The op amp-based current sensing circuit described here is not new, it has been around for some time, but with very little discussion of the circuit itself. Somewhere along the line it was informally given the name: “Current Drive” circuit, so we'll call it that for now. Let's dive into the basic concept first, which is that of an op amp and MOSFET current source (note that bipolar transistors can also be used if you don't mind around 1% error due to base current). Figure 1A shows a basic op amp current source circuit. Flip it upside down so that we can do high-side current sensing in Figure 1B, redrawn in Figure 1C to depict how we will use the shunt voltage as the input voltage, and finally in Figure 1D the final circuit.
This figure depicts the transformation from a basic op amp current source to a high-side current sensing amplifier with a current output.
Figure 2 shows an implementation at low voltages of less than the power supply rating of the op amp. Add a load resistor for voltage to current conversion keeping in mind that you now have a high impedance output, which may be fine where you want the simplest solution.
The Basic Circuit
Figure 2 shows a full circuit for the basic implementation of high-side current sensing. Among the details to consider:
- The op amp must either be rail-to-rail input, or have a common-mode voltage range that includes the positive supply rail. Zero-drift op amps are good choices for minimum offset. However, remember that even with zero-drift rail-to-rail op amps, you are operating in the upper common-mode range which is not the optimum for lowest offset, usually.
- The output node at the MOSFET drain is limited with regard to positive voltage swing to something less than the supply rail that the shunt is on, or put another way, less than the common-mode voltage. Adding a buffer with gain can reduce the voltage swing requirements at this node.
- This circuit does not have a common-mode voltage capability to zero volts as would be necessary for low side sensing or current sensing during a dead short. In the circuit of Figure 2 the maximum common-mode voltage is equal to the maximum supply voltage rating of the op amp.
- This circuit is uni-directional and can only measure current in one direction
- Gain accuracy is a direct function of RIN and RGAIN tolerance. Very high gain accuracy is possible.
- Common-mode rejection ratio (CMRR) is generally determined by the amplifier common-mode rejection capability. The MOSFET can also play a part in CMRR, a leaky or otherwise crummy MOSFET can degrade CMRR.
The simplest realization uses an op amp within its power supply voltage ratings. This one is configured for a gain of 50. Gain is set by RGAIN/RIN.
A fully buffered output is always much more versatile than the high impedance output of Figure 2 and providing a slight gain of 2 in the buffer reduces the dynamic range requirements of the first stage and the MOSFET.
In Figure 3 we also add circuitry to allow bi-directional current sensing. The concept here is to use a current source circuit (remember Figure 1A?) along with an input resistor (RIN2) on U1 non-inverting input equal to RIN (in this case it becomes RIN1). This resistor then develops a voltage drop to offset the output to accommodate the necessary bi-directional output swing. The gain from the REF pin to the overall circuit output is based on the relationship of RGAIN/ROS such that the REF input can be configured to provide unity gain regardless of the gain set by RGAIN/RIN (so long as RIN1 and RIN2 are the same value), operating just like a conventional difference amplifier reference input:
Note that in all subsequent circuits that the bi-directional circuitry is optional and can be omitted for uni-directional operation.
This version adds the buffered output along with bi-directional sensing capability. It provides a reference input that always operates at unity gain even over different gain settings determined by the RIN1 and RIN2 values.
Use at High Common-Mode Voltages
The current drive circuit can be used at nearly any common-mode voltage by floating the circuit and using a MOSFET with an adequate voltage rating, this has become a very common and popular application with circuits shown operating up to hundreds of volts. The voltage rating this circuit is capable of is determined by the voltage rating of the MOSFETs used.
Floating the circuit consists of adding the Zener diode, Z1, across the amplifier and providing it with a source of bias current from ground. The Zener bias can be as simple as a resistor, although this author likes the current mirror technique as it improves the circuit's ability to tolerate variations in load voltage. In doing this we have created a power supply “window” across the op amp that floats at the load voltage.
Another diode, D1, has appeared in the high voltage version. This diode is necessary since a short circuit to ground at the load initially would pull the non-inverting input negative enough compared to the amplifier negative supply rail that it would damage the amplifier. The diode clamps this condition protecting the amplifier.
The high voltage implementation “floats” the op amp and its Zener power supply at the load voltage rail.
Other Lesser Known Uses for the Circuit
I'm not sure anyone uses current sensing MOSFETs anymore. In some lab studies a couple of years ago I satisfied myself that, once calibrated, MOSFET current sensing was both very accurate and linear, although they did have a tempco of around 400 ppm. Nonetheless the optimum circuit configuration forces the sense electrode to operate at the same voltage as the source of the MOSFET while delivering its portion of the current. Figure 5 shows how to accomplish this with the current drive circuit.
MOSFET sense FET circuit
Lastly, for some additional fun with this circuit, and demonstration of how it can be used to “ratio” current see: