200Ω
39.8kΩ
9.9kΩ
VIN
CDAC
(0.3125V to 2.8125V)
20kΩ
40kΩ
66.5kΩ
+5V
100Ω
+2.5V
+2.5V
200Ω
39.8kΩ
9.9kΩ
CDAC
(0.3125V to 2.8125V)
33.2kΩ
100Ω
20kΩ
40kΩ
VIN
+2.5V
+2.5V
200Ω
39.8kΩ
9.9kΩ
VIN
CDAC
(0.3125V to 2.8125V)
33.2kΩ
100Ω
20kΩ
40kΩ
+2.5V
+2.5V
FIGURE 9. Circuit Diagrams Showing External and Internal Resistors.
tied HIGH will power-down the internal reference reducing
the overall power consumption of the ADS7807 by approxi-
mately 5mW.
ZCAP
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).
CAP
(Pin 4)
CDAC
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.
Buffer
Internal
Reference
REF
(Pin 5)
ZREF
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
PWRD 0
REFD 0
PWRD 0
REFD 1
PWRD 1
REFD 0
PWRD 1
REFD 1
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.
ZCAP (Ω)
ZREF (Ω)
1
1
200
6k
200
6k
100M
100M
FIGURE 10. Characteristic Impedances of Internal Buffer.
CAP
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 (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
ADS7807
SBAS022C
15
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