AS1520/AS1521
Data Sheet - Detailed Description
8 Detailed Description
Analog Input
The equivalent input circuit (Figure 20) shows the input architecture: track/hold circuitry, input multiplexer, input com-
parator, switched-capacitor DAC, and internal reference. A flexible serial interface provides easy connections to vari-
ous microprocessors.
Figure 20. Equivalent Input Circuit
REF
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CHOLD
13pF
+
–
–
+
AIN+
AIN-
CSWITCH
11pF
Sample
Switch
Comparator
COM
CHOLD
13pF
RIN
Analog Input
Multiplexer
–
+
CSWITCH
11pF
CSWITCH includes all parasitics
The input tracking circuitry has a 6MHz small-signal bandwidth, thus it is possible to under-sample (digitize high-speed
transient events) and measure periodic signals modulated at frequencies exceeding the AS1520/AS1521 sampling
rate.
Note: To avoid high-frequency signals being aliased into the frequency band of interest, antialias filtering is recom-
mended
Input Protection
Internal protection diodes (which clamp the analog input to VDD1 and GND) allow the channel inputs to swing from
(GND to 0.3V) to (VDD1 + 0.3V) without damaging the devices. However, for accurate conversions near full scale, the
inputs must not exceed VDD1 by more than 50mV or be lower than GND by 50mV.
Note: If the analog input exceeds 50mV beyond the supply voltage, do not allow the input current to exceed 2mA.
Track/Hold
The track/hold stage enters tracking mode on the rising edge of SCLK which clocks in bit MODE of the 8-bit control
byte (see Figure 21 on page 17). The track/hold stage enters hold mode on the falling clock edge after bit PD0 of the 8-
bit control byte has been shifted in.
The time required for the track/hold circuit to acquire an input signal is a function of how quickly the input capacitance
is charged. If the input signal source impedance is high, the acquisition time lengthens. The acquisition time (tACQ) is
the maximum time the device takes to acquire the signal and is also the minimum time needed for the signal to be
acquired.
tACQ is never less than 390ns (AS1520) or 520ns (AS1521), and is calculated by:
tACQ = 7(RS + RIN)18pF
(EQ 1)
Where::
RIN = 800Ω
RS = the source impedance of the input signal.
Note: Source impedances below 2kΩ do not significantly affect the AC performance of the devices.
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