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AS1530/AS1531

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

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 AS1530/AS1531 sampling

rate.

Note: To avoid high-frequency signals being aliased into the frequency band of interest, antialias filtering is recom-

mended

Input Protection

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

the maximum time the device takes to acquire the signal and is also the minimum time needed for the signal to be

acquired.

tACQ = 9(RS + RIN)18pF

(EQ 1)

Where::

Ω

Note: Source impedances below 2k

Ω do not significantly affect the AC performance of the devices.

+

–

+

Comparator

11pF

13pF

–

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

REF

AIN+

11pF

+

13pF

–

Sample

Switch

Analog Input

Multiplexer

AIN-