Datasheet

OPA350
24pF
47 Fm
ADS8317
V
REF
50W
ADS8317
www.ti.com .................................................................................................................................................... SBAS356D –JUNE 2007–REVISED OCTOBER 2009
Noise
The equivalent input circuit for the reference voltage
is presented in Figure 43. At the same time, an
The transition noise of the ADS8317 itself is
equivalent capacitor of 24pF is switched. To obtain
extremely low, as shown in Figure 19 and Figure 36;
optimum performance from the ADS8317, special
it is much lower than competing A/D converters.
care must be taken in designing the interface circuit
These histograms were generated by applying a
to the reference input pin. To ensure a stable
low-noise DC input and initiating 8192 conversions.
reference voltage, a 47μF tantalum capacitor with low
The digital output of the A/D converter varies in
ESR should be connected as close as possible to the
output code because of the internal noise of the
input pin. If a high output impedance reference
ADS8317. This variance is true for all 16-bit,
source is used, an additional operational amplifier
SAR-type A/D converters. Using a histogram to plot
with a current-limiting resistor must be placed in front
the output codes, the distribution should appear
of the capacitors.
bell-shaped with the peak of the bell curve
representing the nominal code for the input value.
The ±1σ, ±2σ, and ±3σ distributions represent 68.3%,
95.5%, and 99.7%, respectively, of all codes. The
transition noise can be calculated by dividing the
number of codes measured by 6, which yields the
±3σ distribution, or 99.7%, of all codes. Statistically,
up to three codes could fall outside the distribution
when executing 1000 conversions. The ADS8317,
with five output codes for the ±3σ distribution, yields
less than ±0.8LSB of transition noise. Remember that
Figure 43. Input Reference Circuit and Interface
to achieve this low-noise performance, the
peak-to-peak noise of the input signal and reference
When the ADS8317 is in power-down mode, the input
must be less than 50μV.
resistance of the reference pin has a value of 5GΩ.
Because the input capacitors must be recharged
Averaging
before the next conversion starts, an operational
amplifier with good dynamic characteristics, such as
The noise of the A/D converter can be compensated
the OPA350, should be used to buffer the reference
by averaging the digital codes. By averaging
input.
conversion results, transition noise is reduced by a
factor of 1/√n , where n is the number of averages.
For example, averaging four conversion results
reduces the transition noise from ±0.8LSB to
±0.4LSB. Averaging should only be used for input
signals with frequencies near DC.
For AC signals, a digital filter can be used to
low-pass filter and decimate the output codes. This
configuration works in a similar manner to averaging;
for every decimation by 2, the signal-to-noise ratio
improves by 3dB.
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