MOSFET and resistor transform unidirectional current-sense amp to bidirectional operation

The LTC6101 amplifier is an example of a very good unidirectional, high-side current-sense amplifier. But that “unidirectional” is sometimes a problem. It means two things: you can't sense negative current, and you can't accurately sense all the way down to zero current. This design idea shows how to use a single MOSFET and a single resistor to solve both problems.

Figure 1 shows the normal operating mode of the LTC6101 amplifier.

Figure 1: An LTC6101 Unidirectional Current Sense Amplifier. Output is ground referenced, cannot sense reverse current; VOUT /ISENSE = RSENSE · R2 / R1 .

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The current to be sensed passes through RSENSE , creating a sense voltage. The amplifier imposes this sense voltage across R1 , forcing a current through its internal FET, and therefore an output voltage across R2 . The gain from sense voltage to output voltage is R2 /R1 , and the overall transimpedance gain of sense current to output voltage is VOUT /ISENSE = RSENSE · R2 /R1 . The function of R3 is to cancel the effects of bias current for improved accuracy.

The circuit works very well down to the point where the output clips at 0 V output. It cannot sense the opposite direction of current, and can accurately “gain up” the applied input-sense voltage only down to its own offset voltage, which may be positive or negative.

Figure 2 shows the bidirectional upgrade.

Figure 2: Adding Q1 and R4 (and upgrading R1 to 1%) creates a bias voltage and input tilt, around which the circuit can now report both positive and negative currents. VOUT = VBIAS ± ISENSE · RSENSE · R2 / R1 .

(Click to Enlarge Image)

The addition of MOSFET Q1 (with some gate bias of 3 to 5 V) and resistor R4 creates an output-bias reference voltage VBIAS (depending on Q1 's VGS ). It also forces a matched current (VBIAS / R4 ) into the drain to apply an accurate input voltage across R3 (upgraded to 1% from Figure 1). This input voltage is now the new null point.

Therefore, with VSENSE = 0 mV, the currents though both left and right legs are identical and the output voltage referred to VBIAS is 0 V. Any applied sense voltage (from a current though RSENSE ) will induce an output voltage with the same gain as previously. But it can be positive or negative with respect to VBIAS , supporting either direction of current flow. That's a lot of additional functionality for a MOSFET and a resistor! Note, however, that effective input-offset voltage is now dependant on the 1% resistors, and will typically be worse than that achieved by the unaided LTC6101.

About the Author
Glen Brisebois, is with Linear Technology Corp, Milpitas, CA.

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