Revision 1.3 Joulescope JS220 User’s Guide Page 33 / 51
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P = V * I = V
2
/ R = I
2
* R
Energy is the integral of power over time.
When we talk about energy, we often mean the energy consumed over a specific time interval, which is
the definite integral:
The Joulescope host software performs the discrete-time Euler integration shown to the right. The
Joulescope software carefully manages the numerical precision to ensure that it computes energy
correctly. IEEE double-precision floating-point numbers do not provide sufficient precision.
Charge is the integral of current over time.
Charge and energy are mathematically similar. Both voltage and current contribute to energy, but only
current contributes to charge.
10.5. High impedance / low leakage
Joulescope measures currents at nanoamp resolutions and accuracies. Many contemporary devices
have input impedances of 1 MΩ to 10 MΩ, which is not good enough. At 3.3V, a 10 MΩ load draws:
3.3V / 10 MΩ = 330 nA
Joulescope features a combined input impedance and leakage current of approximately 1 GΩ. This high
input impedance allows Joulescope to make accurate and precise low current measurements.
10.6. Electrical isolation
The sensor side of Joulescope is electrically isolated from the host computer. This isolation allows you
to insert Joulescope into your test system without worrying about ground issues, including ground loops
and ground noise.
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