How to take time average of high side current sense amplifier output?

[M. Noone]

High - I'm planning on using a MAX4372H
(http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2104) high side current
amplifier to monitor current use in a small motor. I plan on using a
..025 ohm 0805 1% resistor (digi-key WSLA-.025CT-ND, see here:
http://dkc3.digikey.com/PDF/T053/1168.pdf) for current sensing. I will
also have a .1uf ceramic decoupling cap from vcc to gnd. The output
from the MAX4372H will be attached to a 10b ADC.

My primary question is this: I plan on sampling from this ADC at
probabaly about 1khz. When I take each sample I'd like the sample to be
the average current since the last sample was taken - so I'd like it to
be the average current over the last 1/(1khz) or the last 1ms.
Implementation of this is where I get a bit confused. Would this be an
application for a low pass filter? Or something else? Can anybody
suggest a circuit that would work for this?

Secondary question: is there anything wrong with my plan? At that small
of a resistance - do I need to worry about things like the resistance
of the traces, the solder, or the pads? I plan on having the current
sensing resistor directly between the two current sensing pins (RS- and
RS+) - so trace length between the pins and the resistor will be maybe
~1-2mm or so.

Thanks for your help,

-M. Noone

 

[Dan Hollands]

High - I'm planning on using a MAX4372H
(http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2104) high side current
amplifier to monitor current use in a small motor. I plan on using a
.025 ohm 0805 1% resistor (digi-key WSLA-.025CT-ND, see here:
http://dkc3.digikey.com/PDF/T053/1168.pdf) for current sensing. I will
also have a .1uf ceramic decoupling cap from vcc to gnd. The output
from the MAX4372H will be attached to a 10b ADC.

My primary question is this: I plan on sampling from this ADC at
probabaly about 1khz. When I take each sample I'd like the sample to be
the average current since the last sample was taken - so I'd like it to
be the average current over the last 1/(1khz) or the last 1ms.
Implementation of this is where I get a bit confused. Would this be an
application for a low pass filter? Or something else? Can anybody
suggest a circuit that would work for this?

Secondary question: is there anything wrong with my plan? At that small
of a resistance - do I need to worry about things like the resistance
of the traces, the solder, or the pads? I plan on having the current
sensing resistor directly between the two current sensing pins (RS- and
RS+) - so trace length between the pins and the resistor will be maybe
~1-2mm or so.

Thanks for your help,

-M. Noone

An R in the line from maxim to ADC with a cap to ground at the input of the
ADC would "average" out the signal i.e. remove high freq components so that
that you don't get false readings if the sample falls on a current spike.

Only other concern is whether the .025 ohm gives the correct volts needed at
the expected current and to make sure resistor can handle the power. You
will need some way to calibrate the circuit - either with a pot or in the
processing of the digital output. That should take care of the actual
resistor value and anyother minor resistances.

Dan
--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com

 

[M. Noone]

An R in the line from maxim to ADC with a cap to ground at the input of the
ADC would "average" out the signal i.e. remove high freq components so that
that you don't get false readings if the sample falls on a current spike.

Only other concern is whether the .025 ohm gives the correct volts needed at
the expected current and to make sure resistor can handle the power. You
will need some way to calibrate the circuit - either with a pot or in the
processing of the digital output. That should take care of the actual
resistor value and anyother minor resistances.

Dan
--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com

Hi Dan - can you reccomend how to calculate values for the resistor and
capacitor?

About the choice of 0.025 ohms - max current should be under 500ma, but
I want to be able to sense past that. Vref on the ADC is 2.048V. so to
calculate max current: Imax * 0.025 * 100 = 2.048, which gives Imax =
819ma. The current sense resistor is rated for an 0.125W. So
0.819^2*.025 = 0.0168W, so I have plenty of room there.

Thanks,

-M. Noone

 

[Mark]

regarding your question about the low resistance and the traces....

look up "Kelvin connection"

you need to understand and use this type of connection for this
application....

this type of connection will eliminate the problem of the trace
resistance

Mark