cycle. This capacitor also provides compensation for the
output of the buffer. Using a capacitor any smaller than 1µF
can cause the output buffer to oscillate and may not have
sufficient charge for the CDAC. Capacitor values larger than
2.2µF will have little affect on improving performance. See
Figures 10 and 11.
REFD
REFD HIGH will power-down the internal 2.5V reference. All
other analog circuitry, including the reference buffer, will be
active. REFD should be HIGH when using an external
reference to minimize power consumption and the loading
effects on the external reference. See Figure 10 for the
characteristic impedance of the reference buffer’s input for
both REFD HIGH and LOW. The internal reference con-
sumes approximately 5mW.
7000
6000
5000
4000
3000
2000
1000
0
LAYOUT
POWER
For optimum performance, tie the analog and digital power
pins to the same +5V power supply and tie the analog and
digital grounds together. As noted in the electrical character-
istics, the ADS7807 uses 90% of its power for the analog
circuitry. The ADS7807 should be considered as an analog
component.
0.1
1
10
100
“CAP” Pin Value (µF)
The +5V power for the A/D converter should be separate
from the +5V used for the system’s digital logic. Connecting
FIGURE 11. Power-Down to Power-Up Time vs Capacitor
Value on CAP.
VDIG (pin 28) directly to a digital supply can reduce converter
performance due to switching noise from the digital logic. For
best performance, the +5V supply can be produced from
whatever analog supply is used for the rest of the analog
signal conditioning. If +12V or +15V supplies are present, a
simple +5V regulator can be used. Although it is not sug-
gested, if the digital supply must be used to power the
converter, be sure to properly filter the supply. Either using a
filtered digital supply or a regulated analog supply, both VDIG
and VANA should be tied to the same +5V source.
The output of the buffer is capable of driving up to 1mA of
current to a DC load. Using an external buffer will allow 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 will cause performance degra-
dation of the converter.
REFERENCE
AND POWER-DOWN
GROUNDING
Three ground pins are present on the ADS7807. DGND is the
digital supply ground. AGND2 is the analog supply ground.
The ADS7807 has analog power-down and reference power
down capabilities via PWRD (pin 25) and REFD (pin 26),
respectively. PWRD and REFD HIGH will power-down all
analog circuitry maintaining data from the previous conver-
sion 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 50µW. Power recovery is
typically 1ms, using a 2.2µF capacitor connected to CAP.
Figure 11 shows power-down to power-up recovery time
relative to the capacitor value on CAP. With +5V applied to
AGND1 is the ground to which all analog signals internal to the
A/D converter are referenced. AGND1 is more susceptible to
current induced voltage drops and must have the path of
least resistance back to the power supply.
All the ground pins of the A/D converter should be tied to an
analog ground plane, separated from the system’s digital
logic ground, to achieve optimum performance. Both analog
and digital ground planes should be tied to the “system”
ground as near to the power supplies as possible. This helps
to prevent dynamic digital ground currents from modulating
the analog ground through a common impedance to power
ground.
VDIG, the digital circuitry of the ADS7807 remains active at all
times, regardless of PWRD and REFD states.
PWRD
PWRD HIGH will power-down all of the analog circuitry
except for the reference. Data from the previous conversion
will be maintained in the internal registers and can still be
read. With PWRD HIGH, a convert command yields mean-
ingless data.
SIGNAL CONDITIONING
The FET switches used for the sample hold on many CMOS
A/D converters release a significant amount of charge injec-
tion which can cause the driving op amp to oscillate. The
ADS7807
16
SBAS022B
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