Power analyzer

Christoph TackChristoph Tack wrote 10/20/2019 at 09:26 • 3 min read • Like

One of the applications of a differential voltage probe is to measure voltage over a sense resistor.  For low power applications, the current drawn can be quite small, which leads us to using a high value sense resistor.  Unfortunately this also increases the source resistance of our voltage supply.  So when the current suddenly rises (e.g. because the device wakes up), this might lead to a sharp drop in supply voltage to the device, even powering it down in the worst case.
The Elektor MicroSupply (Elektorlabs 9/10 2019) is Elektor's solution to measure power of low power devices.  It has the following features:

The Elektor designer also thought of using a single instrumentation amplifier with switchable sense resistors, but they didn't see the benefit from it.

Cheaper implementation

Single differential amplifiers with multiple sense resistors

High side potentiostat to overcome the burden voltage (=voltage drop over the sense resistor).

The working principle of this power supply is quite simple.  Some calculations:

So the power supply output voltage is equal to the analog input voltage V1.

The sense resistor R2 is switchable with the gate of M1.  There could be many PMOS-transistor / sense resistor combinations mounted in parallel with M1 & R2.

Low current measuring techniques

Many sources refer to the transimpedance amplifier.  The advantage is that it avoids big sense resistors and big burden voltages.  The disadvantage is that a negative power supply rail is needed.

Commercial solutions