Dale(294engr]
04-02-2004, 21:14
Allegro engineering sent me a preliminary advisory:
Allegro advises against use of 150uF filter on output pin !
Their concern: 150uF may cause unstable output or damage output stage.
They suggest adding integrator high Rin Op Amp with appropriate feedback.
I don't see ~1000 teams going that route.
Their 2nd recommendation: 411K series w/.1uF (10nF) on the output to A/D
(411K likely cause significant voltage division error unless Rin ~5-10M)
150uF presumably was added to "filter" noise from the 2kHz PWM motor.
I have been unable to obtain output stage detail to determine actual liability.
Anyone know the input resistance of the RC A/D ?
Allegro data sheet: 1-2 ohm load spec
guided me to place a 274 ohm resistor in series w/150uF to gnd
to effect ~125 mS (3 time constant) noise filter
which was born out in scope tests.
(274 ohms serves to limit current a bit isolating Out + establish RC)
Their engineer indicated output stage is a constant current output
which I would assume should have no problem with the large Cap., TBD ??
Caution:
2nd bend of brittle lead from outward angled plastic case & they break off!!
(observe caution pulling lead out of the PCBoard to make any changes)
(better to cut than pull tiny leads)
Allegro advises against use of 150uF filter on output pin !
Their concern: 150uF may cause unstable output or damage output stage.
They suggest adding integrator high Rin Op Amp with appropriate feedback.
I don't see ~1000 teams going that route.
Their 2nd recommendation: 411K series w/.1uF (10nF) on the output to A/D
(411K likely cause significant voltage division error unless Rin ~5-10M)
150uF presumably was added to "filter" noise from the 2kHz PWM motor.
I have been unable to obtain output stage detail to determine actual liability.
Anyone know the input resistance of the RC A/D ?
Allegro data sheet: 1-2 ohm load spec
guided me to place a 274 ohm resistor in series w/150uF to gnd
to effect ~125 mS (3 time constant) noise filter
which was born out in scope tests.
(274 ohms serves to limit current a bit isolating Out + establish RC)
Their engineer indicated output stage is a constant current output
which I would assume should have no problem with the large Cap., TBD ??
Caution:
2nd bend of brittle lead from outward angled plastic case & they break off!!
(observe caution pulling lead out of the PCBoard to make any changes)
(better to cut than pull tiny leads)