Level-shifting nixes need for dual power supply
Ron Olmstead, Westcor, Sunnyvale, CA -- EDN, January 6, 2000
The AD736 true-rms-to-dcconverter is useful for many applications that require precise calculation of the rms value of a waveform. This converter can determine the true rms value, the average rectified value, or the absolute value of a myriad input waveforms. Basically, all applications require both a positive and a negative power supply. According to the data sheet, you can use the device with a single supply by ac-coupling the input signal and biasing the common pin above ground. However, the ability to process only ac signals is a major performance limitation. You can lift this limitation by using a level-shifting approach (Figure 1). This approach requires more circuitry, but it removes the ac-only input-waveform restriction.
The circuit consists of three sections. The first is a differential amplifier that adds the level-shifting offset, VREF, to the input waveform. This amplifier's primary function is to level-shift the waveform, but it can also provide gain and filtering if necessary. The output of the op amp needs to swing to the value of VREF minus the peak negative swing of the input waveform times the gain of the op amp (VREF-(A*VN)) and to the value of VREF plus the peak positive swing of the input voltage times the gain of the op amp (VREF+(A*VIN)). By adjusting the value of VREF and the gain of the op amp, you can eliminate the need for an expensive rail-to-rail op amp and can then use any single-supply op amp. All three sections use the same level-shifting offset, VREF.
The second section is the rms-to-dc-converter stage. The output of this stage is the dc (rms) value of the input waveform plus the offset value (VREF). The input voltage divider reduces the amplitude of the input waveform. For successful rms-to-dc conversion, the circuit must keep the voltage going into the AD736 within the specified range, which is 1V rms for a VCC of ±5 to ±16V. If amplitude reduction is unnecessary, you can eliminate these resistors and simply ground Pin 1 of the AD736. The offset voltage needs to connect to the AD736 (Figure 1). This connection provides a reference for the circuit that is above ground. The AD736 cannot provide accurate calculations for inputs that go below or even equal the converter's negative rail, -VS. VREF should be greater than the peak negative swing of the input waveform. VCC should be greater than VREF plus the peak positive swing of the input voltage.
The third section of the circuit is a level-shifting circuit, which subtracts VREF from the output of the AD736. The last-stage differential amplifier can provide any necessary gain, and you can use this gain to eliminate the need for a rail-to-rail op amp.
The application of the circuit in Figure 1 is to measure the current draw of a power supply and detect overcurrent conditions. For this application, only a positive power supply was available. The input op amp raises the amplitude of the input signal and filters out any noise greater than 5 kHz. The power-supply input is a three-phase 60-Hz signal, so the ripple frequency is 360 Hz. By providing gain in this first stage and a 5V level shift, any single-supply op amp is suitable. Also, a rail-to-rail op amp is unnecessary. The circuit divides down the output of the first stage to be sure not to exceed the input voltage range of the AD736. The output amplifier provides gain to the dc signal and level-shifts the signal back to a ground-referenced signal. Again, the gain of this op amp produces a signal with an amplitude suitable for use with any single-supply op amp. (DI #2466)
