SBAS363 – DECEMBER 2009
For optimum performance, care should be taken with the physical layout of the ADS8331/32 circuitry. This
consideration is particularly true if the reference voltage is low and/or the conversion rate is high. With a
conversion clock of 12MHz, the ADS8331/32 makes a bit decision every 83ns. That is, for each subsequent bit
decision, the capacitor array must be switched and charged, and the input to the comparator settled to a 16-bit
level, all within one conversion clock cycle.
The basic SAR architecture is sensitive to spikes on the power supply, reference, and ground connections that
occur just prior to latching the comparator output. 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
spikes might originate from switching power supplies, digital logic, and high-power devices, to name a few
potential sources. This particular source of error can be very difficult to track down if the glitch is almost
synchronous to the converter CCLK signal because the phase difference between the two changes with time and
temperature, causing sporadic misoperation.
With this possibility in mind, power to the ADS8331/32 should be clean and well-bypassed. A 0.1μF ceramic
bypass capacitor should be placed as close as possible to the ADS8331/32 package. 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 22μF 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 the op amp can drive the bypass capacitor without oscillation (the series resistor can help in this case).
Although the ADS8331/32 draws very little current from the reference on average, there can still be
instantaneous current demands placed on the external input and reference circuitry.
The OPA365 or OPA211 from Texas Instrumets provide optimum performance for buffering the signal inputs; the
OPA350 can be used to effectively buffer the reference input.
Also, keep in mind that the ADS8331/32 offers no inherent rejection of noise or voltage variation in regards to the
reference input. This consideration 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, voltage variation resulting from the line frequency (50Hz or 60Hz) can be difficult to remove.
The AGND pin on the ADS8331/32 should be placed on a clean ground point. In many cases, this location is the
analog ground. Avoid connecting the AGND pin too close to the grounding point for a microprocessor,
microcontroller, or digital signal processor. If needed, run a ground trace directly from the converter to the
power-supply connection point. The ideal layout includes an analog ground plane for the converter and
associated analog circuitry.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): ADS8331 ADS8332
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