Datasheet

REFERENCE
The ADS8513 can be operated with its internal 2.5V reference or an external reference. By applying an external
REF
CAP
BUF
ADS8513
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reference voltage to the REF pin, the internal reference voltage is overdriven. The voltage at the REF input is
internally buffered by a unity gain buffer. The output of this buffer is present at the BUF and CAP pins.
The REF pin is the output of the internal 2.5V reference or the input for an external reference. A 1 µ F to 2.2 µ F
tantalum capacitor should be connected between this pin and ground. The capacitor should be placed as close
to the ADS8513 as possible.
When using the internal reference, the REF pin should not be connected to any type of significant load. An
external load causes a voltage drop across the internal 4k Ω resistor that is in series with the internal reference.
Even a 40M Ω external load to ground causes a decrease in the full-scale range of the converter by 6LSBs.
The range for the external reference is 2.3V to 2.7V. The voltage on REF determines the full-scale range of the
converter and the corresponding LSB size. Increasing the reference voltage increases the LSB size in relation to
the internal noise sources which, in turn, can improve signal-to-noise ratio. Likewise, decreasing the reference
voltage reduces the LSB size and signal-to-noise ratio.
The ADS8513 is factory-tested with 2.2 µ F capacitors connected to pin 6 (CAP) and pin 7 (REF). Each capacitor
should be placed as close as possible to its pin. The capacitor on pin 7 band-limits the internal reference noise.
A lower-value capacitor can be used, but it may degrade SNR and SINAD. The capacitor on pin 6 stabilizes the
reference buffer and provides a switching charge to the CDAC during conversion. Capacitors smaller than 1 µ F
can cause the buffer to become unstable and may not hold sufficient charge for the CDAC. The parts are tested
to specifications with 2.2 µ F, so larger capacitors are not necessary. The equivalent series resistance (ESR) of
these compensation capacitors is also critical. The total ESR must be kept under 3 Ω . See the Typical
Characteristics section concerning how ESR affects performance.
The voltage on the BUF pin is the output of the internal reference buffer. This pin is used to provide +2.5V to the
analog input or inputs for the various input configurations. The BUF output can provide up to 1mA of current to
an external load. The load should be constant because variable load could affect the conversion result by
modulating the BUF voltage. Also note that the BUF output shows significant glitches as each bit decision is
made during a conversion. Between conversions, the BUF output is quiet.
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