ALD500ASC datasheet(3/11 Pages) ALD

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ALD500AU/ALD500A/ALD500

Advanced Linear Devices

(integrate cycle)

(1)

VX = VINT - (VREF . tDINT / RINT . CINT)

(deintegrate cycle)

(2)

Combining equations 1 and 2 results in:

VIN / VREF = -tDINT / tINT

(3)

where:

= An offset voltage used as starting voltage

VINT = Voltage change across CINT during tINT and

during tDINT (equal in magnitude)

VIN

= Average, or an integrated, value of input voltage

to be measured during tINT (Constant VIN)

tINT = Fixed time period over which unknown voltage is

integrated

tDINT = Unknown time period over which a known

reference voltage is integrated

VREF = Reference Voltage

CINT = Integrating Capacitor value

RINT = Integrating Resistor value

Actual data conversion is accomplished in two phases: Input

Signal Integration Phase and Reference Voltage Deintegration

Phase.

The integrator output is initialized to 0V prior to the start of

Input Signal Integration Phase. During Input Signal Integration

Phase, internal analog switches connect VIN to the buffer

input where it is maintained for a fixed integration time period

(tINT). This fixed integration period is generally determined by

a digital counter

controlled by a crystal oscillator. The

application of VIN causes the integrator output to depart 0V at

a rate determined by VIN and a direction determined by the

polarity of VIN.

The Reference Voltage Deintegration Phase is initiated

immediately after tINT, within 1 clock cycle. During Reference

Voltage Deintegration Phase, internal analog switches connect

a reference voltage having a polarity opposite that of VIN to

the integrator input. Simultaneously the same digital counter

controlled by the same crystal oscillator used above is used to

start counting clock pulses.

The Reference Voltage

Deintegration Phase is maintained until the comparator output

inside the dual slope analog processor changes state, indicating

the integrator has returned to 0V. At that point the digital

counter is stopped. The Deintegration time period (tDINT), as

measured by the digital counter, is directly proportional to the

magnitude of the applied input voltage.

After the digital counter value has been read, the digital

counter, the integrator, and the auto zero capacitor are all

reset to zero through an Integrator Zero Phase and an Auto

Zero Phase so that the next conversion can begin again. In

practice, this process is usually automated so that analog-to-

digital conversion is continuously updated. The digital control

is handled by a microprocessor or a dedicated logic controller.

The output, in the form of a binary serial word, is read by a

microprocessor or a display adapter when desired.

Figure 2. Basic Dual-Slope Converter

CINT

VINT

RINT

SWITCH DRIVER

CONTROL

LOGIC

POLARITY CONTROL

REF

SWITCHES

INTEGRATOR

COMPARATOR

PHASE

CONTROL

ANALOG

INPUT

(VIN)

VIN

≈ VFULL SCALE

VIN

≈ 1/2 VFULL SCALE

tINT

tDINT

VINT = 4.1V MAX

Figure 2. Basic Dual-Slope Converter

VOLTAGE

REFERENCE

COUT

POLARITY

DETECTION

tDINT

≈ 0

MICROCONTROLLER

(CONTROL LOGIC

+ COUNTER)

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