ADS8345_15 Datasheet, PDF (16/24 Pages) Texas Instruments – 16-Bit, 8-Channel Serial Output Sampling ANALOG-TO-DIGITAL CONVERTER

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FFFCH FFFDH FFFEH FFFFH 0000H 0001H 0002H 0003H 0004H
Code
FIGURE 11. Histogram of 5000 Conversions of a DC Input at
the Code Center, 2.7V operation external clock
mode. V_REF = V_COM = 1.25V.
AVERAGING
The noise of the A/D converter can be compensated by
averaging the digital codes. By averaging conversion results,
transition noise will be reduced by a factor of 1/√n, where n
is the number of averages. For example, averaging 4 conver-
sion results will reduce the transition noise by 1/2 to
±0.25LSBs. 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 works in a similar
manner to averaging: for every decimation by 2, the
signal-to-noise ratio will improve 3dB.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS8345 circuitry. This is particularly
true if the reference voltage is LOW and/or the conversion
rate is HIGH.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connec-
tions, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “windows”
in which large external transient voltages can easily affect
the conversion result. Such glitches might originate from
switching power supplies, nearby digital logic, and high-
power devices. The degree of error in the digital output
depends on the reference voltage, layout, and the exact
timing of the external event. The error can change if the
external event changes in time with respect to the DCLK
input.
With this in mind, power to the ADS8345 should be clean and
well bypassed. A 0.1µF ceramic bypass capacitor should be
placed as close to the device as possible. In addition, a 1µF
to 10µF capacitor and a 5Ω or 10Ω series resistor may be
used to low-pass filter a noisy supply.
The reference should be similarly bypassed with a 0.1µF
capacitor. Again, a series resistor and large capacitor can be
used to low-pass filter the reference voltage. If the reference
voltage originates from an op amp, make sure that it can
drive the bypass capacitor without oscillation (the series
resistor can help in this case). The ADS8345 draws very little
current from the reference on average, but it does place
larger demands on the reference circuitry over short periods
of time (on each rising edge of DCLK during a conversion).
The ADS8345 architecture offers no inherent rejection of
noise or voltage variation in regards to the reference input.
This is of particular concern when the reference input is tied
to the power supply. Any noise and ripple from the supply will
appear directly in the digital results. While high-frequency
noise can be filtered out as discussed in the previous
paragraph, voltage variation due to line frequency (50Hz or
60Hz) can be difficult to remove.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power-supply
entry point. The ideal layout will include an analog ground
plane dedicated to the converter and associated analog
circuitry.
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ADS8345
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