AN840 Datasheet, PDF (2/4 Pages) Silicon Laboratories – Redefining a New State-of-the-Art in Microampere Current-Sense Amplifiers

AN840
3. Bidirectional Current Sense Amplifiers
While uni-directional CSAs are primarily used in those applications where current is delivered to a load, there are
many applications where it is necessary to measure current in both directions. Some applications where bi-
directional current-sense monitoring/amplification are needed include: smart battery packs and chargers, portable
computers, super capacitor charging/discharging devices, and general-purpose current-shunt measurements.
Prior to the advent of bi-directional CSAs, uni-directional CSAs were used; however, it was necessary to use two
uni-directional CSAs in order to measure current in both directions. The RS+/RS- input pair of CSA #1 is wired
normally for measuring current to the load whereas, for CSA #2, the RS+/RS- input pair would be wired anti-phase
with respect to CSA #1 for measuring current back to the source. Significant disadvantages to using this
configuration besides the cost of two CSAs are that the technique requires twice the printed-circuit-board (pcb)
area, ties up two ADC inputs, and requires additional microcontroller coding and machine cycles.
To save on additional computing resources, pcb area, and component costs, a straight-forward modification to the
uni-directional CSA configuration yields a bi-directional CSA as shown in Figure 2 for the TS1101.
Figure 2. Typical Application for a Bidirectional High-Precision Current Sense Amplifier (TS1101)
As shown in Figure 2, the internal amplifier was reconfigured for fully differential input/output operation and a
second low-threshold p-channel FET (M2) was added. The operation of this bidirectional CSA is identical to that of
the unidirectional CSA previously discussed when VRS– > VRS+. In the implementation shown in Figure 2, when
M1 is conducting current, the internal amplifier holds M2 OFF. When M2 is conducting current, the amplifier holds
M1 OFF. In either case, the disabled FET does not contribute to the resultant output voltage.
For both types of unidirectional or bidirectional CSAs, gain error accuracy is a measure of how well controlled is the
ratio of ROUT to RGAIN, especially over temperature. In a monolithic implementation, gain error accuracy can be
<0.5% using novel circuit techniques. In a discrete CSA circuit, it would be quite difficult to achieve this level of
performance over temperature with standard 1% tolerance and 100 ppm/°C temperature coefficient resistors.
While some CSAs are only available with fixed-gain options, many CSAs offer different gain options tailored to
specific application requirements.
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