Datasheet
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REFERENCE
The ADS8506 can operate with its internal 2.5-V reference or an external reference. By applying an external
Buffer
CDAC
Internal
Reference
Z
REF
Z
CAP
CAP
(Pin 4)
REF
(Pin 5)
Z
CAP
Ω
Z
REF
Ω
PWRD 0
REFD 0
1
6 k
PWRD 0
REFD 1
1
100 M
PWRD 1
REFD 0
200
6 k
PWRD 1
REFD 1
200
100 M
REF
CAP
ADS8506
SLAS484B – SEPTEMBER 2007 – REVISED DECEMBER 2007
reference to pin 5, the internal reference can be bypassed. The reference voltage at REF is buffered internally
with the output on CAP (pin 4).
The internal reference has an 8 ppm/ ° C drift (typical) and accounts for approximately 20% of the full-scale error
(FSE = ± 0.5% for low grade, ± 0.25% for high grade).
The ADS8506 also has an internal buffer for the reference voltage. Figure 46 shows characteristic impedances at
the input and output of the buffer with all combinations of powerdown and reference down.
Figure 46. Characteristic Impedances of the Internal Buffer
REF (pin 5) is an input for an external reference or the output for the internal 2.5-V reference. A 2.2- µ F tantalum
capacitor should be connected as close as possible to the REF pin from ground. This capacitor and the output
resistance of REF create a low-pass filter to bandlimit noise on the reference. Using a smaller value capacitor will
introduce more noise to the reference, degrading the SNR and SINAD. The REF pin should not be used to drive
external AC or DC loads, as shown in Figure 46 .
The range for the external reference is 2.3 V to 2.7 V and determines the actual LSB size. Increasing the
reference voltage increases the full-scale range and the LSB size of the converter which can improve the SNR.
CAP (pin 4) is the output of the internal reference buffer. A 2.2- µ F tantalum capacitor should be placed as close
as possible to the CAP pin from ground to provide optimum switching currents for the CDAC throughout the
conversions cycle. This capacitor also provides compensation for the output of the buffer. Using a capacitor any
smaller than 1 µ F can cause the output buffer to oscillate and may not have sufficient charge for the CDAC.
Capacitor values larger than 2.2 µ F have little affect on improving performance. ESR is the total equivalent series
resistance of the compensation capacitor (CAP pin). See Figure 46 and Figure 47 .
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