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.
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
consumes approximately 5mW.
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.
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 specifica-
tions, the ADS7806 uses 90% of its power for the analog
circuitry. The ADS7806 should be considered as an analog
component.
7000
6000
5000
4000
3000
2000
1000
0
The +5V power for the A/D should be separate from the +5V
used for the system’s digital logic. Connecting 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 what-
ever 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 suggested, 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.
0.1
1
10
100
“CAP” Pin Value (µF)
FIGURE 11. Power-Down to Power-Up Time vs Capacitor
Value on CAP.
GROUNDING
Three ground pins are present on the ADS7806. DGND is the
digital supply ground. AGND2 is the analog supply ground.
AGND1 is the ground to which all analog signals internal to
the A/D are referenced. AGND1 is more susceptible to current
induced voltage drops and must have the path of least
resistance back to the power supply.
REFERENCE
AND POWER DOWN
The ADS7806 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.
See Figure 11 for power-down to power-up recovery time
relative to the capacitor value on CAP. With +5V applied to
VDIG, the digital circuitry of the ADS7806 remains active at
all times, regardless of PWRD and REFD states.
All the ground pins of the A/D 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.
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
amount of charge injection due to the sampling FET switch
on the ADS7806 is approximately 5-10% of the amount on
similar ADCs with the charge redistribution DAC (CDAC)
architecture. There is also a resistive front end which attenu-
ates any charge which is released. The end result is a
minimal requirement for the drive capability on the signal
conditioning preceding the A/D. Any op amp sufficient for
the signal in an application will be sufficient to drive the
ADS7806.
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.
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
®
ADS7806
16
BB [ BURR-BROWN CORPORATION ]
