iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC5/36
AB Signal Path
The AB Signal Path consists of the blocks between
the analog PGAs and the output generator. The front
end of the AB signal path is shown below.
Analog Offset
Correction
Digital Offset
Correction
Digital Gain
Match
Correction
Analog Gain
Digital Phase Correction
SIN+
SIN–
Sin Output
Cos Output
ADC
ADC
aoffs
aoffc
±1.4V max
±5800 max
(digital)
doffs
doffc
dgains
dgainc
again
dphase
COS+
COS–
Figure 3: AB Signal Path Front End
an output value of approximately ±5800 ADC in-
crements. Above these values, ADC distortion in-
creases and produces a corresponding reduction in
interpolation accuracy.
Analog Gain
Analog gain is applied to the raw Sin/Cos sensor
inputs by two programmable gain amplifiers
(PGAs) with a range of 6 – 45 dB in 3dB steps. The
actual analog gain, again, is determined by the pa-
rameter AGAIN in the PAR block in internal
memory. The same gain is applied to both channels.
Digital Offset Correction
Individual fine offset correction is next applied to
the digital Sin and Cos ADC outputs. A range of
fine offset correction of ±125mV in 244µV steps at
the ADC inputs is available. The actual digital offset
corrections for the Sin and Cos channels, doffs and
docffc respectively, are determined by the parame-
ters DOFFS and DOFFC in the PAR block in inter-
nal memory.
Analog Offset Correction
Individual analog offset correction is applied to the
output of the two analog PGAs This is a coarse cor-
rection with a range of ±3.1V in 100mV steps. The
actual analog offset corrections for the Sin and Cos
channels, aoffs and aocffc respectively, are deter-
mined by the parameters AOFFS and AOFFC in the
PAR block in internal memory.
The effective digital offset correction referenced to
the input Sin/Cos signals can be calculated as
The effective analog offset correction referenced to
the input Sin/Cos signals, aoffins and aoffinc is a
function of the analog gain and can be calculated as
doffs
doffc
doffins
; doffinc
again
again
aoffs
aoffc
Digital Gain Match Correction
aoffins
; aoffinc
again
again
Next, digital gain match correction is applied to ei-
ther the Sin or Cos channel to correct any mismatch
between the two signal amplitudes. The actual digi-
tal gain values for the Sin and Cos channels, dgains
and dgainc respectively, are determined by the
DGAIN parameter in the PAR block in internal
memory. A range of ±1.25 in steps of 0.000244 at
the ADC inputs is available.
Analog-to-Digital Converters (ADCs)
The two ADCs convert the conditioned analog Sin
and Cos signals into 14-bit digital values for further
processing. The remainder of the AB signal path is
completely digital.
The ADCs in the iC-TW8 operate best with inputs
limited to ±1.4V maximum, which corresponds to
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