15
®
ADS7806
CAP
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 conversion
cycle. This capacitor also provides compensation for the
FIGURE 10. Characteristic Impedances of Internal Buffer.
PWRD 0
PWRD 0
PWRD 1
PWRD 1
REFD 0
REFD 1
REFD 0
REFD 1
Z
CAP
(
Ω
)
1
1
200
200
Z
REF
(
Ω
)
6k
100M
6k
100M
CDAC
CAP
(Pin 4)
Z
CAP
Buffer
Internal
Reference
REF
(Pin 5)
Z
REF
FIGURE 9. Circuit Diagrams Showing External and Internal Resistors.
pin 5, the internal reference can be bypassed; REFD (pin 26)
tied HIGH will power-down the internal reference reducing
the overall power consumption of the ADS7806 by approxi-
mately 5mW.
The internal reference has approximately 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 ADS7806 also has an internal buffer for the reference
voltage. See Figure 10 for characteristic impedances at the
input and output of the buffer with all combinations of
power down and reference down.
REF
REF (pin 5) is an input for an external reference or the output
for the internal 2.5V 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. See
Figure 10.
The range for the external reference is 2.3V to 2.7V and
determines the actual LSB size. Increasing the reference
voltage will increase the full scale range and the LSB size of
the converter which can improve the SNR.
39.8k
Ω
200
Ω
V
IN
CDAC (High Impedance)
(0.3125V to 2.8125V)
+5V
9.9k
Ω
100
Ω
20k
Ω
40k
Ω
+2.5V
66.5k
Ω
+2.5V
39.8k
Ω
200
Ω
CDAC (High Impedance)
(0.3125V to 2.8125V)
V
IN
9.9k
Ω
33.2k
Ω
20k
Ω
40k
Ω
+2.5V
100
Ω
+2.5V
39.8k
Ω
200
Ω
V
IN
CDAC (High Impedance)
(0.3125V to 2.8125V)
9.9k
Ω
33.2k
Ω
20k
Ω
40k
Ω
+2.5V
100
Ω
+2.5V
