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# how to write equation with voltage from channel 1 as input in math function of adalm 2000 (scopy)?

Sir

i want to plot current across a  diode  and resistor  .but i am confused to plot vi chara by math function.what is" to and t1"" in math function used for.is their any other way to plot current directly.what is meant by usage of shunt resistor to plot current.

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• Hello,

You can't measure current directly in Scopy, but you can use a shunt resistor. This means that you have to connect a resistor  in series with the load ( in this case the diode) and measure the voltage drop across it. To obtain the current apply Ohm's law (V=i*R).

t0 and t1 are the variable signals that you get on oscilloscope CH1 and CH2 respectively.

Suppose that you measure the voltage across the shunt resistor on CH1, in the math channel you will divide t0 ( voltage represented on CH1) to the resistance of the shunt resistor and obtain the current through the load.

https://wiki.analog.com/university/courses/electronics/ohm_law

https://wiki.analog.com/university/courses/electronics/electronics-lab-2

https://wiki.analog.com/university/courses/electronics/labs

Regards,

Andreea

• sir i need to plot vi chara of components in my pcb( to find the damage of my board) .can i use any type of shunt resistor?( when i used high value of resistance since the current is minute it is not able to plot value by math function)is their any problem of loading of ckt or impedence matching problem related with resistor usage.

• It is difficult for us to provide a detailed response not knowing more details about the circuit you are trying to measure. But in general, if the current is very small, then as you indicated a larger value is needed for the shunt.

The ADALM2000 differential inputs are useful for measuring the voltage difference across a shunt but there is little voltage gain. On the lowest voltage range, +/- 2.5 V, the smallest voltage that can be resolved (LSB) is 1.4 mV. The very wide bandwidth of the ADALM2000 is also problematic from a noise point of view. Thus this is not necessarily a good method of measuring current especially low value currents.

The ideal solution, most like an actual ammeter, is to have a 0 ohm shunt. To use a low value shunt you need to add some gain. For this purpose, ADI makes specific current shunt monitor ICs such as the AD8210 which we include in the ADALP2000 Analog Parts Kit: https://wiki.analog.com/university/tools/adalp2000/parts-index

This IC has a fixed gain of 20 but there are other parts in the same family with other gains. A gain of 20 will allow you to use a shunt 20 times smaller in value with the ADALM2000.

Another method of adding gain is to use an instrument amplifier such as the AD8226, also in the Kit. Using an inamp you can set the gain by changing the gain setting resistor. Gains of 100 or more are possible.

Hopes this helps.

Doug

• It is difficult for us to provide a detailed response not knowing more details about the circuit you are trying to measure. But in general, if the current is very small, then as you indicated a larger value is needed for the shunt.

The ADALM2000 differential inputs are useful for measuring the voltage difference across a shunt but there is little voltage gain. On the lowest voltage range, +/- 2.5 V, the smallest voltage that can be resolved (LSB) is 1.4 mV. The very wide bandwidth of the ADALM2000 is also problematic from a noise point of view. Thus this is not necessarily a good method of measuring current especially low value currents.

The ideal solution, most like an actual ammeter, is to have a 0 ohm shunt. To use a low value shunt you need to add some gain. For this purpose, ADI makes specific current shunt monitor ICs such as the AD8210 which we include in the ADALP2000 Analog Parts Kit: https://wiki.analog.com/university/tools/adalp2000/parts-index

This IC has a fixed gain of 20 but there are other parts in the same family with other gains. A gain of 20 will allow you to use a shunt 20 times smaller in value with the ADALM2000.

Another method of adding gain is to use an instrument amplifier such as the AD8226, also in the Kit. Using an inamp you can set the gain by changing the gain setting resistor. Gains of 100 or more are possible.

Hopes this helps.

Doug

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