Voltage/current Spike Averaging Circuit

[noiprocs]

averaging circuit

I'm currently designing a circuit that will be used as a signal conditioner for a data-logger. It will be measuring the current consumption of a battery powered device. I plan to use a 4ohm resistor to convert the current into about 100mV (at 25mA) and then amplify with an opamp.

Thats all OK, heres the problem:
The device switches on/off a radio chip that uses 25mA. It uses about 10uA when it's turned off. It turns on for 100us; the sample period of the data-logger is 500us! Obviously the data-logger could completely miss the spike! What I would like to do is build some form of averaging circuit so that the spike will always be captured (maybe in 2 samples) so I am still able to work out the energy consumed by the device.

Does anyone have any ideas or calculations that I should be following?

thanks

 

[LvW]

current spike

I am not sure if I got all your requirements exactly. Nevertheless, my question:
Did you consider already a kind of MEDIAN filter ?
That´s a nonlinear device which is able to cancel impulsive noise spikes without influencing signal edges (which normally is the price for low pass filtering) ?

 

[FvM]

analog voltage averaging circuits

I understand, that want to measure the true average current rather than the peak current. In this case,a simple low pass would help.

 

[noiprocs]

canceling spikes in signals

My requirements are to measure the total power consumption over a long period of time.

A median filter looks like it will completely remove the spike which is not what i actually want to do.

FvM - I agree, a low pass filter may be the best option - I would need to ensure that the spike was flattened only to a level where I can measure it at a low sample rate. For each sample I would be able to work out the average power consumption over that sample period.

Do you have any idea how I would calculate the cutoff of an RC?

 

[FvM]

voltage spike measurement circuit

The minimum time constant depends on the acceptable error in average current measurement. You can calculate it exactly from the low pass pulse response. For a first order low pass, the maximum error is equal to the ratio of sampling period and time constant in a rough estimation, 5 ms gives 10% and 50 ms 1 % maximum error with 500 us sampling period. If you dont need a specific time resolution, first order should be O.K.