Datasheet

10W
50W
OPA365
24pF
1000pF
1nF
10W
50W
OPA365
24pF
1000pF
+IN
-IN
ADS8317
10W
50W
OPA365
24pF
1000pF
50W
24pF
+IN
-IN
ADS8317
Single-Ended
Differential
V
CM
+
ADS8317
SBAS356D –JUNE 2007–REVISED OCTOBER 2009 .................................................................................................................................................... www.ti.com
The input current on the analog inputs depends on a REFERENCE INPUT
number of factors: sample rate, input voltage, and
The external reference sets the analog input range.
source impedance. Essentially, the current into the
The ADS8317 operates with a reference in the range
ADS8317 charges the internal capacitor array during
of 0.1V to V
DD
/2. There are several important
the sample period. After this capacitance has been
implications to this specification.
fully charged, there is no further input current. The
source of the analog input voltage must be able to
As the reference voltage is reduced, the analog
charge the input capacitance (24pF) to 16-bit settling
voltage weight of each digital output code is reduced.
level within 4.5 clock cycles. When the converter
This reduction is often referred to as the least
goes into the hold mode, or while it is in the
significant bit (LSB) size and is equal to the reference
power-down mode, the input impedance is greater
voltage divided by 65,536. This relationship means
than 1GΩ.
that any offset or gain error inherent in the A/D
converter appears to increase (in terms of LSB size)
Care must be taken regarding the absolute analog
as the reference voltage is reduced. For a reference
input voltage. The +IN input should always remain
voltage of 2.5V, the value of the LSB is 76.3μV, and
within the range of GND – 300mV to V
DD
+ 300mW.
for a reference voltage of 1.25V, the LSB is 38.15μV.
The –IN input should always remain within the range
of GND – 300mV to 4V. Outside of these ranges, the
The noise inherent in the converter also appears to
converter linearity may not meet specifications. To
increase with a lower LSB size. With a 2.5V
obtain maximum performance from the ADS8317, an
reference, the internal noise of the converter typically
input circuit such as that shown in Figure 42 is
contributes only 5LSB peak-to-peak of potential error
recommended.
to the output code. When the external reference is
1.25V, the potential error contribution from the
internal noise is almost two times larger (9LSB). The
errors arising from the internal noise are Gaussian in
nature and can be reduced by averaging consecutive
conversion results.
For more information regarding noise, consult
Figure 18, Peak-to-Peak Noise for a DC Input vs
Reference Voltage. Note that the Effective Number Of
Bits (ENOB) figure is calculated based on the
converter signal-to-(noise + distortion) ratio with a
2kHz, 0dB input signal. SINAD is related to ENOB as
follows:
SINAD = 6.02 × ENOB + 1.76
With lower reference voltages, extra care should be
taken to provide a clean layout including adequate
bypassing, a clean power supply, a low-noise
reference, and a low-noise input signal. Due to the
lower LSB size, the converter is also more sensitive
to external sources of error, such as nearby digital
signals and electromagnetic interference.
Figure 42. Single-Ended and Differential Methods
of Interfacing the ADS8317
18 Submit Documentation Feedback Copyright © 2007–2009, Texas Instruments Incorporated
Product Folder Link(s): ADS8317