Datasheet

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3000
0.1
4000
5000
7000
6000
2000
0
1000
1 10 100
CAP − Pin Value − mF
3000
0.1
4000
5000
7000
6000
2000
0
1000
1 10 100
Power−Up Time − ms
REFERENCE AND POWER-DOWN
The ADS8506 has analog power-down and reference power down capabilities via PWRD (pin 25) and REFD (pin
PWRD
REFD
LAYOUT
POWER
ADS8506
SLAS484B – SEPTEMBER 2007 – REVISED DECEMBER 2007
Figure 47. Power-Down to Power-Up Time vs Capacitor Value on CAP
The output of the buffer is capable of driving up to 1 mA of current to a DC load. Using an external buffer allows
the internal reference to be used for larger DC loads and AC loads. Do not attempt to directly drive an AC load
with the output voltage on CAP. This causes performance degradation of the converter.
26), respectively. PWRD and REFD high powers down all analog circuitry maintaining data from the previous
conversion in the internal registers, provided that the data has not already been shifted out through the serial
port. Typical power consumption in this mode is 50 µ W. Power recovery is typically 1 ms, using a 2.2- µ F
capacitor connected to CAP. Figure 47 shows power-down to power-up recovery time relative to the capacitor
value on CAP. With +5 V applied to V
DIG
, the digital circuitry of the ADS8506 remains active at all times,
regardless of PWRD and REFD states.
PWRD high powers down all of the analog circuitry except for the reference. Data from the previous conversion
is maintained in the internal registers and can still be read. With PWRD high, a convert command yields
meaningless data.
REFD high powers down the internal 2.5-V reference. All other analog circuitry, including the reference buffer, is
active. REFD should be high when using an external reference to minimize power consumption and the loading
effects on the external reference. See Figure 46 for the characteristic impedance of the reference buffer's input
for both REFD high and low. The internal reference consumes approximately 5 mW.
For optimum performance, tie the analog and digital power pins to the same +5-V power supply and tie the
analog and digital grounds together. As noted in the electrical characteristics, the ADS8506 uses 90% of its
power for the analog circuitry. The ADS8506 should be considered as an analog component.
The +5-V power for the A/D converter should be separate from the +5 V used for the system's digital logic.
Connecting V
DIG
(pin 28) directly to a digital supply can reduce converter performance due to switching noise
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