8.2.1.1 Design Requirements

The high-side current-sensing circuit (Figure 31) is commonly used in a battery charger to monitor charging current to prevent overcharging. A sense resistor R_SENSE is connected to the battery directly. This system requires an op amp with rail-to-rail input. The LMV93x-Q1-N are ideal for this application because its common-mode input range extends up to the positive supply.

8.2.1.2 Detailed Design Procedure

As seen in Figure 31, the I_CHARGE current flowing through sense resistor R_SENSE develops a voltage drop equal to V_SENSE. The voltage at the negative sense point will now be less than the positive sense point by an amount proportional to the V_SENSE voltage.

The low-bias currents of the LMV93x-Q1-Q1 cause little voltage drop through R₂, so the negative input of the LMV93x-Q1 amplifier is at essentially the same potential as the negative sense input.

The LMV93x-Q1 will detect this voltage error between its inputs and servo the transistor base to conduct more current through Q1, increasing the voltage drop across R₁ until the LMV93x-Q1 inverting input matches the noninverting input. At this point, the voltage drop across R₁ now matches V_SENSE.

I_G, a current proportional to I_CHARGE, will flow according to the following relation:

I_G also flows through the gain resistor R₃ developing a voltage drop equal to:

The other channel of the LMV93x-Q1 may be used to buffer the voltage across R3 to drive the following stages.

8.2.1.3 Application Curve

Figure 32 shows the results of the example current sense circuit.

NOTE: the error after 4 V where transistor Q1 runs out of headroom and saturates, limiting the upper output swing.

Figure 32. Current Sense Amplifier Results

8.2.2.1 Design Requirements

Because the LMV931-N-Q1, LMV932-N-Q1, LMV934-N-Q1 input common-mode range includes both positive and negative supply rails and the output can also swing to either supply, achieving half-wave rectifier functions in either direction is an easy task. All that is needed are two external resistors; there is no need for diodes or matched resistors. The half-wave rectifier can have either positive or negative going outputs, depending on the way the circuit is arranged.

8.2.2.2 Detailed Design Procedure

In Figure 33 the circuit is referenced to ground, while in Figure 34 the circuit is biased to the positive supply. These configurations implement the half-wave rectifier because the LMV93x-Q1-N can not respond to one-half of the incoming waveform. It can not respond to one-half of the incoming because the amplifier cannot swing the output beyond either rail therefore the output disengages during this half cycle. During the other half cycle, however, the amplifier achieves a half wave that can have a peak equal to the total supply voltage. R_I should be large enough not to load the LMV93x-Q1-N.

8.2.2.3 Application Curve

Figure 35. Output of Ground-to-Rail Circuit

Figure 36. Output of Rail-to-Ground Circuit

8.2.3.1 Design Requirements

Using three of the LMV93x-Q1-N amplifiers, an instrumentation amplifier with rail-to-rail inputs and outputs can be made as shown in Figure 37.

8.2.3.2 Detailed Design Procedure

In this example, amplifiers on the left side act as buffers to the differential stage. These buffers assure that the input impedance is very high. They also assure that the difference amp is driven from a voltage source. This is necessary to maintain the CMRR set by the matching R₁-R₂ with R₃-R₄. The gain is set by the ratio of R₂ / R₁ and R₃ must equal R₁ and R₄ equal R₂. With both rail-to-rail input and output ranges, the input and output are only limited by the supply voltages. Remember that even with rail-to-rail outputs, the output can not swing past the supplies so the combined common-mode voltages plus the signal must not be greater that the supplies or limiting will occur.

8.2.3.3 Application Curve

Figure 38 shows the results of the instrumentation amplifier with R₁ and R₃ = 1 K, and R₂ and R₄ = 100 kΩ, for a gain of 100, running on a single 5-V supply with a input of V_CM = V_S/2. The combined effects of the individual offset voltages can be seen as a shift in the offset of the curve.

Figure 38. Instrumentation Amplifier Output Results