Datasheet

ADS7807
15
SBAS022D
www.ti.com
The internal reference has approximately an 8ppm/°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 ADS7807 also has an internal buffer for the reference
voltage. Figure 10 shows characteristic impedances at the
input and output of the buffer with all combinations of power-
down and reference down.
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
FIGURE 9. Circuit Diagrams Showing External and Internal Resistors.
REF
REF (pin 5) is an input for an external reference or the output
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, as
shown in 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.
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
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.
39.8kΩ200Ω
V
IN
CDAC
(0.3125V to 2.8125V)
+5V
9.9kΩ
100Ω
20kΩ
40kΩ
+2.5V
66.5kΩ
+2.5V
39.8kΩ200Ω
CDAC
(0.3125V to 2.8125V)
V
IN
9.9kΩ
33.2kΩ
20kΩ
40kΩ
+2.5V
100Ω
+2.5V
39.8kΩ200Ω
V
IN
CDAC
(0.3125V to 2.8125V)
9.9kΩ
33.2kΩ
20kΩ
40kΩ
+2.5V
100Ω
+2.5V
CDAC
CAP
(Pin 4)
Z
CAP
Buffer
Internal
Reference
REF
(Pin 5)
Z
REF