AD2S83

REV. E

–8–

COMPONENT SELECTION

The following instructions describe how to select the external

components for the converter in order to achieve the required

bandwidth and tracking rate. In all cases the nearest “preferred

value” component should be used, and a 5% tolerance will not

degrade the overall performance of the converter. Care should

be taken that the resistors and capacitors will function over the

required operating temperature range. The components should

be connected as shown in Figure 1.

Free PC compatible software is available to help users select the

optimum component values for the AD2S83, and display the transfer

gain, phase and small step response.

For more detailed information and explanation, see the Circuit

Functions and Dynamic Performance section.

1. HF Filter (R1, R2, C1, C2)

The function of the HF filter is to remove any dc offset and

to reduce the amount of noise present on the signal inputs to

the AD2S83, reaching the Phase Sensitive Detector and

affecting the outputs. R1 and C2 may be omitted—in which

case R2 = R3 and C1 = C3, calculated below—but their use

is particularly recommended if noise from switch mode

power supplies and brushless motor drive is present.

Values should be chosen so that

15 k

Ω≤ R1= R2 ≤ 56 kΩ

C1

= C2 =

1

2

π R1 f

REF

(Hz)

This filter gives an attenuation of three times at the input to

the phase sensitive detector.

2. Gain Scaling Resistor (R4) (See Phase Sensitive Demodula-

tor section.)

If R1, C2 are used:

R4

=

E

DC

100

× 10

1

3

Ω

where 100

If R1, C2 are not used:

R4

=

E

DC

100

× 10

= 160

= 40

= 10

= 2.5

= Scaling of the DC ERROR in volts/LSB

3. AC Coupling of Reference Input (R3, C3)

Select R3 and C3 so that there is no significant phase shift at

the reference frequency. That is,

R 3

= 100 kΩ

C 3

>

1

R 3

× f

REF

F

with R3 in

Ω.

4. Maximum Tracking Rate (R6)

The VCO input resistor R6 sets the maximum tracking rate

of the converter and hence the velocity scaling as at the max

tracking rate, the velocity output will be 8 V.

Decide on your maximum tracking rate, “T,” in revolutions

per second. When setting the value for R6, it should be

remembered that the linearity of the velocity output is

specified across 0 kHz–500 kHz and 500 kHz–1000 kHz.

The following conversion can be used to determine the

corresponding rps:

rps

=

VCO Rate (Hz )

2

N

Note that “T” must not exceed the maximum tracking rate

or 1/16 of the reference frequency.

R6

=

6.81

× 10

10

T

× n

Ω

where n = bits per revolution

= 1,024 for 10 bits resolution

= 4,096 for 12 bits

= 16,384 for 14 bits

= 65,536 for 16 bits

5. Closed-Loop Bandwidth Selection (C4, C5, R5)

ensuring that the ratio of reference frequency to band-

width does not exceed the following guidelines:

Resolution

Ratio of Reference Frequency/Bandwidth

10

2.5 : 1

12

4

: 1

14

6

: 1

16

7.5 : 1

Typical values may be 100 Hz for a 400 Hz reference fre-

quency and 500 Hz to 1000 Hz for a 5 kHz reference

frequency.

b.

Select C4 so that

C4

=

21

R6

× f

BW

2

F

with R6 in

c.

C5 is given by

C5

= 5 × C4

d.

R5 is given by

R5

=

4

2

×π × f

Ω

6. VCO Phase Compensation

The following values of C6 and R7 should be connected as

close as possible to the VCO output, Pin 41.

C6

= 390 pF, R7 = 3.3 kΩ

7. VCO Optimization

To optimize the performance of the VCO a capacitor, C7,

should be placed across the VCO input and output, Pins 40

and 41.

C7

= 150 pF