AS1520/AS1521
Data Sheet - Detailed Description
A 0.01µF bypass capacitor plus the internal 17kΩ reference resistor at REFADJ form an R/C filter with a 170µs time
constant. To achieve full 10-bit accuracy, 7 time constants (1.4ms) are required after power-up if the bypass capacitor
is fully discharged between conversions. Waiting this 1.4ms in reduced-power mode instead of normal operation mode
can further reduce power consumption. This is achieved by using the sequence shown in Figure 30 on page 21.
Figure 31 on page 21 shows the AS1521 power consumption for conversions using full power-down mode (PD1 = PD0
= 0 (see Table 12), an external reference, and the maximum clock speed. One dummy conversion to power-up the
device is required, but no wait-time is necessary to start the second conversion, thereby achieving lower power con-
sumption up to the full sampling rate.
Figure 30. Full Power-Down Timing Waveforms (AS1521)
1
0 0
1
1 0
Reduced-
1
0 0
Full Power-
1
DIN
Full Power-
Down
Power Mode
Down
REFADJ
1.22V
1.22V
γ = R/C = 17Ωk x 0.01µF
REF
2.5V
2.5V
2.2mA
2.2mA
2.2mA
IVDD1+IVDD2
+IVDD3
0.4mA
Normal Mode
Conversion
Normal Mode
Conversion
Normal Mode
Dummy
Conversion
0mA
0mA
Reduced
Power Mode
Full Power-
Down
Full Power-
Down
Figure 31. Average Supply Current vs. Sampling Rate (AS1521, FULLPD, and External Reference)
100000
1 Channel
100
0.1
0.001
0.01
0.1
1
10
100
Sampling Rate (ksps)
Reference
The AS1520/AS1521 can operate with the internal or an external reference.
Internal Reference
The internal reference is selected by placing a capacitor between REFADJ and GND. The internally trimmed 1.22V
bandgap voltage available at REFADJ is buffered with a gain of 2.045V/V to pin REF, where 2.5V are available. A
decoupling capacitor is needed at pin REF.
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