Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The LMV61x devices bring performance, economy, and ease-of-use to low-voltage, low-power systems. They provide rail-to-rail input and rail-to-rail output swings into heavy loads.
Because the LMV61x 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.
In Figure 30 the circuit is referenced to ground, while in Figure 31 the circuit is biased to the positive supply. These configurations implement the half-wave rectifier because the LMV61x can not respond to one-half of the incoming waveform. It can not respond to one-half of the incoming because the amplifier can not 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. RI must be large enough not to load the LMV61x.
Some manufactures make rail-to-rail op amps out of op amps that are otherwise non-rail-to-rail by using a resistive divider on the inputs. The resistors divide the input voltage to get a rail-to-rail input range. The problem with this method is that it also divides the signal, so to get the obtained gain, the amplifier must have a higher closed-loop gain. This raises the noise and drift by the internal gain factor and lowers the input impedance. Any mismatch in these precision resistors reduces the CMRR, as well. The LMV61x is rail-to-rail and therefore doesn’t have these disadvantages.
Using three of the LMV61x amplifiers, an instrumentation amplifier with rail-to-rail inputs and outputs can be made as shown in Figure 32.
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 and require no precision matched resistors in the input stage. 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 occurs.
The high-side, current-sensing circuit (Figure 33) is commonly used in a battery charger to monitor charging current to prevent overcharging. A sense resistor RSENSE is connected to the battery directly. This system requires an op amp with rail-to-rail input. The LMV61x are ideal for this application because its common-mode input range goes up to the rail.
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As seen in (Figure 33), the ICHARGE current flowing through sense resistor RSENSE develops a voltage drop equal to VSENSE. The voltage at the negative sense point is now less than the positive sense point by an amount proportional to the VSENSE voltage.
The low-bias currents of the LMV61x cause little voltage drop through R2, so the negative input of the LMV61x amplifier is at essentially the same potential as the negative sense input.
The LMV61x detects 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 LMV61x inverting input matches the noninverting input. At this point, the voltage drop across R1 now matches VSENSE.
IG, a current proportional to ICHARGE, flows according to Equation 1.
IG also flows through the gain resistor R3 developing a voltage drop equal to Equation 2.
The other channel of the LMV61x may be used to buffer the voltage across R3 to drive the following stages.