Current Measurement – Why Cap Across Shunt Resistor, Input Bypass Caps, and Small Resistors?

amplifiercapacitorcurrent measurementresistorsshunt

I want to build a sensitive current sensing circuit using shunt, and since I also repair MacBooks occasionally, I have some contact with the schematics of real working expensive devices (although whoever follows Louis Rossmann, knows there are issues there too sometimes, but it's not the point here).

While browsing through the schematics of a macbook, I've noticed an interesting thing: most current sensing circuits are simply shunt resistors with each of their sides connected to current sense amplifier such as INA210. All cool and easy, no magic outside Hogwarts detected.

But when it comes to PPBUS (main power line) and charging power line, there is a 0.02R SHUNT, and sides of the shunt go through 10R RESISTOR each, then there is a CAPACITOR between the inputs of 0.047uF and a 0.1uF CAP on each input line to the ground. And only then the lines go into an IC. Why so much mess, while on other lines it's simply shunt straight to amp. Should I have something similar?

Here's a screenshot of that part of the circuit:

A1466 2015-2017

There is some info here about the small resistors, but it doesn't exactly compare what happens with resistors and without, like with: this happens. Without: this happens.

I still didn't understand how some 10 Ohm resistor is supposed to make things better if input impedance is like a megaohm or greater.

Besides, there is also a capacitor question (why cap across? why bypass caps there? what value? what for? should I do it too?)

Best Answer

The basic purpose of a pair of resistors and a capacitor across the differential amplifier is to filter out the noise.

Measuring current is often noisy, and measuring critical current path requires clean input.

If you follow the design recommendation for Current Monitor IC INA219 in the datasheet, you'll find the exact same configuration:

INA219 filter

I'll copy the verbatim as-is from the datasheet below:

The internal ADC is based on a delta-sigma (ΔΣ) front-end with a 500-kHz (±30%) typical sampling rate. This architecture has good inherent noise rejection; however, transients that occur at or very close to the sampling rate harmonics can cause problems. Because these signals are at 1 MHz and higher, they can be dealt with by incorporating filtering at the input of the INA219. The high frequency enables the use of low-value series resistors on the filter for negligible effects on measurement accuracy. In general, filtering the INA219 input is only necessary if there are transients at exact harmonics of the 500-kHz (±30%) sampling rate (>1 MHz). Filter using the lowest possible series resistance and ceramic capacitor. Recommended values are 0.1 to 1 μF. Figure 14 shows the INA219 with an additional filter added at the input.


Is it a good idea to always include a noise filter?

It depends on several factors:

  1. Your amplifier input bias current. If your amp has a relatively high input bias, then some current will leak into the amp, reducing measurement accuracy. The situation worsens if you're trying to measure low current (μA or nA). INA219 has 100 nA input bias current, so measuring tens of mA to several amperes will be good enough.
  2. Your noise frequency. The low pass filter cannot, well, filter out the low-frequency noise.

But yeah, adding low-pass filter is generally a good practice.