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ALD500

PRECISION INTEGRATING ANALOG PROCESSOR

Advanced Linear Devices 

APPLICATIONS AND DESIGN NOTES

Determination and Selection of System Variables

The procedure outlined below allows the user to determine the

values for the following ALD500AU/ALD500A/ALD500 system

design variables:

(1) Determine Input Voltage Range

(2) Clock Frequency and Resolution Selection

(3) Input Integration Phase Timing

(4) Integrator Timing Components (RINT, CINT)

(5) Auto Zero and Reference Capacitors

(6) Voltage Reference

System Timing

Figure 3 and Figure 4 show the overall timing for a typical

system in which ALD500AU/ALD500A/ALD500 is interfaced

to a microcontroller. The microcontroller drives the A, B inputs

with I/O lines and monitors the comparator output, COUT,

using an I/O line or dedicated timer-capture control pin. It may

be necessary to monitor the state of the comparator output in

addition to having it control a timer directly during the Reference

Deintegration Phase.

There are four critical timing events: sampling the input

polarity; capturing the deintegration time; minimizing overshoot

and properly executing the Integrator Output Zero Phase.

Selecting Input Integration Time

For maximum 50/60 cycle noise rejection, Input Integration

Time must be picked as a multiple of the period of line

frequency. For example, tINT times of 33msec, 66msec and

100 msec maximize 60Hz line rejection, and 20msec, 40

msec, 80msec, and 100 msec maximize 50Hz line rejection.

Note that tINT of 100 msec maximizes both 60 Hz and 50Hz

line rejection.

INT and DINT Phase Timing

The duration of the Reference Deintegrate Phase (DINT) is a

function of the amount of voltage charge stored on the

integrator capacitor during INT phase, and the value of VREF.

The DINT phase must be initiated immediately following INT

phase and terminated when an integrator output zero-crossing

is detected. In general, the maximum number of counts

chosen for DINT phase is twice to three times that of INT phase

with VREF chosen as a maximum voltage relative to VIN. For

example, VREF = VIN(max)/2 would be a good reference

voltage.

Integrating Resistor (RINT )

The desired full-scale input voltage and amplifier output

current capability determine the value of RINT. The buffer and

integrator amplifiers each have a full-scale current of 20µA.

The value of RINT is therefore directly calculated as follows:

RINT = VIN MAX / 20 µA

where:

VIN MAX = Maximum input voltage desired

(full count voltage)

RINT = Integrating Resistor value

For minimum noise and maximum linearity, RINT should be in

the range of between 50kΩ to 150kΩ .

Integrating Capacitor (CINT)

The integrating capacitor should be selected to maximize

integrator output voltage swing VINT, for a given integration

time, without output level saturation. For +/-5V supplies,

recommended VINT range is between +/- 3 Volt to +/-4 Volt.

Using the 20µA buffer maximum output current, the value of

the integrating capacitor is calculated as follows:

CINT = (tINT) . (20 x 10-6) / VINT

where: tINT = Input Integration Phase Period

VINT = Maximum integrator output

voltage swing

It is critical that the integrating capacitor must have a very low

dielectric absorption, as charge loss or gain during conversion

directly converts into an error voltage. Polypropylene capacitors

are recommended while Polyester and Polybicarbonate

capacitors may also be used in less critical applications.

Reference (CREF) and Auto Zero (CAZ) Capacitors

CREF and CAZ must be low leakage capacitors (e.g.

polypropylene types). The slower the conversion rate, the

larger the value CREF must be. Recommended capacitor

values for CREF and CAZ are equal to CINT. Larger values for

CAZ and CREF may also be used to limit roll-over errors.

Calculate VREF

The reference deintegration voltage is calculated using:

VREF = (VINT) . (CINT) . (RINT) / 2(tINT)

Converter Noise

The converter noise is the total algebraic sum of the integrator

noise and the comparator noise. This value is typically 14 µV

peak to peak. The higher the value of the reference voltage,

the lower the converter noise. Such sources of noise errors

can be reduced by increased integration times, which effectively

filter out any such noise. If the integration time periods are

selected as multiples of 50/60Hz frequencies, then 50/60Hz

noise is also rejected, or averaged out. The signal-to-noise

ratio is related to the integration time (tINT) and the integration

time constant (RINT) (CINT) as follows:

S/N (dB) = 20 Log ((VINT / 14 x 10-6) . tINT /(RINT . CINT))

This converter noise can also be reduced by using multiple

samples and mathematically averaged. For example, taking

16 samples and averaging the readings result in a mathematical

(by software) filtering of noise to less than 4µV.

ALD500AU/ALD500A/ALD500

Advanced Linear Devices

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