AD7262BSTZ datasheet(21/33 Pages) AD

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AD7262

Data Sheet

Rev. B | Page 20 of 32

Comparator Application Details

The comparators on the AD7262/AD7262-5have been designed

with no internal hysteresis, allowing users the flexibility to add

external hysteretic if required for systems operating in noisy

environments. If the comparators on the AD7262/AD7262-5

are used with external hysteresis, some external resistors and

capacitors are required, as shown in Figure 28. The value of RF

and RS, the external resistors, can be determined using the

following equation, depending on the amount of hysteresis

required in the application:

HYS

where Cx_CxVCC = CA_CBVCC or CC_CDVCC.

The amount of hysteresis chosen must be sufficient to eliminate

the effects of analog noise at the comparator inputs, which may

affect the stability of the comparator outputs. The level of

hysteresis required in any system depends on the noise in the

system; thus, the values of RF and RS need to be carefully selected

to eliminate any noise effects. To increase the level of hysteresis in

the system, increase the value ofRS or RF. For example,RF = 10 MΩ,

RS = 1 kΩ give 330 μV of hysteresis with a Cx_CxVCC of 3.3 V; if

hysteresis is increased to 1 mV, RS = 3.1 kΩ. Incertainapplications,

a load capacitor (100 pF) may be required on the comparator

outputs to suppress high frequency transient glitches.

Cx–

Cx+

SENSOR

COUTx

Figure 28. Recommended Comparator Connection Diagram

APPLICATION DETAILS

The AD7262/AD7262-5 have been specifically designed to meet

the requirements of any motor control shaft position feedback

loop. The devices can interface directly to multiple sensortypes,

including optical encoders, magneto resistive sensors, and Hall

effect sensors. Flexible analog inputs that incorporate program-

mable gain ensure that identical board design can be used for a

variety of sensors, which results inreduced designcycles and costs.

The two simultaneous sampling ADCs are used to sample the

sine and cosine outputs from the sensor. No external buffering

is required between the sensor/transducer and the analog inputs

of the AD7262/AD7262-5. The on-chip comparators can be

used to monitor the pole sensors, which can be Hall effect sensors

or the inner tracks from an optical encoder.

Figure 29 shows how the AD7262/AD7262-5 can be used in a

typical application. An optical encoder is shown in Figure 29,

but other sensor types could as easily be used. Figure 29 indicates

a typical application configuration only, and there are several

other configurations that render equally effective results.

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