The circuit in Figure 7-17 uses a single supply to half-wave rectify a sinusoid centered about ground. R_i limits current into the amplifier caused by the input voltage exceeding the supply voltage. Full-wave rectification is provided by the circuit in Figure 7-19.

Figure 7-17 Half-Wave Rectifier With Input Current Protection (R_i)

Figure 7-18 Half-Wave Rectifier Waveform

In Figure 7-23, dielectric absorption and leakage is minimized by using a polystyrene or polyethylene hold capacitor. The droop rate is primarily determined by the value of C_HOLD and diode leakage current. The ultra-low input current of the LMC648x has a negligible effect on droop. For applications requiring ultra-low input bias current, see the OPA928.

Figure 7-19 Full-Wave Rectifier With Input Current Protection (R_I)

Figure 7-21 Large Compliance Range Current Source

Figure 7-23 Low-Voltage Peak Detector With Rail-To-Rail Peak Capture Range

Figure 7-20 Full-Wave Rectifier Waveform

Figure 7-22 Positive Supply Current Sense

The high CMRR (82dB) of the LMC648x allows excellent accuracy throughout the rail-to-rail dynamic capture range of the circuit.

Figure 7-24 Rail-to-Rail Sample and Hold

The low-pass filter circuit in Figure 7-25 can be used as an anti-aliasing filter with the same voltage supply as the A/D converter.

Filter designs can also take advantage of the LMC648x ultra-low input current. The ultra-low input current yields negligible offset error even when large value resistors are used. This in turn allows the use of smaller valued capacitors that take less board space and cost less.

Figure 7-25 Rail-to-Rail, Single-Supply Low-Pass Filter