Bidirectional Current Sense Amplifiers (CSA) are capable of measuring current flowing in either direction. The quiescent output level corresponds to zero current. One, sometimes two output reference pins are available for configuring the zero current voltage. A bidirectional CSA can function in unidirectional mode by setting the quiescent output at or close to either of the supply rails. There are many ways of driving the reference pins. The most common methods include using a reference IC, a voltage divider, or a voltage divider followed by a buffer. Output impedance of the driving source impacts the CSA output and may cause significant error if not designed properly. A voltage divider has the advantage of being versatile and inexpensive. It also tends to occupy less area. As a result, it finds adoption in many applications.
All trademarks are the property of their respective owners.
One way to classify a Current Sense Amplifier (CSA) is based on whether it is able to measure current in both directions. Then it can be put into one of two categories - unidirectional or bidirectional. A unidirectional device only linearly responds to current flowing in one direction, with its output moving in one direction in proportion to the input differential signal. A current flowing in the opposite direction causes the output to collapse to one of the supply rails, normally ground. Figure 1-1 illustrates such a scenario.
For a bidirectional CSA, current flowing in either direction is allowed. The output of the device moves off of a quiescent output level, in proportion to the input differential signal. The fact that bidirectional CSA output is able to move up toward supply or down toward ground implies that the quiescent output level corresponds to zero current. In these devices, there is typically one or two output reference pins. The output is level-shifted by driving the reference pins with a suitable source. Figure 1-2 shows the same bidirectional input is accurately reproduced. A bidirectional CSA can be configured as unidirectional by setting the quiescent output at or close to either supply rail.
This report reviews TI’s bidirectional CSA then examines different ways of configuring the output reference, associated performance tradeoffs, and the reasons behind these tradeoffs. Next, the impact of the resistor divider driving reference pins in common CSA architectures is explored. The goal is to help designers make an informed decision when choosing a reference driving circuit that meets performance requirements, and is economical at the same time.
A CSA measures target current by deriving a small differential signal (voltage or current) that is proportional to the magnitude of the current. Signal conditioning circuity then turns this small differential signal into a stable and noise-free output for further processing down the signal chain. For shunt-based current sensors, either non-isolated or isolated, the input differential signal is created by inserting a shunt resistor in the path of the target current. Magnetic sensors work without making physical contact between the sensor IC and the target current. For example, the magnetic field generated by the load current can be sensed by a Hall sensor, which is then conditioned and amplified by a low-noise amplifier.
Figure 2-1 shows a block diagram of a typical bidirectional CSA with a single reference pin. The input stage is responsible for extracting the differential input signal while rejecting the typically very high input common-mode voltage. The input stage can take on many forms, including but not limited to, voltage feedback, current feedback, and isolated technology. The output stage takes care of output drive capabilities to interface effectively with downstream circuitry.
The output stage is typically a classic difference amplifier. To enable bidirectional measurement capability, the output stage is equipped with a reference pin. By providing a positive reference voltage to the reference pin, the output is level shifted to a desired quiescent output voltage. Typically, when a positive differential input is applied, the output moves away from the quiescent voltage, toward the supply. Conversely, when a negative differential input is applied, the output moves away from the quiescent voltage toward ground.
Matching of the resistor network is important. One of the parameters that reflect how well the resistors match is Reference Voltage Rejection Ratio (RVRR). This parameter measures net change (relative to Vref) in output voltage for a given amount of change in reference voltage. If RVRR is listed in the data sheet and is input referred, the device gain should be used as a multiplier in calculating the corresponding change in output.
Some bidirectional devices come with two reference pins which are connected internally to form a voltage divider. Figure 2-2 shows such an arrangement.
As an example, for INA240, the reference divider is made up of two equal-value resistors. Real-world differences directly influence the reference voltage. For this reason, the divider accuracy is specified in the data sheet, However, if the two reference pins are shorted together and driven with a voltage source, then the divider function is not used. The divider accuracy specification is not a concern in this situation.
A common scheme of creating a reference voltage, called splitting the supply, is shown in Figure 2-3. One of the reference pins is connected to the device power supply, while the other connected to ground. This results in a reference voltage that is half of the supply. In similar fashion, this scheme can be used to create customized references, with voltage rails at different potentials.
The two-pin arrangement brings flexibility without incurring additional error compared with external resistor dividers. When the two reference pins are shorted together, they function exactly the same as a single pin and can be treated as such.