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amplifying dc signal for measuring

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elpajuo

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I have a low current circuit where I want to log the current every 10s for a long period of time

The current jumps from about 100uA to 25mA.

I have a data logger that can log from 0 to 20 V, so I can add a 1 ohm resistor with the volt logger to measure current but it needs to be much higher.

If I amplify the current with a gain stage of 200 from 25mA to 5A, how can I revert back to normal operating current of the circuit? do I need a buffer stage?

My focus is on being able to read when the 25mA signal is on with the datta logger, the 100uA can be shown as 0 and does not matter

Input appreciated
 

alexan_e

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You can use a differential amplifier assuming that the input voltage is in the opamp range

DiffAmp.jpg
The gain is set by Rf/R1

or you can use an Instrumentation Amplifier like AD620
(AD620)

You can use a shunt resistor that creates a voltage drop with the current and then use any of the above to amplify the voltage
Alex
 

elpajuo

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Thanks for the reply

Ok, so I can use the op amp to make the gain into something noticeable for the datta logger.

I am trying to measure how many times a battery powered device goes into sleep mode (100uA) and transmit mode (25mA) over a long period of time with the data logger. So once I amplify the signal to eg 5V how can I lower it back to the 25mV range in order for the circuit not to get damaged?



This is what I am thinking of


I only have access to the two battery terminals which receive a constant voltage under normal operation.
 

alexan_e

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So once I amplify the signal to eg 5V how can I lower it back to the 25mV range in order for the circuit not to get damaged?
What will be damaged?

For example with the 1ohm resistor and a differential amplifier with a gain of 50 you can get 0.025*1R=0.025*50=1.25v
this can be easily detected from your logger.

Alex

I mean something like
bat.gif

assuming that the battery - is also the ground
 

starsunmoon

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I was thinking using a low input offset comparator - if you just need to know low (uA) or high (25 mA) levels.
 

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