10

Advanced Linear Devices

ALD500RAU/ALD500RA/ALD500R

APPLICATIONS AND DESIGN NOTES

Determination and Selection of System Variables

The procedure outlined below allows the user to determine the

values for the following ALD500RAU/ALD500RA/ALD500R

system design variables:

(1) Determine Input Voltage Range

(2) Clock Frequency and Resolution Selection

(3) Input Integration Phase Timing

(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 ALD500RAU/ALD500RA/ALD500R is

interfaced to a microcontroller. The microcontroller drives the

A, B inputs with I/O lines and monitors the comparator output,

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

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

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

line rejection.

function of the amount of voltage charge stored on the

phase and terminated when an integrator output zero-crossing

is detected.

In general, the maximum number of counts

voltage.

The desired full-scale input voltage and amplifier output

integrator amplifiers each have a full-scale current of 20

µA.

where:

(full count voltage)

= Integrating Resistor value

the range of between 50k

Ω to 150kΩ .

The integrating capacitor should be selected to maximize

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

Using the 20

µA buffer maximum output current, the value of

the integrating capacitor is calculated as follows:

=

Input Integration Phase Period

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.

polypropylene types). The slower the conversion rate, the

The reference deintegration voltage is calculated using:

The ALD500RAU/ALD500RA/ALD500R requires an external

metal film 100K

Ω resistor, 50 ppm/C. Any other loading must

be high impedance (

≥100MΩ).

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

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 4

µV.