AD2S80A
4. Maximum Tracking Rate (R6)
COMPONENT SELECTION
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.
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.
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.
6. 32 ×1010
R6 =
Ω
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.
T × n
where n = bits per revolution
= 1,024 for 10 bits resolution
For more detailed information and explanation, see section “CIR-
CUIT FUNCTIONS AND DYNAMIC PERFORMANCE.”
= 4,096 for 12 bits
= 16,384 for 14 bits
= 65,536 for 16 bits
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.
5. Closed-Loop Bandwidth Selection (C4, C5, R5)
a. Choose the closed-loop bandwidth (fBW) required
ensuring that the ratio of reference frequency to band-
width does not exceed the following guidelines:
Resolution
Ratio of Reference Frequency/Bandwidth
2.5 : 1
10
12
14
16
4
6
: 1
: 1
: 1
Values should be chosen so that
7.5
C1 = C2 = 15 kΩ ≤ R1 = R2 ≤ 56 kΩ
Typical values may be 100 Hz for a 400 Hz reference frequency
and 500 Hz to 1000 Hz for a 5 kHz reference frequency.
1
C1 = C2 =
b. Select C4 so that
21
2π R1 fREF
C4 =
F
and fREF = Reference frequency
(Hz)
2
R6× fBW
with R6 in Ω and fBW, in Hz selected above.
This filter gives an attenuation of three times at the input to
the phase sensitive detector.
c. C5 is given by
2. Gain Scaling Resistor (R4)
If R1, C2 are used:
C5 = 5 × C4
d. R5 is given by
4
EDC
100 ×10–9
1
3
R5 =
Ω
R4 =
×
Ω
2 × π × fBW × C5
where 100 × 10–9 = current/LSB
If R1, C2 are not used:
6. VCO Phase Compensation
The following values of C6 and R7 should be fitted.
EDC
100 ×10–9
C6 = 470 pF, R7 = 68 Ω
R4 =
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.
where EDC = 160 × 10–3 for 10 bits resolution
= 40 × 10–3 for 12 bits
= 10 × 10–3 for 14 bits
= 2.5 × 10–3 for 16 bits
If fitted, the following values of R8 and R9 should be used:
R8 = 4.7 MΩ, R9 = 1 MΩ potentiometer
= 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,
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.
R3 = 100 kΩ
1
C3 >
F
The potentiometer may be replaced with select on test resistors
if preferred.
R3 × fREF
with R3 in Ω.
–8–
REV. B
ADI [ ADI ]
