Datasheet
OPA350
24pF
47 Fm
ADS8326
V
REF
50W
ADS8326
SBAS343C –MAY 2007–REVISED SEPTEMBER 2009..................................................................................................................................................
www.ti.com
REFERENCE INPUT
The external reference sets the analog input range.
The ADS8326 operates with a reference in the range
of 0.1V to V
DD
. There are several important
implications to this.
As the reference voltage is reduced, the analog
voltage weight of each digital output code is reduced.
This is often referred to as the least significant bit
Figure 42. Input Reference Circuit and Interface
(LSB) size and is equal to the reference voltage
divided by 65,536. This means that any offset or gain
When the ADS8326 is in Power-Down mode, the
error inherent in the A/D converter will appear to
input resistance of the reference pin will have a value
increase (in terms of LSB size) as the reference
of 5GΩ. Since the input capacitors must be
voltage is reduced. For a reference voltage of 2.5V,
recharged before the next conversion starts, an
the value of the LSB is 38.15μV, and for a reference
operational amplifier with good dynamic
voltage of 5V, the LSB is 76.3μV.
characteristics must be used to buffer the reference
The noise inherent in the converter will also appear to
input.
increase with a lower LSB size. With a 5V reference,
the internal noise of the converter typically contributes
Noise
only 1.5LSB peak-to-peak of potential error to the
The transition noise of the ADS8326 itself is
output code. When the external reference is 2.5V, the
extremely low, as shown in Figure 20 (+5V) and
potential error contribution from the internal noise will
Figure 37 (+2.7V); it is much lower than competing
be two times larger (3LSB). The errors arising from
A/D converters. These histograms were generated by
the internal noise are Gaussian in nature and can be
applying a low-noise DC input and initiating 8192
reduced by averaging consecutive conversion results.
conversions. The digital output of the A/D converter
For more information regarding noise, see Figure 15,
will vary in output code because of the internal noise
Peak-to-Peak Noise for a DC Input vs Reference
of the ADS8326. This is true for all 16-bit, SAR-type
Voltage. Note that the Effective Number Of Bits
A/D converters. Using a histogram to plot the output
(ENOB) figure is calculated based on the converter
codes, the distribution should appear bell-shaped with
signal-to-(noise + distortion) ratio with a 1kHz, 0dB
the peak of the bell curve representing the nominal
input signal. SINAD is related to ENOB as follows:
code for the input value. The ±1σ, ±2σ, and ±3σ
SINAD = 6.02 × ENOB + 1.76
distributions will represent 68.3%, 95.5%, and 99.7%,
respectively, of all codes. The transition noise can be
With lower reference voltages, extra care should be
calculated by dividing the number of codes measured
taken to provide a clean layout including adequate
by 6, which yields the ±3σ distribution, or 99.7%, of
bypassing, a clean power supply, a low-noise
all codes. Statistically, up to three codes could fall
reference, and a low-noise input signal. Due to the
outside the distribution when executing 1000
lower LSB size, the converter is also more sensitive
conversions. The ADS8326, with < 3 output codes for
to external sources of error, such as nearby digital
the ±3σ distribution, yields < ±0.5LSB of transition
signals and electromagnetic interference.
noise. Remember, to achieve this low-noise
The equivalent input circuit for the reference voltage
performance, the peak-to-peak noise of the input
is presented in Figure 42. During the conversion
signal and reference must be < 50μV.
process, an equivalent capacitor of 24pF is switched
on. To obtain optimum performance from the
Averaging
ADS8326, special care must be taken in designing
The noise of the A/D converter can be compensated
the interface circuit to the reference input pin. To
by averaging the digital codes. By averaging
ensure a stable reference voltage, a 47μF tantalum
conversion results, transition noise is reduced by a
capacitor with low ESR should be connected as close
factor of 1/√n , where n is the number of averages.
as possible to the input pin. If a high output
For example, averaging four conversion results
impedance reference source is used, an additional
reduces the transition noise from ±0.5LSB to
operational amplifier with a current-limiting resistor
±0.25LSB. Averaging should only be used for input
must be placed in front of the capacitors.
signals with frequencies near DC.
For AC signals, a digital filter can be used to
low-pass filter and decimate the output codes. This
works in a similar manner to averaging; for every
decimation by 2, the signal-to-noise ratio improves by
3dB.
22 Submit Documentation Feedback Copyright © 2007–2009, Texas Instruments Incorporated
Product Folder Link(s): ADS8326