. The exact time depends on the
specific data rate and the operation that resulted in
the start of a conversion; see Table 16 for specific
values.
ANALOG INPUT IMPEDANCE
The ADS1246/7/8 inputs are buffered through a
high-impedance PGA before they reach the ΔΣ
modulator. For the majority of applications, the input
current leakage is minimal and can be neglected.
However, because the PGA is chopper-stabilized for
noise and offset performance, the input impedance is
best described as small absolute input current. The
absolute current leakage for selected channels is
Figure 52. Reference Input Multiplexer
approximately proportional to the selected modulator
clock. Table 7 shows the typical values for these
The reference input circuit has ESD diodes to protect
currents with a differential voltage coefficient and the
the inputs. To prevent the diodes from turning on,
corresponding input impedances over data rate.
make sure the voltage on the reference input pin is
not less than AVSS – 100mV, and does not exceed
AVDD + 100mV, as shown in Equation 3:
AVSS – 100mV < (V
REFP
or V
REFN
) < AVDD + 100mV (3)
Table 7. Typical Values for Analog Input Current Over Data Rate
(1)
CONDITION ABSOLUTE INPUT CURRENT EFFECTIVE INPUT IMPEDANCE
DR = 5SPS, 10SPS, 20SPS ± (0.5nA + 0.1nA/V) 5000MΩ
DR = 40SPS, 80SPS, 160SPS ± (2nA + 0.5nA/V) 1200MΩ
DR = 320SPS, 640SPS, 1kSPS ± (4nA + 1nA/V) 600MΩ
DR = 2kSPS ± (8nA + 2nA/V) 300MΩ
(1) Input current with V
CM
= 2.5V. T
A
= +25°C, AVDD = 5V, and AVSS = 0V.
Copyright © 2008–2011, Texas Instruments Incorporated Submit Documentation Feedback 25
Product Folder Link(s): ADS1246 ADS1247 ADS1248