AC-DC conversion circuit based on absolute value circuit

Function of the circuit

The linearity and accuracy of the rectifier circuit composed of diodes are not ideal when used to measure the voltage of AC signals or convert AC signals into DC signals . This circuit uses an absolute value circuit composed of an OP amplifier. Because it converts the input signal into the average value by averaging capacitors, it can obtain high accuracy even when the input voltage is very small. The average value can be used to measure the sine wave voltage, but it is difficult to measure the effective value of the pulse waveform by using the average value .

How the Circuit Works

OP amplifier A1 is an AC amplifier with an amplification factor of 10. The required amplification factor depends on the size of the input signal . The amplification factor A can be calculated using the formula A=1+(R3/R2). The low frequency FL of A1 is about 1.6HZ (FZ=1/2πC1.R2 ≈1.6), and C1 and C3 are also related to the low frequency. Their capacities are both 10UF. Because two capacitors are connected in series, the total capacity is 5UF, and its FL is about 0.5HZ. OP amplifiers A2 and A3 are standard absolute value circuits. A2 is a negative output half-wave rectifier circuit, and its output is added to the input signal for full-wave rectification. The ratio of the adding resistors R6 and R7 is very important. Their ratio is 2:1, so 150K and 75K resistors in the E series are selected. C4 is an equalizing capacitor. The capacitance must be determined according to the frequency range of the input signal. If the capacitance is too small, rectification ripple will be generated. On the other hand, the output response also depends on C4. If fast measurement is required, its capacitance cannot be too large. The output is the average value of the full-wave rectification. For a sine wave, it is equal to 2/π (0.636EI) of the peak voltage. This should be noted.

The upper frequency limit is limited by the high frequency characteristics of the OP amplifier, and the matching circuit can reach 100KHZ.

Application Notes

In order to speed up the output response of the AC-DC conversion circuit, C4 is made smaller. If ripple is to be considered, C4 can be removed and a 12DB~24DB/OCT low-pass filter can be added to the output end of this circuit to achieve good results.

Notes

Frequency Limits of Absolute Value Circuits

When using the OP amplifier's rectification circuit or absolute value circuit to measure a very low-level signal, the diode's forward voltage drop can be ignored, and the temperature characteristics are also very good. However, since its working principle is to use the OP amplifier's open-loop gain to be extremely large , when the frequency increases, the loop gain will decrease, causing the rectification performance to deteriorate. Assuming that the diode's forward voltage drop VF is 1V and the OP amplifier's open-loop gain is A0, the error voltage △E can be calculated using the following formula : △E=VF/A0

When A0=40DB, △E=10MV, but for general OP amplifier, when A0=40DB, its frequency is very low and it should be noted.

If you want to improve the accuracy, you can reduce the VF value according to the above formula or choose an OP amplifier with a larger A0 at high frequency.

When it is not required to convert the absolute value output into DC, there will be a problem of waveform synthesis, because the standard absolute value circuit adds the output of the half-wave rectifier circuit to the input waveform, while the OP amplifier has a phase lag, and there is a phase difference between the two, which cannot perform waveform synthesis well. The following measures can be taken to improve it: use a high-speed OP amplifier to reduce phase lag; add a capacitor or a low-pass filter to the input signal channel to align the phase.