200Ω
39.8kΩ
9.9kΩ
VIN
CDAC (High Impedance)
(0.3125V to 2.8125V)
20kΩ
40kΩ
66.5kΩ
+5V
100Ω
+2.5V
+2.5V
200Ω
39.8kΩ
9.9kΩ
CDAC (High Impedance)
(0.3125V to 2.8125V)
33.2kΩ
100Ω
20kΩ
40kΩ
VIN
+2.5V
+2.5V
200Ω
39.8kΩ
9.9kΩ
VIN
CDAC (High Impedance)
(0.3125V to 2.8125V)
33.2kΩ
100Ω
20kΩ
40kΩ
+2.5V
+2.5V
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.
ZCAP
CAP
(Pin 4)
CDAC
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).
Buffer
Internal
REF
Reference
(Pin 5)
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.
ZREF
PWRD 0
REFD 0
PWRD 0
REFD 1
PWRD 1
REFD 0
PWRD 1
REFD 1
REF
REF (pin 5) is an input for an external reference or the output
ZCAP (Ω)
ZREF (Ω)
1
1
200
6k
200
for the internal 2.5V reference. A 2.2µF 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.
6k
100M
100M
FIGURE 10. Characteristic Impedances of Internal Buffer.
CAP
CAP (pin 4) is the output of the internal reference buffer. A
2.2µF 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
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.
®
ADS7806
15
BB [ BURR-BROWN CORPORATION ]
