AD2S80A

–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.

PG compatible software is available to help users select the optimum

component values for the AD2S80A, and display the transfer gain,

phase and small step response.

For more detailed information and explanation, see section “CIR-

CUIT FUNCTIONS AND DYNAMIC PERFORMANCE.”

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 AD2S80A, 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

(Hz)

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

the phase sensitive detector.

2. Gain Scaling Resistor (R4)

If R1, C2 are used:

R4

=

100

×

1

3

Ω

where 100

If R1, C2 are not used:

R4

=

E

DC

100

× 10

= 40

= 10

= 2.5

= Scaling of the DC ERROR in volts

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. Note that “T” must not exceed the maximum

tracking rate or 1/16 of the reference frequency.

R6

=

6. 32

× 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 frequency

and 500 Hz to 1000 Hz for a 5 kHz reference frequency.

b. Select C4 so that

C4

=

21

R6

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 fitted.

C6

= 470 pF, R7 = 68 Ω

7. Offset Adjust

Offsets and bias currents at the integrator input can cause an

additional positional offset at the output of the converter of

1 arc minute typical, 5.3 arc minutes maximum. If this can be

tolerated, then R8 and R9 can be omitted from the circuit.

If fitted, the following values of R8 and R9 should be used:

R8

= 4.7 MΩ, R9 = 1 MΩ potentiometer

To adjust the zero offset, ensure the resolver is disconnected

and all the external components are fitted. Connect the COS

pin to the REFERENCE INPUT and the SIN pin to the

SIGNAL GROUND and with the power and reference

applied, adjust the potentiometer to give all “0s” on the

digital output bits.

The potentiometer may be replaced with select on test resistors

if preferred.

CC

k

R

R

k

CC

Rf

REF

12

15

12

56

12

1

21

==

Ω ≤

=

≤

Ω

==

π

REV. D