8.1.1 Half-Wave Rectifier With Rail-to-Ground Output Swing

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. R_I must be large enough not to load the LMV61x.

Figure 30. Half-Wave Rectifier With Rail-to-Ground Output Swing Referenced to Ground

Figure 31. Half-Wave Rectifier With Negative-Going Output Referenced to V_CC

8.1.2 Instrumentation Amplifier With Rail-to-Rail Input and Output

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.

Figure 32. Rail-to-Rail Instrumentation Amplifier

8.2.1 High-Side Current Sensing

Figure 33. High-Side, Current-Sensing Schematic

8.2.1.2 Detailed Design Procedure

As seen in (Figure 33), 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 is now less than the positive sense point by an amount proportional to the V_SENSE 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 V_SENSE.

I_G, a current proportional to I_CHARGE, flows according to Equation 1.

Equation 1. I_G = V_RSENSE / R₁ = ( R_SENSE × I_CHARGE ) / R₁

I_G also flows through the gain resistor R₃ developing a voltage drop equal to Equation 2.

Equation 2. V₃ = I_G × R₃ = ( V_RSENSE / R₁ ) × R₃ = ( ( R_SENSE × I_CHARGE ) / R₂ ) × R₃

Equation 3. V_OUT = (R_SENSE × I_CHARGE ) × G

where

• G = R₃ / R₁

The other channel of the LMV61x may be used to buffer the voltage across R3 to drive the following stages.

8.2.1.2.1 Application Curve

Figure 34. High-Side, Current-Sensing Results