iC-MQ PROGRAMMABLE 9-BIT
Sin/Cos INTERPOLATION IC WITH RS422 DRIVER
Rev D4, Page 37/39
APPLICATION HINTS
In-circuit programming of the EEPROM
A phase error between the sine and cosine signals
Access to the EEPROM is unhindered when the iC- (a deviation in phase shift from the ideal 90°) has the
MQ supply voltage is kept below power down reset most marked influence on the absolute angle error at
threshold VDDoff. In this case an EEPROM which op- 0°, 90°, 180° and 270°. The greatest effect on the tran-
erates at a supply voltage of 2.5 V and above is re- sition distance is noted at 45°, 135°, 225° and 315°.
quired. If 3.3 V are necessary to power the EEPROM, iC-MQ’s phase correction feature permits a step size of
iC-MQ’s supply voltage can be raised at a maximum 0.64° so that incorrect compensation by 1 LSB would
to power on threshold VDDon; this must occur without increase the absolute angle error by ca. 0.64°. The
overshooting.
transition distance would then vary by +/- 1.1 %.
The supply voltage provided by pins VDDS and GNDS In a perfect signal conditioning procedure it can be as-
can be used to power the EEPROM; shutdown only sumed that the residual error constitutes half a com-
occurs with reverse polarity. Here, the load-dependent pensation step respectively. With this, in theory iC-
voltage drop at both switches must be taken into ac- MQ would achieve an absolute angle accuracy of ca.
count; see Vs(VDDS) and Vs(GNDS) in the Electrical 0.5°, with the transition distance varying by ca. +/-
Characteristics, C01 and C02.
1.5 %. The linearity error of the interpolator must also
be taken into consideration; this increases the absolute
angle error by ca. 0.12° and the variation in transition
Absolute angle accuracy and edge jitter
The precise setting of the signal conditioning unit for distance by 0.4 %. With ideal, almost static input sig-
correction of the analog input signals is crucial to the nals iC-MQ then obtains an absolute angle accuracy
result of interpolation; the absolute angle error ob- of 0.62° and a variation in transition distance of under
tained determines the minimum signal jitter. Here, the 2 %.
effect on the transition distance of the A/B output sig-
nals is not always the same but instead dependent on Information on the demo board
the absolute phase angle of the input signals. The fol- The default delivery status of demo board EVAL MQ1D
lowing gives an example for an interpolation factor of is such that it expects differential sine/cosine signals at
100, i.e. 400 edges per sine period.
inputs X3 to X6 with an amplitude of 125 mV, i.e.
The offset error in the cosine signal has the strongest
effect on the absolute angle error at 90° and 270°; at 0°
and 180° its influence on the transition distance is the
most marked. With a range setting of OR1 = OR2 = 00
and VOSSC = 01 the offset error can be compensated
for by an increment of 3.9 mV. If the offset has been
compensated for incorrectly by one step (1 LSB), the
absolute angle error would increase by ca. 0.45° and
V(X4) = 2.5 V + 0.125 Vsin(ϕt)
V(X3) = 2.5 V − 0.125 Vsin(ϕt)
V(X5) = 2.5 V + 0.125 Vsin(90 + ϕt)
V(X6) = 2.5 V − 0.125 Vsin(90 + ϕt)
the transition distance vary by approximately +/- 0.8 %. Outputs PA, NA, PB and NB generate a differential
Similar conditions apply to the sine signal, with the sole A/B signal with an angle resolution of 4 (an interpo-
difference that the maxima would be shifted by 90°.
lation factor of 1). When high sine input frequencies
are applied or the resolution is increased, the mini-
An error in amplitude has the strongest effect on the mum phase distance (MTD), short-circuit current limit
absolute angle error at 45°, 135°, 225° and 315°; the (SIK) and driver slew rate (SSR) must be adjusted to
biggest change in the transition distance can be ob- meet requirements. For example, a minimum phase
served at 0°, 90°, 180° and 270°. iC-MQ can compen- distance of MTD = 8 should be selected with a resolu-
sate for the amplitude ratio in steps of 1.5 % so that tion of 200 (an interpolation factor of 50) when input
incorrect compensation by 1 LSB would increase the frequencies of up to 20 kHz are to be applied.
absolute angle error by ca. 0.42°. The transition dis-
tance would then vary by +/- 1.5 %.
ICHAUS [ IC-HAUS GMBH ]
