AS1528 datasheet(10/21 Pages) AMSCO

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AS1528/AS1529

Datasheet - D e t a i l e d D e s c r i p t i o n

8 Detailed Description

The AS1528/AS1529 employ a successive approximation conversion (SAR) technique and integrated track/hold cir-

cuitry to convert analog signals into 10-bit digital output. The serial interface provides easy interfacing to microproces-

sors. Figure 17 shows the simplified internal structure for the AS1529 (2-channels, single ended) and the AS1528

(1-channel, true differential).

True Differential Analog Input Track/Hold

The equivalent circuit of Figure 17 shows the device input architecture which is composed of track/hold circuitry, input

multiplexer, comparator, and switched-capacitor DAC. The track/hold circuitry enters its tracking mode on the rising

edge of CNVST. The positive input capacitor is connected to AIN1 or AIN2 (AS1529) or AIN+ (AS1528). The negative

input capacitor is connected to GND (AS1529) or AIN- (AS1528).

Figure 17. Equivalent Input Circuit

The track/hold circuitry enters its hold mode on the falling edge of CNVST and the difference between the sampled

positive and negative input voltages is converted. The time required for the track/hold to acquire an input signal is

determined by how quickly its input capacitance is charged. If the input signal’s source impedance is high, the acquisi-

tion time lengthens, and CNVST must be held high for a longer period of time. The acquisition time (tACQ) is the maxi-

mum time needed for the signal to be acquired, plus the power-up time. tACQ is calculated by:

tACQ = 9 x (RS + RIN) x 20pF + tPWR

(EQ 1)

Where:

RS is the source impedance of the input signal;

RIN = 1.5k

Ω;

tPWR of 1µs is the power-up time of the device.

Note: tACQ is never less than 1.4µs and any source impedance below 300. does not significantly affect the AS1528/

AS1529 AC performance. A high-impedance source can be accommodated either by lengthening tACQ or by

placing a 1µF capacitor between the positive and negative analog inputs.

Selecting AIN1 or AIN2 (AS1529)

Select one of the AS1529 two positive input channels using the CNVST pin (see page 3). If AIN1 is selected (see Fig-

ure 18), drive CNVST high to power up the AS1529 and place the track/hold circuitry in track mode with AIN1 con-

nected to the positive input capacitor. Hold CNVST high for tACQ to fully acquire the signal. Drive CNVST low to place

the track/hold circuitry in hold mode. The AS1529 then performs a conversion and shutdown automatically. The MSB is

available at DOUT after 3.7µs. Data can then be clocked out using SCLK. Clock out all 12 bits of data before driving

CNVST high for the next conversion. If all 12 bits of data are not clocked out before CNVST is driven high, AIN2 is

selected for the next conversion.

–

Comparator

Hold

10-Bit Capacitive DAC

Track

AIN2

AIN1/AIN+

VDD/2

RIN-

CIN+

REF

GND

GND/AIN-

CIN-

Hold

RIN+

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