Current Sense Amplifiers

Owned by Kevin Li
Last updated: 2021-08-23
5 min read

Current sense amplifiers amplify the small voltages generated across a shunt resistor so that they can be processed effectively by low-voltage ADCs, such as those typically embedded into a microcontroller. These devices commonly take the form as specialized versions of the following two op-amp circuits:

  • Differential Amplifier

  • Instrumentation Amplifier

Differential Amplifier

The figure below shows a typical differential amplifier.

Basic Differential Amplifier Schematic

With the condition R1=R3 and R2=R4 enforced, the gain can be determined by the expression:

Instrumentation Amplifier

An instrumentation amplifier is a type of differential amplifier with an input buffer, eliminating the need for input impedance matching. This makes it suitable for use in measurement and test equipment applications. Instrumentation amplifiers are used where great accuracy and stability of the circuit are required. Characteristics of an instrumentation amplifier include:

  • Very low DC offset

  • Low drift

  • Low noise

  • High open-loop gain

  • High common-mode rejection ratio

  • High input impedance

The figure below shows a typical differential amplifier.

The gain of the circuit is given by the following expression:

Impedance Matching

Impedance matching is the act of designing the input impedance of a device to correspond to the output impedance in order to maximize the power transfer or minimize signal reflection. Differential amplifiers are constructed without the consideration of impedance matching. Instrumentation amplifiers, however, are designed to practice impedance matching.

Current Sense Configurations

The two current sense topologies are:

  • High-side sensing

  • Low-side sensing

Topology

Advantages

Disadvantages

Topology

Advantages

Disadvantages

High-side Sensing

  • The load and system reference are fixed at ground independent of the monitored current

  • Bus and shorts to ground are easily detected

  • Common mode voltage is close to bus, requiring the (potential) need to shift the output down to near the system reference level

Low-side Sensing

  • Reference is fixed at ground and has a low input common mode voltage

  • The load is lifted above ground

  • Short circuits to ground of the voltage bus around the shunt resistor are not detectable

Filtering

The voltage output is generally noisy in applications such as power supplies and DC-DC converters. It is thus necessary to filter the current sensing input path so that the measurement accuracy can be improved. Such filters must be successfully implemented by choosing the right component values. If the wrong component values are selected, non-desired offset voltages and gain errors might be introduced, which compromise circuit performance.

Two filtering architectures can be used:

  • Common mode filter

  • Differential mode filter

These two methods of filtering can be combined for more efficient results. The common mode filter increases ESD immunity or filtered temporary overvoltage. The differential mode filter helps to smooth spiky load currents.

Differential Mode Filtering

The figure below shows an example of differential mode filtering for a current sense amplifier.

The cut-off frequency is given by the equation below:

Common Mode Mode Filtering

The figure below shows an example of common mode filtering for a current sense amplifier.

The cut-off frequency is given by the equation below:

Common and Differential Mode Filter

The common mode and differential mode filter may be merged together to produce the filter as shown in the figure below:

Filtering Example

Consider the power supply below which delivers 5V at 3A. The current is monitored as shown in the figure below.

Considering a shunt resistor of 0.05Ω, the theoretical voltage seen at the input of the current sense amplifier is 150mV. Due to the switching mode of the power supply, the provided 5V has an undesirable high frequency. The differential voltage without filtering is shown in the figure below.

With a differential mode filter, the differential voltage is shown in the figure below.

With a common mode filter, the differential voltage is shown in the figure below.

Â